grounding fundamentals course presentation
DESCRIPTION
Grounding Fundamentals Course PresentationTRANSCRIPT
Grounding Fundamentals
Instructor: Allan Bozek, P.Eng.www EngWorks cawww.EngWorks.ca
1 5 EICCEUs1. 5 CEUs
Introduction
• Introductions• Introductions• Please introduce yourself – name, job title and
experienceexperience• Sign-in sheet circulated, everyone please sign
in and return• Emergency response requirements• Please turn off all cell phones or turn to silentPlease turn off all cell phones or turn to silent
mode• Washrooms and Breaks
2www.EngWorks.ca Grounding Fundamentals 2
Safety TopicStatic Electricity and Refuellingy g
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Safety TopicStatic Electricity and Refuellingy g Some statistics Petroleum Equipment Institute reports 175 fires since
19921992 50% of the accidents occurred when the refueler returned
to their vehicle Women account for 75% of all static ignition firesWomen account for 75% of all static ignition fires
Safety Guidelines when refueling Turn off engine
D 't k Don't smoke Never re-enter your vehicle while refueling. Do not overfill or top off your tank
If a fire starts Do not remove the nozzle from the vehicle or try to stop
the flow of gasoline. Immediately leave the area and call g yfor help
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Learning Objectives1. To understand why we ground2 To describe the difference between grounding and2. To describe the difference between grounding and
bonding3. To apply the safety requirements as defined by the3. To apply the safety requirements as defined by the
Canadian Electrical Code and the IEEE as they relate to grounding
4. To select the appropriate systems grounding scheme for an industrial facility Sizing of components How it impacts the overall design of a facility
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Learning Objectives5. To implement a static electricity control and
lightning protection systemg g p y6. To avoid the problems typically associated with the
grounding of sensitive electronic systemsg g y Ground loops Methods of noise mitigation
7. To design a ground grid for a high voltage industrial substation Concept of ground potential rise and touch and step
potential
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Agenda Overview1. History of Grounding2 System grounding2. System grounding Generator and UPS systems grounding
3 Equipment bonding3. Equipment bonding4. Static Protection 5 Lightning Protection5. Lightning Protection6. Grounding of Electronic and Instrumentation
SystemsSystems7. Station Ground Grid Systems Design8 T t i l8. Tutorialwww.EngWorks.ca Grounding Fundamentals 7
Introduction
Section 1
Edison's Pearl Street Generation Station
Pearl Street Generation station was initiallystation was initially constructed in 1882 to provide DC current for lighting systems in New York's financial district
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Edison’s Floating Approach to DC Systemsy Original design used an earth ground for DC
lighting systemslighting systems Several incidents associated with “stray
currents” forced Edison to revise his plancurrents forced Edison to revise his plan One dead horse
W k b th ti t ti ld f l Workers nearby the generating station could feel the currentBelieved the there was a “devil in the wire”Believed the there was a devil in the wire
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Edison’s Floating Approach to DC Systemsy
Current Flow
G LL
+
Gen LLIntendedReturnPath
-
UnintendedReturn Path
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Human Sensitivity to Electricity
Physiological Reaction to y gElectric Current Range from minor muscular
contraction to ventricular fibrillationFunction of body weighty gCurrent magnitudeCurrent duration
H b d b id d 1000Ω i t
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Human body can be considered a 1000Ω resistor
Human Sensitivity to Electricity
Direct Current Alternating Current
Human Response (ma)g
(ma)Men Women Men Women
Slight Sensation on Hand 1 0.6 0.4 0.3“Let Go” Threshold 6.2 3.5 1.1 0.7Shock – Not Painful 9 6 1.8 1.2Painful Shock – Muscular 62 41 9 6Control LossSevere Shock –Breathing Difficult
90 60 23 15
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Edison's’ Isolated 3 Wire System Edison later adopted a 3-
wire system that did not Positiveyrely on a earth path for return G1 LL
+100V
Allowed two circuits to be run with three wires
Circuit was isolated from
G1-
+
Neutral200V
Circuit was isolated from ground
All currents within the circuit G2 LL
+
-100V
could be measured and accounted for Negative
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Shock Current Path A shock current path
requires two pointsSingle point ofcontactq p
One point for the current to enter and the second to exitto exit
Voltage difference is required for current to flo
G1 L
N lflow An isolated system under
normal operating conditions
Neutral
Isolated GroundS tinsures a single point of
contactSystem
No Shock Current Exists
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Shock Current Path Under fault conditions,
an isolated system Single point oft tA id t lan isolated system
ground creates a shock hazard
contactAccidentalGround
G1 LNeutral
Alternate circuit pathleads to shock hazardleads to shock hazard
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Ground Fault Detection An isolated system cannot detect the presence
of a ground faultof a ground fault
Fuse
AccidentalGroundCircuit protection
cannot detectG1 L
Neutral
the accidentalground
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System Overvoltage and Surges An isolated system cannot dissipate a high
voltage surgevoltage surge Usually results in equipment damage
Lightning
Equipment insulationis stressed as the
Fuse
LightningStrike
is stressed as the high voltage surge finds its way to
G1 L
Neutralground
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The Intentional Grounding of Circuits Elihu Thompson Founder of Thompson-Houston
IndustriesLater merged with Edison General and
became General Electric Author of over 700 patents
Advocated AC systems should be intentionally earthedbe intentionally earthed Proposed as a safeguard against a
breakdown in insulation of a primary circuit conductor
Proposal created a large amount of controversyy
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Grounding Premise An intentionally grounded circuit provides a circuit
path back to the source in the event of an paccidental ground
Allows the circuit protective devices to function ppreventing the circuit from becoming a safety hazard
Low Impedancepath to source G1 L
Fuse
AccidentalGround
path to sourceallows fuse tooperate
G1 L
Neutral
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History of Grounding
Practice of earthing the secondary (neutral) conductor was banned by the New York Board ofconductor was banned by the New York Board of Fire Underwriters Speculation that Thomas Edison was behind the p
scenes with his patented 3 wire un-grounded circuit AIEE (Precursor to the IEEE) recommended that
low voltage AC systems be grounded where alow voltage AC systems be grounded where a reliable ground connection could be secured Advocated a solid connection without a fuse on the
t l ineutral wire
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History of Grounding NFPA later resolved that grounding the secondary
circuit was the only way of absolutely insuring the f t f th i itsafety of the circuit The debate continued from 1903 – 1913 when it was
passed into law Secondaries of all circuits 550V or less must be groundedRecommended that all circuits 300V or less be grounded
Original rule has not been changed in substance Original rule has not been changed in substance since the original 1913 rule in the NEC Section 10 of the CEC Part 1 also adheres to the
fundamental premise of the rule
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Canadian Electrical Code - Part 1 CSA C22.1-06
Minimum safety standards for installation and maintenance of electrical equipment Compliance will ensure a
safe installationSection 10 deals Section 10 deals specifically with grounding and bondingand bonding Significant re-write in 2006 Minor updates in 2009p
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Canadian Electrical Code - Part 1 CSA C22.1-06Scope and object: Rules 10-000 and Rule 10-002Protect life from the danger of shockgLimit the voltage on a circuitFacilitate operation of protective devices
System and circuit grounding: Rules 10-100 to 10-116All circuits must be grounded with the exception of:Electric Arc furnacesCranes installed in Class III locationsCranes installed in Class III locations Isolated systems in patient care areasCircuits less than 50V
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CEC Handbook Provides background
information and commentary th R l f th C dion the Rules of the Canadian
Electrical Code, Part I Intended to provide a clearer Intended to provide a clearer
understanding of the safety requirements of the CodeI t i f ti Incorporates information on: Rational IntentIntent Field Considerations
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IEEE Standard 142(Green Book)( )
Recommended practices and methods associated with grounding Systems grounding Equipment grounding and bonding Equipment grounding and bonding Static and lightning protection Grounding electrode design Grounding of electronic equipment
Applies to industrial and commercial power systemscommercial power systems Utility grounding methods are not
covered Recommended Purchase
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Commonly Used Grounding Terms and Definitions
Neutral Point Neutral Conductor
Metallic
Neutral GroundDevice
MetallicEnclosure
Grounding Conductor
Bonding ConductorStray Current
EarthGroundingElectrode
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Earth Conducting body of varying resistance Earthing – A connection to earthEarthing A connection to earthInterchangeable with the term ground
Earth
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Earth
Ground A conducting connection by which an electrical circuit
is connected to earth Grounding Electrode – a conductor buried in earth and
used for collecting or dissipating ground current to earth Grounding Conductor – conductor used to connect the g
service equipment to a ground electrode
Grounding G gConductor
Grounding
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GroundingElectrode
Bonding Low impedance path created by joining all non-
current-carrying metal parts to ensure electrical ti itcontinuity
Bonding Conductor – conductor that connects the non-current carrying parts of electrical equipment, raceways or enclosures
B di C d tBonding Conductor(Equipment ground conductor)
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Neutral Point The point of a symmetrical system which is normally
at zero voltageg Neutral Conductor – a system conductor, other than a
phase conductor that provides a return path for current to the sourcethe source
Neutral Point
Neutral GroundNeutral ConductorDevice
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Definitions Ground Fault Current – ground current resulting
from any phase-conductor-to-earth faulty p Normal – brief flow of current that occurs until the
protective device opens Abnormal – continuous flow of current from a phase
conductor to ground Often referred to as the Zero Sequence Current Often referred to as the Zero Sequence Current
Neutral grounding devices include grounding resistors, grounding transformers, ground-faultresistors, grounding transformers, ground fault neutralizers, reactors, capacitors, or a combination of these components
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Ground Fault Current
Metallic
IntendedG d F lt
MetallicEnclosureNeutral Ground
Device Ground FaultCurrent Path Ground Fault
Earth
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Stray Current The uncontrolled flow of current over and through
the earth results in undesired safety and system performance
characteristics
Stray Current
Earth
Stray Current
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Earth
Systems Grounding
Section 2
Purpose of a Systems Ground
“System grounding, or the intentional connection f h t l d t t th i fof a phase or neutral conductor to earth, is for
the purpose of controlling the voltage to earth, or ground within predictable limits”or ground, within predictable limits
Most system faults are ground fault related
IEEE 142 Green Book
Most system faults are ground fault related
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Systems Ground A systems ground will: Control the voltage to ground to prevent stressing g g p g
equipment insulation Allow the operation of ground fault detection protection
d idevices Reduce the risk of fire and shock hazard to persons who
might come in contact with live conductorsmight come in contact with live conductors In some cases provide service continuity
Allow the ground fault to be isolated and repaired at a convenient titime
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Concept of a System Ground A grounding system
consists of all SystemsGround 1
interconnected grounding connections in a specific
YGround 1
SystemsGround 2
power system and is isolated from adjacent; grounding systems through Y
YYSystemsgrounding systems through
a high impedance Isolation occurs via an M M M
Y Y
Y
SystemsGround 3
Isolation occurs via an ungrounded transformer winding connection
PPSystemsGround 4
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Transformer Winding Connections ∆ (delta) Connections Isolates the power system p y
from ground Important is creating “zones of
protection”
Y (wye) Connections Y point provides a neutral point
for managing ground faultsfor managing ground faults Opportunity for multiple
voltages
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System Grounding Classifications
Ungrounded Solid Ground
Resistance Ground Reactance Ground
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System Grounding Classifications
Systems Grounding
Ungrounded Grounded
ImpedanceGrounded
SolidGrounded
Resistance Reactance
LowResistance
HighResistance
Reactance TunedReactance
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Reactance
Ungrounded Historically was used on power systems where a
high level of process continuity was requiredg p y q Exists in many process facilities designed prior to 1980
Advantagesg Single ground fault does not does not allow current to flow
Allowed for a controlled shutdown for fault repairs
f Eliminates the need for elaborate protection schemes Grounding system cost is minimized A
NG B
C
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Ungrounded Disadvantages On a ground faults, the voltage to ground for the remaining g , g g g
phases is elevated by 73%Higher insulation rating required for system components
T i t lt b bl Transient overvoltages can be a problemVoltages up to 6X system voltage stresses insulation eventually
leading to a second ground fault and subsequently a phase to phase fault
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Ground Fault Voltage Shift
Normal Operating Conditions A
A
IA
N
VAG
IA IB
BCIB
I
N
A
BC
VCG VBGIC
CA CB CC
IC
NVANVCN
A
VCAVAB
VAG
GVBN
If CA = CB = CC then IA+ IB + IC = 0 BC
GVCA AB
VBC
VCGVBG
N
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A B C A B C
Ground Fault Voltage Shift
Ground Fault Phase C A
VAG
IB
IG
A
VBGB
C G VCG=0
IA
B
VB
IA
I
N
A
VCA
VANC
CA CB
IBNVBN
IG
IA + IB = IGBC
CAVAB
V
VCG=0
VAG NG
VCN
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BCG VBC
VBG
Intermittent Ground Faults Intermittent or restriking
type ground faults on A yp gisolated grounded systems can cause severe
A
B
IA
N
system overvoltages Up to 6 or 8 times line to
line voltage
BC
CA CB
IB
IG line voltage Will eventually lead to an
insulation failure resulting in G Breakdown in insulation ga phase to phase fault
Must be detected and corrected ASAP
results in phase to phase fault IG = ISC
corrected ASAP
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Ungrounded System Ground Fault Detection Scheme10-106 Alternating-current systems (see Appendix B)(2) Wiring systems supplied by an ungrounded supply shall ( ) g y pp y g pp ybe equipped with a suitable ground detectiondevice to indicate the presence of a ground fault.
Ground
L L L 0VLight DimsOrExtinguishes
Fault
Extinguishes
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Ground fault Detection Scheme
Solid Ground A solid grounded system is one in which the neutral
points have been intentionally connected to earth points have been intentionally connected to earth ground with a conductor having no intentional impedance Often referred to as effective grounding
N
A
NG B
C
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Solid Ground
Uniground SystemUsed in Industrial Systems
Multi-grounded SystemUsed by Utilities in Rural
Distribution SystemsDistribution Systems
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Solid Ground Advantages Partially reduces the problem of transient over-voltages
R d d i l ti l l i dReduced insulation level required Ground faults do not shift the system neutral Simple ground relay schemes provide for circuit protection
Disadvantages Damage at the fault may be excessive Arc flash hazard due to high ground fault current levels Difficult to coordinate ground fault protection
Magnitude of the fault current is unknown
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CEC DefinitionEffective Grounding - a path to ground from circuits,
equipment, or conductor enclosures that is q p ,permanent and continuous and has carrying capacity ample to conduct safely any currents liable to be imposed upon it
CEC Rule 10-500 in Appendix B states that the complete fault path of the circuit conductor together with the bonding returnpath of the circuit conductor, together with the bonding return, should have an impedance that allows at least five times the current setting of the overcurrent device to flow when a fault of negligible impedance occurs
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Solid Ground
A
NBC
N
IO/C fuse may notVAN
NVBN
VCN
IGO/C fuse may notclear arcing ground fault
HighG
HighImpedanceGround fault
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High Resistance Ground System is grounded through a high-impedance
resistor High-impedance grounding typically limits ground fault
current to 25 A or less Typically used on low voltage (600V or less) systems
under 3000 Amps
N
A
G BC
2 - 25A
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High Resistance Ground Scheme
1000 KVA Xfmr25kV 600V
51G AL25kV – 600V5.75% ZY 5 Amp
NGR
NGR5A
75kVAStarter
45kVA
5A
M MMLP HTPU
/H
25HPInjection
Pump
75HPRecyclePump30kWLighting
150HPCooling
Fan Heat Trace
75kVA
X 2X 2
Ground
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u pPumpUnit
HeaterPanel
Fan Heat TracePanelFault
High Resistance Ground Advantages Allows system to operate under a ground fault conditiony p g Reduces arc flash energy associated with a ground fault Insures a ground fault of a known magnitude
Aids in protective relay coordination and limiting equipment damage
Reduces transient ground fault overvoltagesReduces transient ground fault overvoltages Allows easy identification and isolation of the ground fault
location Disadvantages Neutral shift on ground fault
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Low Resistance Ground System is grounded through a low-impedance
resistor Low-resistance grounding typically limits ground fault
current to 400A or less for a short period of time (10 sec) Typically used on medium and high voltage industrial
power distribution systems
N
A
G BC
25 - 400A
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Low Resistance Ground Advantages Allows protective relay devices to quickly clear a ground p y q y g
fault Limits damage to equipment and reduces overheating and
h i l t d tmechanical stress on conductors Disadvantages
Ne tral oltage shift of limited d ration Neutral voltage shift of limited duration
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Low Resistance Ground Scheme
Trip Upstream Breaker
Y
51
NGR
Trip setting ~ 20%of NGR rating
Y400A NGR
13.8kV
NGR400A Trip Downstream Breaker
M51
Y600V5A NGRSGR
(Secondary GroundResistor)
XFMR
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M MAlternate Arrangement
LR Grounding Resistor
Connection to Neutral Point on Transformer
Connection to ground
Resistors
Current Transformer
51
NGR400A
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LR Grounding Transformer
Ground resistor
SGRXFMR
51
SGR(Secondary Ground
Resistor)
XFMR
Grounding Transformer
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Alternate Grounding Schemes Corner-of-the-Delta System Applicable to low-voltage I
A
pp gsystems
Not widely used in industrial t
B
Csystems
Delta One Phase Grounded
I
Delta One Phase Grounded at Midpoint Applicable to single phase
240VApplicable to single phase 120/240V loads
G 240V
120V120V
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Reactance Grounding Ground fault current is a
function of the neutral reactance Typically results in higher
N
ground fault currents than a resistance grounded system25 – 60% of three phase fault
51
Reactor5 60% o t ee p ase au tcurrent
Primarily used by Utilities on multi grounded systems on
IG
multi-grounded systems on systems above 5kV
Seldom used in industrial plant applications
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Resonant Grounding Tuning reactor is used to ground
the neutral point to ground Reactor is tuned to match the system
capacitanceResults in a very low value of ground
N
fault current 75% of line to ground faults are self-
extinguishingC l t l i d t
51
Reactor Complex controls are required to
constantly match the reactance to the system capacitanceP i il d h d d I
Ground FaultNeutralizer
Primarily used on overhead and transmission lines above 15kV
Rarely used in industrial applications
IG
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Grounding System Comparison
Condition Un-grounded
Solid Ground
Low Resistance
High Resistanceg
Immunity to transient overvoltages Worst Good Good Best
Arc Fault DamageArc Fault Damage Protection Worst Poor Better Best
Safety to Personnel Worst Better Good Best
Service Reliability Worst Good Better Best
Continued operation Better Poor Poor Bestafter initial ground fault Better Poor Poor Best
Ground fault locating Not Possible Good Better Best
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Ground Fault Sensing
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Ground Fault SensingGround ReturnGround fault current isGround fault current is
measured in the neutral to ground connection Phase A
Applicable only at a source transformer or generator 51G Phase B
Phase C
Neutral
gOften used for ground
fault alarm sensing on LV di t ib ti tdistribution systems
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Ground Fault SensingZero Sequence RelayMeasures zero sequence orMeasures zero sequence or
ground currents by sensing the magnetic fields surrounding th h d t l
Phase A
the phase and neutral conductorsShould cancel under normal
Phase BPhase C
Neutral
conditions
Often used in motor protection and feeder breaker relays 51Gand feeder breaker relays
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Ground Fault SensingDifferentialPhase current and neutralPhase current and neutral
current values are measured and ground fault current is
l l t d th diffPhase A
calculated as the differenceUsed in applications where
current transformers are Phase BPhase C
Neutral
required for phase overcurrent relaysHi h i d t ti
51G
Phase C
High accuracy in detecting ground faults
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High Resistance Ground Detection Scheme
1000 KVA Xfmr25kV 600V
51G AL
25kV – 600V5.75% ZY 5 Amp
NGRPulsing ResistorNGR5A
Pulsing readingon phase indicates 75kVA
Starter45kVA
ClampOn CTon phase indicates
ground faultM MM
LP HTPU/H
25HPInjection
Pump
75HPRecyclePump30kWLighting
150HPCooling
Fan Heat Trace
75kVA
X 2X 2
Ground
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u pPumpUnit
HeaterPanel
Fan Heat TracePanelFault
High Resistance Ground fault Detection Systemy
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Ground Fault Relay SettingsAlarm only on continuous rated ground resistor
applicationsppAlarm setting at 80% of maximum current level allowed by
ground resistorAbove system charging current level
Trip on short time duty ground resistor applicationsHigh resistance ground applicationsHigh resistance ground applicationsTrip at 80% of maximum current level allowed by resistor
Low resistance ground applicationsg ppTrip at 20% of maximum current level allowed by ground resistor
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Low Resistance Ground Detection Scheme
Trip Upstream Breaker
Y51G
NGR
p pTrip setting ~ 20%of NGR rating
Y400A NGR
NGR400A Trip Downstream Breaker
Trip
13.8kV51G
Trip Trip Trip51G
Trip
ZCT
ZCT
M
51G 51G 51G M
51G
ZCT - Zero Sequence CTs
ZCTZCT ZCT
ZCT
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GF Relay Time Coordination Curves
Settings for ground-fault relays can berelays can be determined during the relay coordination studyy y
GF curves are plotted on the coordination diagrams Set parameters include
ti d t l ltime and current level
Ground Fault coordination curves
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Ground Fault coordination curves
CEC Requirements AssociatedCEC Requirements Associated with Systems Grounding
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CEC Code Requirements10-1102 – Installation of Neutral Grounding Devices1) Neutral grounding devices can only be installed on1) Neutral grounding devices can only be installed on
systems where line to neutral loads are not servedNo single phase loads from a resistance grounded system
2) S t ith lt 5kV h ll b d i d2) Systems with voltages > 5kV shall be de-energized on detection of a ground fault
a) Electrical systems operating at 5 kV or less are permitted to remain ) y p g 5 penergized if the ground fault current is controlled at 10A or less
i. Audible alarm is required
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CEC Code Requirements3) Where line-to-neutral loads are served, the
system must be de-energized on occurrence of a:system must be de energized on occurrence of a:1) Ground fault2) Grounded neutral on the load side of the NGR3) Break in the continuity of the conductor connecting the
NGR to ground
Apparent conflict between subsection 1) and subsection 3)
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NGR with Isolated System Neutral
AHTCkt
HTCkt
HTCkt N
HT HT HT HT
Trip main breaker
51 BC
HTCkt
HTCkt
HTCkt
HTCkt
NGR
IG
Rule 10-1102 requires the system tobe de-energized on detection of G gground current
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Neutral Ground Devices10-1104 NGRs must be approved for the application
CAN/CSA-C22.2 No. 0.4 – Bonding and Grounding of Electrical E i (P i G di )Equipment (Protective Grounding)
CAN/CSA-C22.2 No.14 – Industrial Control Equipment CAN/CSA-C22.2 No. 94 – Special Purpose Enclosures
Must be continuously rated where provisions are not made to interrupt the fault Maximum temperature allowed is 375°C Maximum temperature allowed is 375°C
Where not continuously rated, the time rating of the device must be coordinated with the protective devices of the systemy
Must have an insulation voltage equal to the line-to-neutral system voltage
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Location of Grounding Devices10-1106 All live parts must be enclosed Must be placed in a location accessible to qualified Must be placed in a location accessible to qualified
personnel Must be placed in a location where it can dissipate Must be placed in a location where it can dissipate
the heat under ground fault conditions Warning signs must be provided indicating the g g p g
system is impedance grounded and located at: Transformer or generator, or both Consumers service switchgear Supply authorities metering equipment
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Grounding Conductors System grounding conductors must be copper Solid grounded systems sized as per CEC Table 17g y p
Based on the ampacity of the largest service conductor
No splicing is permitted
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CEC Code Requirements
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NGR Conductors10-1108 conductors connecting the NGR to the Neutral
point of the system must be identified as white or p ygrey Must not be grounded Sized to conduct the rated current of the device
No less than #8 AWG
Conductor connecting the NGR to the system ground Conductor connecting the NGR to the system ground electrode may be insulated green or bare
Made of copperpp
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NGR Conductors
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Bonding of Conduit Enclosing a Grounding or Bonding Conductorg g Magnetic effect of metal conduit can increase the impedance
of the grounding circuit by a factor of 40! Not an issue with PVC or aluminium conduits Not an issue with PVC or aluminium conduits
Problem can be mitigated by bonding the grounding conductor to the metal conduit at both ends Allow the metal conduit to carry a portion of the ground current Allow the metal conduit to carry a portion of the ground current New CEC rule 10-806 makes this mandatory
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Sizing and Specification of NeutralSizing and Specification of Neutral Ground Resistors
NGR Sizing Criteria NGRs are sized based on the following criteria Charging currentCharging currentHRG - Maximum ground current must be greater than
3X the charging current for the systemLRG – Charging current not a factor
Temperature riseBased on how long the fault is allowed to persist
– Continuous E t d d ti (1 i t )– Extended time (1 minute)
– 10 seconds
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NGR Sizing Criteria
RNGR =VLL
√RNGR
√3IG
RNGR ≤XCO
3IG ≥ 3ICO
NGR ≤ 351
NGRI
RNGR = Resistor Size (Ohms)
WNGR = IG2RNGR
IG
NGR ( )IG = Maximum Ground Current (Amps)ICO = System Charging Current (Amps)W = Resistor Size (Watts)
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WNGR = Resistor Size (Watts)
NGR Sizing CriteriaSecondary Ground Resistory
RSGR = RNGR
N2
51 N =VLN(Pri)VLN(Sec)
SGRXFMR ISGR = NIG
KVA = PNGR = IGVLN(Pri)
RNGR = Equivalent Primary Resistance (Ohms)RNGR = Equivalent Primary Resistance (Ohms) I M i G d C t (A )IG = Maximum Ground Current (Amps)ISGR = Maximum Ground Current (Amps)N = Turns ratio
www.EngWorks.ca Grounding Fundamentals 88
PNGR = Resistor Power Rating (Watts)
Charging Current - Estimation Resistor must be sized to ensure that the ground
fault current limit is greater than the system's total g ycapacitance-to-ground charging current
System Voltage
Charging Current (3ICO) Amps per 1000 kVA of System Capacity
480 0.1 – 2.0
600 0.1 – 2.0
2400 2.0 – 5.0
4160 2.0 – 5.0
13800 5.0 – 10.0
www.EngWorks.ca Grounding Fundamentals 89
Typical Charging Currents based on Voltage Level
Charging Current – More Detailed Analysis
System Voltage
Component Type Typical Charging Current
600V Cable 3/C - 250 – 500MCM 0.15A/1000ft
3/C - #1 – 4/0AWG 0.02A/1000ft
Transformers 0.02A/MVA
Motors 0.01A/1000HP
4160V Cable 3/C - 500–1000MCM Shielded 0.58A/1000ft
3/C – 1/0 – 350MCM Shielded 0.23/1000ft
Non Shielded 0.1A/1000ft
T f 0 05A/MVATransformers 0.05A/MVA
Surge Suppressor. 1.35A per Set
Motors 0.1A/1000HP
13800V Cable 3/C - 600–1000MCM Shielded 0.65A/1000ft
3/C – 250 – 350MCM Shielded 0.75/1000ft
3/C - #1 – 4/0AWG Shielded 0.65A/1000ft
Transformers 0.05A/MVA
Surge Suppressor 2 25A per SetSurge Suppressor 2.25A per Set
Motors 0.15A/1000HP
www.EngWorks.ca Grounding Fundamentals 90
Charging Current CalculationExamplepROT → IG ≥ 3ICO
I 3(4 78A) 14 34A 15A NGR more
Component Charge C t
Qty Total Ch i Y
SurgeSuppressor
12 MVA
IG ≥ 3(4.78A) = 14.34A 5 G o eappropriate size
Current Charging Current
Transformer 0.05A /MVA
17.5 0.875A
Y10A NGR
4160V
12 MVA
500MCM Cable
0.58A /1000ft
4200 ft 2.43A
250MCM 0.23A 600 ft 0.13A1 5MVA
1200ft500MCM600ft
250MCMSurge2MVA
1500
ft50
0MC
M
1500
ft50
0MC
M
2MVACable /1000ftSurge Suppressor
1.35A /Set
1 1.35A
T t l 4 78A
M600V2A NGRY
1.5MVA
3000HP
SurgeSuppressor
2MVAYY
2MVA
Total 4.78A
www.EngWorks.ca Grounding Fundamentals 91
M M
Charging Current Test Procedure
Connect an ammeter to ground through a resistance, switch and g ,a fuse
Increase the resistance to maximum level and close the di t
A
IA
Ndisconnect
Slowly reduce the resistance to zero Ammeter will indication charging
BC
CA CB CC
IB
IC A Ammeter0-10A Ammeter will indication charging
current (3ICO) All three phases should be
measured and the average used
CA CB CC
G
0 10A
gas the system charging current
G
www.EngWorks.ca Grounding Fundamentals 92
Cable Insulation Ratings on Resistance Grounded Systemsy Low Voltage Systems (≤ 600V) 100% insulation rating acceptable for all applications% g p pp Refer to Standata CEC 12
Medium Voltage Systems (IEEE Recommendations)g y ( ) 100% insulation level required where clearing time will not
exceed one minute 133% insulation level required where clearing time will not
exceed one hour 173% insulation level required where clearing time173% insulation level required where clearing time
exceeds one hour
www.EngWorks.ca Grounding Fundamentals 93
NGR Ratings Based on the criteria defined in IEEE 32 - Standard
Requirements, Terminology, and Test Procedure for Neutral Grounding DevicesCurrentCurrent through the device during a ground fault conditionCurrent through the device during a ground fault condition
VoltageV = IR at 25ºCMay need to be de-rated at elevations above 1000m
FrequencyCircuit Voltage of SystemCircuit Voltage of System
ServiceNEMA Type 1 for Indoor ApplicationsNEMA Type 3 for Outdoor Applications
www.EngWorks.ca Grounding Fundamentals 94
NGR RatingsBasic Impulse Insulation Level
System Insulation Class
Class BILClass BIL
1.2kV 452 5kV 602.5kV 605kV 75
8 7kV 958.7kV 9515kV 10023kV 15023kV 150
www.EngWorks.ca Grounding Fundamentals 95
NGR Ratings Time Rating and Permissible Temperature Rise under fault
conditionsTime Rating Permissible Temperature Rise
(Rise Above 30ºC Ambient)
Ten Seconds (Short Time) (NGRs 760ºCTen Seconds (Short Time) (NGRs used with Protective Relay)
760 C
One Minute (Short Time) 760ºC
Ten Minutes (Short Time) (seldom specified)
610ºC
Extended Time (GF 610ºCExtended Time (GF allowed to persist > 10min)
610 C
Steady State (Continuous) 385ºC*
www.EngWorks.ca Grounding Fundamentals 96
*CSA permissible rise is 375ºC over 40ºC Ambient
NGR Monitoring
Broken Spot Weld
NGR Thermal Failure
www.EngWorks.ca Grounding Fundamentals 97
Broken Resistor Wire
NGR Monitor The NGR monitor
measures changes in NGR resistance, current in the neutral, and neutral-to-ground voltageg g Anomalies are detected and
an alarm or trip signal is activatedactivated
www.EngWorks.ca Grounding Fundamentals 98
NGR Sizing Tutorial
NGR Sizing Tutorial Modular Substation incorporating
5 kV Switchgear and MCCs 600 V Switchgear and MCCs 600 V Switchgear and MCCs
Grounding system consists of: Power Distribution System Ground
5kV L i t d t 5kV Low resistance ground system 600V High resistance ground system
Objective Size the grounding resistors for the 5kV LRG system and the
600V HRG system Assume 1.5A charging current for the 600V System Assume 8A charging current for the 5kV System
www.EngWorks.ca Grounding Fundamentals 100
Substation Single Line
YLRG NGR
M
5kV
MM M
Y600V
HRG NGR
MM
M M
~=
=~
www.EngWorks.ca Grounding Fundamentals 101
UPSPP
NGR Sizing TutorialAnswers
Sizing the NGRs
RNGR =VLL
√RNGR
√3IG
RNGR ≤XCO
3IG ≥ 3ICO
NGR ≤ 351
NGR
RNGR = Resistor Size (Ohms)
WNGR = IG2RNGRIG
NGR ( )IG = Maximum Ground Current (Amps)ICO = System Charging Current (Amps)W = Resistor Size (Watts)
www.EngWorks.ca Grounding Fundamentals 103
WNGR = Resistor Size (Watts)
HRG Sizing
ROT → IG ≥ 3ICO ICO = 1.5AG CO
IG ≥ 4.5 → Choose 5A as the HRG Current RatingCO
RNGR = VLL
√3IGWNGR = IG
2RNGR
RNGR = 600V√3 x 5A
WNGR = 5A2 x 69.3Ω
RNGR = 69.3Ω WNGR = 1733watts
www.EngWorks.ca Grounding Fundamentals 104
LRG Sizing
ROT → IG-Trip Setting ≥ 3ICO to avoid nuisance tripping ICO = 8AT i i i hl 20% f h LRG i i
I 120A Ch 125A h LRG C R i
Trip setting is roughly 20% of the LRG resistor sizeIG-Trip Setting ≥ 24A to avoid nuisance tripping
IG ≥ 120A → Choose 125A as the LRG Current Rating
RNGR =VLL
√WNGR = IG
2RNGRRNGR √3IG
RNGR =4160V
WNGR IG RNGR
WNGR = 125A2 x 19.2ΩRNGR √3 x 125A
RNGR = 19.2Ω
NGR
WNGR = 300kW
www.EngWorks.ca Grounding Fundamentals 105
NGR
System Grounding Application Summary
Solid Systems Ground Industrial applications 208V or lesspp Commercial Applications 600V or less
High Resistance Ground (5-15A)g ( ) Industrial applications 600V or less
CEC allows HRG to be used on applications up to 5kV
Low Resistance Ground (100 – 400A) Industrial applications 5kV – 34.5kV
G d f lt t ti id d b Z S CTGround fault protection provided by Zero Sequence CTs on individual equipment items
GF relays set to trip at 10 -20% of maximum ground fault current
www.EngWorks.ca Grounding Fundamentals 106
Obtaining a Systems Neutral
Application of GroundingGrounding
Transformers
Obtaining a Systems Neutral Often there are cases
where a systems neutral point must be established for the purposes of: Servicing line to neutral
Y Servicing line to neutral
loads Establishing a systems
d i t t d th13.8kV
ground point to ground the system through a HRG, LRG or solid ground
ti
M
connectionExample: Conversion of a
isolated ground system to a high resistance ground systemhigh resistance ground system
www.EngWorks.ca Grounding Fundamentals 108
Grounding Transformers Grounding transformers are the standard means of
obtaining a systems neutral Provide a low impedance path for ground fault currents
Zig-Zag transformerOft f d t di t f Often referred to as a grounding transformer
Specialized transformer with no secondary winding Wye-delta transformer configuration Wye delta transformer configuration Delta winding is left unconnected
www.EngWorks.ca Grounding Fundamentals 109
Grounding Transformer Schemes
ABBC
I
I
G
I
GG
Zig Zag TransformerWye-DeltaTransformer
www.EngWorks.ca Grounding Fundamentals 110
Transformer Connection The grounding transformer
is connected to the main bus and serves as the return path for any unbalanced or ground fault
Yunbalanced or ground fault currents A NGR is then connected to
th t l i t f th13.8kV
the neutral point of the grounding transformer establishing a connection to
d
M
ground
www.EngWorks.ca Grounding Fundamentals 111
LRG
Specifying a Grounding Transformer Parameters for specifying a grounding transformer Primary Voltagey g BIL (Basic Impulse Level) rating
Defined by IEEE standards (refer to IEEE 141 Red book)
Transformer impedanceTypically very high (up to 100%) to minimize magnetizing current
flows
Continuous neutral current ratingApplicable to four wire application
F l d d i Fault current and duration If a LRG scheme of limited duration is used, ( typically 10 – 60
seconds) the grounding transformer does not need a continuous duty rating
www.EngWorks.ca Grounding Fundamentals 112
G di f G tGrounding of GeneratorsSection 3
Generator Grounding Generators differ from transformers in several ways Less able to withstand the heating and mechanical effects g
of a short circuit Will have a higher initial ground fault current than three
h d tphase ground current Can develop third harmonic voltages and currents Less able to withstand voltage surges Less able to withstand voltage surges
www.EngWorks.ca Grounding Fundamentals 114
Objective of Generator Neutral Grounding
Minimize the damage associated with internal ground faults
Limit mechanical stresses in the generator for external ground faults
Limit temporary and transient overvoltages on the
t i l ti tgenerator insulation system Provide a means of system
ground fault protectionground fault protection
www.EngWorks.ca Grounding Fundamentals 115
Systems Ground Incorporating Generation
SystemGround #1
YNGR
M
5kV
SystemGround #2
M
Y600V Normal Bus
G G
M M G
SystemGround #3
www.EngWorks.ca Grounding Fundamentals 116
600V Emergency Bus
Generator Ground Fault
IGF
400ANGR
2 X IGF2 X IGF
Stator Ground Fault near
400ANGR
IGF
Breaker Closed
Stator Ground Fault nearGenerator terminals
Initial ground fault current results in 2 X 400A flowing into fault
www.EngWorks.ca Grounding Fundamentals 117
Generator Ground Fault
IGF
400ANGR
IGFIGF
Stator Ground Fault near
400ANGR
Breaker Open
Stator Ground Fault nearGenerator terminals
Upon breaker trip, ground fault current continues to flow due to the residual magnetism and inertia of the machine
www.EngWorks.ca Grounding Fundamentals 118
the residual magnetism and inertia of the machine
Ground Fault Magnitude Magnitude of a ground fault is determined by the
impedance of the generator or transformer windingp g g Maximum ground fault will occur on the system bus Maximum theoretical ground fault current in the Maximum theoretical ground fault current in the
generator will occur at the generator terminals Closer the stator fault is to the generator terminals, the
higher the fault Resulting damage is a function of current and time
www.EngWorks.ca Grounding Fundamentals 119
Solid System and Generator Ground
NOT RECOMMENDED NOT RECOMMENDED Results in very high ground fault currents resulting in
extensive damageg Risk of abnormal third-harmonic currents when more than
one generator is connected in parallel Increased magnetic core losses in both generator and transformer Increased magnetic core losses in both generator and transformer
www.EngWorks.ca Grounding Fundamentals 120
Low Voltage Emergency Generator Scheme
BondingConductor
Normal Bus Emergency BusNormal Bus Emergency Bus
Gnd Gnd
www.EngWorks.ca Grounding Fundamentals 121
Single Unparalleled Generator GroundingSolid Grounded with Neutral
EquipmentGround NeutralGroundconductor
NeutralConnectedTo ground
3 pole
Normal Bus Solid Neutral
TransferSwitch
NeutralConductor
Emergency Bus51G
Emergency Bus
GndNN GndZero SequenceCT is bypassedresulting in falset i
www.EngWorks.ca Grounding Fundamentals 122
trip
Single Unparalleled Generator GroundingSolid Grounded with Neutral Connection of the neutral to ground at the generator
can cause problemsp Allows stray current to flow between the neutral and the
ground conductors Allow zero sequence (ground fault current) to flow in the
neutral causing nuisance tripping of the main breaker Prevent ground fault relays from detecting a ground fault Prevent ground fault relays from detecting a ground fault
A neutral should not be connected to ground on the load side of a service disconnectload side of a service disconnect
www.EngWorks.ca Grounding Fundamentals 123
Option 1 – Switch Neutral
EquipmentGround NeutralGroundconductor
NeutralConnectedTo ground
4 Pole
Normal Bus Neutral SwitchedWith loadconductors
TransferSwitch
NeutralConductor
Emergency BusGFP
Emergency Bus
GndNN Gnd
www.EngWorks.ca Grounding Fundamentals 124
Option 2 – Connect Generator Neutral with Transformer Neutral
EquipmentGround
G tGroundconductor
3 pole
GeneratorNeutral connectedto transformerneutral in transfer
Normal BusTransferSwitch
neutral in transferswitch
NeutralConductor
Emergency Bus51G
Emergency Bus
GndNN GndZero SequenceCT read fullneutral current
l
www.EngWorks.ca Grounding Fundamentals 125
value
Additional References IEEE 446 Orange Book Provides application information pp
for the system grounding and transfer switching of standby generators 600V or lessgenerators 600V or less
www.EngWorks.ca Grounding Fundamentals 126
Single Unparalleled Generator GroundingHigh Resistance Groundedg
HRG
HRG
HRG
BondingConductor
Normal Bus Emergency BusNormal Bus Emergency Bus
Gnd Gnd
www.EngWorks.ca Grounding Fundamentals 127
HRG Source and Generator Grounding
HRG
HRG
HRG
Advantages Ground fault current limited to a very low valuey
Disadvantage Selective tripping on downstream breakers is not practical
www.EngWorks.ca Grounding Fundamentals 128
LRG Source and Generator Grounding
LRG
LRG
LRG
Advantages Allows selective tripping of downstream feeders
Disadvantage Damage can occur to the generator from high ground fault currents Variations in fault current can cause relay coordination problems Variations in fault current can cause relay coordination problems
www.EngWorks.ca Grounding Fundamentals 129
LRG Source and HRG Generator Groundingg
HRG
LRG
HRG
Advantages Allows selective tripping of downstream feeders
R d d l l f f lt t t th t i i i i d Reduced level of fault current to the generators minimizing damage Disadvantage
System is high resistance grounded when the generator is operating alone – makes selective tripping impossible
www.EngWorks.ca Grounding Fundamentals 130
Source and Generator Grounded with Artificial Neutral
LRG
Advantages Allows selective tripping of downstream feeders
All ith t t id Allows either source or generator to provide power Disadvantage
Damage can occur to the generator from restriking and intermittent ground faults
www.EngWorks.ca Grounding Fundamentals 131
LRG Source and Hybrid LRG/HRG Generator Groundingg
HRGLRG
LRG
HRGLRG
Ground fault will causet b k tgenerator breaker to
trip and open LRG circuit
Advantages Allows selective tripping of downstream feeders Allows generator to operate without the source transformer energized
Disadvantage Additional complexity in the grounding and relaying system Additional complexity in the grounding and relaying system
www.EngWorks.ca Grounding Fundamentals 132
Unit Connected Generator Grounding
HRG
LRGLRGLRG
Advantages Allows selective tripping of downstream feeders Allows generator to operate without the source transformer energized
Disadvantage Cost of the additional transformer Cost of the additional transformer
www.EngWorks.ca Grounding Fundamentals 133
E i t B diEquipment BondingSection 4
System Grounding Grounding and bonding have distinct meanings
within the context of the CEC Grounding refers to a conductive path direct to the
grounding electrodeg g Low impedance path to ground Conductors are sized to carry the expected fault current Insure the operation of protective devices in the circuit
should a fault occur
www.EngWorks.ca Grounding Fundamentals 135
Equipment Bonding Refers to the interconnection and connection to earth
of all normally non-current carrying metal partsy y g p Insures that all metal parts remain at ground potential Reduces the shock hazard to personnel Provides a low impedance return path for ground currents
Allows the circuit protection device to operate
Minimize the fire and explosion hazard Minimize the fire and explosion hazard Reduce accumulated static charges
www.EngWorks.ca Grounding Fundamentals 136
Ground Return Path through Earth
Insufficient current to operate protection device
~Line
Metallic Enclosure
S V
Neutral
GroundFault
Neutral
Short circuit musttake high impedance
High Impedance Ground Path
take high impedancepath to source
www.EngWorks.ca Grounding Fundamentals 137
Ground Return with Metallic Path
High Current Operates Protection Device
~Line
Metallic Enclosure
S V
Neutral
GroundFault
Neutral
Low Impedance Path through Bonding Conductor
High Impedance Ground Path
Low Impedance Path through Bonding Conductor
www.EngWorks.ca Grounding Fundamentals 138
Bonding Fundamentals To reduce electrical shock exposure: the impedance of the bonding conductor must be capable p g p
of carrying the fault current Must provide a lower impedance than all other parallel
thpaths For fire protection:
M st be able to cond ct the a ailable gro nd fa lt c rrent Must be able to conduct the available ground fault current without excessive temperature rise or arcingJoints and connections are critical components
Overcurrent Protection Operation: Provide a low impedance current path back to the source
www.EngWorks.ca Grounding Fundamentals 139
Bonding – CEC Requirements 10-400 All exposed non-current carrying metal parts of fixed
equipmentq p Supplied by a conduit wiring system Supplied by a wiring system that contains a bonding
conductor Located in a wet location In a hazardous location In a hazardous location Operates at more than 150V to ground
Examples Examples Distribution equipment, motor and generator frames Lighting fixtures housingsLighting fixtures housings
www.EngWorks.ca Grounding Fundamentals 140
Bonding Methods Bonding conductor in a cable or raceway Rigid metal conduit
Bonding conductor is required if the conduit is in underground service or installed in concrete slabs
EMT conduit EMT conduit Bonding conductor required if installed in concrete or masonry slabs
Sheath of a mineral insulated cable if manufactured of copper or aluminum
CEC Not acceptable Metal armor of liquid tight flex or cable assemblies Metal armor of liquid tight flex or cable assemblies Conduit made of stainless steel
www.EngWorks.ca Grounding Fundamentals 141
Bonding Methods - Effectiveness Cable or Conduit DC Resistance
Ω/1000ftVoltage Drop V/1000A/100ft
1-1/4” Rigid Steel Conduit 0.0108 11
1-1/4” EMT 0.0205 22
1-1/4” Flexible Conduit 0.435 436
3/C St l A d C bl 553/C Steel Armored Cable 55
3/C Steel Armored Cable with Ground Conductor
11Conductor
3/C Aluminum Armored Cable 0.286 151
3/C Aluminum Armored Cable with 123/C Aluminum Armored Cable with Ground Conductor
12
www.EngWorks.ca Grounding Fundamentals 142
Bonding Conductors Bonding conductors may be: Be copper or other corrosion resistant material
Aluminium conductors are acceptableMay be insulated or bare Insulated bonding conductors shall be coloured greeng g
May be spliced or tapped as required If installed to supplementary bond a raceway: Must be insulated Must be run in the same raceway
M st be protected against mechanical inj r if Must be protected against mechanical injury if: Copper - Smaller than #6 AWG Aluminum – Smaller than #4 AWGAluminum Smaller than #4 AWG
www.EngWorks.ca Grounding Fundamentals 143
Bonding ConductorsEquipment and Racewaysq p y
www.EngWorks.ca Grounding Fundamentals 144
Bonding of Cable Trays Rule 12-2208 of the CEC requires that cable trays be
bonded to groundg If the metal supports for cable tray are in good contact with
the grounded structural metal frame of a building, the tray h ll b d d t b b d d t dshall be deemed to be bonded to ground
If not in direct contact, a bonding conductor must be installed and the tray bonded to the conductor at intervalsinstalled and the tray bonded to the conductor at intervals not exceeding 15mSized as per CEC table 16 based on the largest ungrounded
conductor in the trayconductor in the tray
A bonding conductor may also be required in the cases that the tray supports single conductor cables of a three phase system
www.EngWorks.ca Grounding Fundamentals 145
Bonding of Single Conductor Cables Separate ground conductor required to bond the metallic
equipment at either end Must follow the same routing as the phase conductors
www.EngWorks.ca Grounding Fundamentals 146
Bonding Considerations Bonding connections require a clean surface Paint must be removed from connection points
Connections between dissimilar metals should be avoided Potential for deterioration of the connection due to galvanic Potential for deterioration of the connection due to galvanic
action Mechanical strength may often determine the size of
d tconductor Electrical continuity of expansion joints Cable tray connections Cable tray connections
www.EngWorks.ca Grounding Fundamentals 147
Equipotential Bonding Practice of bonding all exposed and extraneous
conductive parts (Ref CEC 10-406)p ( ) Purpose is to ensure that under fault conditions, all
conductive parts remain at the same potential Applies to Metallic water and sewer piping
G Gas piping HVAC ducting Exposed metal equipment and structures Exposed metal equipment and structures Raised computer floors
www.EngWorks.ca Grounding Fundamentals 148
Equipotential Bonding CEC requires a minimum #6 AWG conductor
www.EngWorks.ca Grounding Fundamentals 149
Bonding of Portable Equipment Non-current carrying metal parts of portable
equipment must be bonded when:q p Equipment is used in a hazardous location Equipment is used in wet or damp locations Equipment operates at more than 150V to ground When the equipment is provided with a grounding means
Th l ith dThree prong plug with ground
www.EngWorks.ca Grounding Fundamentals 150
Grounding of Portable Equipment Exceptions apply to double insulated equipment
productsp Additional insulation barrier added to the electrical device Will be marked with a double insulated symbol
Ground may omitted if a Class A ground fault circuit interrupter is used
www.EngWorks.ca Grounding Fundamentals 151
GFCI Schematic
Designed to provide protection against electric shock Designed to provide protection against electric shock from leakage current flowing to ground
Provide supplementary protection but are not a pp y psubstitute for insulation and grounding protection
www.EngWorks.ca Grounding Fundamentals 152
Ground Fault Circuit Interrupters GFCI Class A Primarily used for personnel protection Typically trip at 5ma Time to trip based on the formula
T =20I
1.43 T in secondsI fault current between 4mA and 260 mA
GFCI Class B (Ground Fault Equipment Protectors) Used for equipment protection
Heat trace circuits in hazardous locationsHeat trace circuits in hazardous locations 30ma trip level
www.EngWorks.ca Grounding Fundamentals 153
GFCI – Where Required Outdoor receptacles Wet locations Wet locations Health care facilities Panels supplying power for buildings or projects Panels supplying power for buildings or projects
under construction Heat trace systems Heat trace systems
www.EngWorks.ca Grounding Fundamentals 154
Static Grounding
Section 5
Did the Cellphone Cause the Ignition?
www.EngWorks.ca Grounding Fundamentals 156
Static Hazards in Industry Aviation Industry Static charges are built up during flight and on the ground
Manufacturing Paper and Printing
P d b lt i llPower and conveyor belts moving over pulleys Paint operations
Transfer of fluids
Coal, Flour and Grain Industry Movement and accumulation of dust and particles
P h i l P i R fi i d Petrochemical Processing, Refining and Transportation Movement of materials Movement of materials
www.EngWorks.ca Grounding Fundamentals 157
Reasons for Static Grounding Reduce the risk of fires and explosions Improve process and quality control Improve process and quality control Reduce the operating costs associated with storing
flammable materialsflammable materials Minimize the potential for damage to sensitive
electronic equipmentq p Loss of electronic data
Comply with hazardous goods transport and storage p y g p gregulations
Reduce the cost of insurance
www.EngWorks.ca Grounding Fundamentals 158
Energy from a Static Discharge
10
CH4/AirH2/Air
Typical range ofE = CV2 X 10-9
)
Typical range of spark dischargeenergy from a human body Where
C = Capacitance in pFV = Voltage in V
Material Dust Dust
1.0
Ene
rgy
(mJ)
Stoichiometric
V = Voltage in VE = Energy in mJ
Cloud LayerCoal 60 mJ 560 mJ
0.1Igni
tion Stoichiometric
CH4/Air Mixture0.274 mJ
Grain 30 mJ -
Sulfur 15 mJ 1.6 mJ020 40 60 80
StoichiometricAir/H2 Mixture0.017 mJ
www.EngWorks.ca Grounding Fundamentals 159
20 40 60 80Fuel (% Volume) Energy Required for Dust Ignition
Typical Values of Static Voltages and Capacitancesp
Equipment Voltage Object Capacitance Energy
Carpet Walk
12 kV Human Being 200 pF 28.8 mJWalkFabric on Fabric
25 kV Automobile 500 pF 312.5 mJ
Tank Truck 25 kV Tank Truck 1000 pF 625 mJ
Tank Truck 25 kV 3.6m Tank with Insulated Lining
100000 pF 62,500 mJ
Lining
www.EngWorks.ca Grounding Fundamentals 160
Static Charge GenerationStatic electricity is generated by the movement of
dissimilar poor conducting materials in close contactdissimilar poor conducting materials in close contactNon conductive fluids or powders in motion are a frequent cause of static
Static charge increases as the velocity of movement is increased. Anything which generates eddies, turbulence or
discontinuities in flowFiltersFiltersChanges in piping cross sectional area
www.EngWorks.ca Grounding Fundamentals 161
Static Charge GenerationTriboelectric Effectcontact electrification in which certain materials become
electrically charged when coming into contact with another and are then separated
www.EngWorks.ca Grounding Fundamentals 162
Electrostatic Charge Dissipation Electrostatic charges continually leak away from a
charged bodyg y Termed electrostatic dissipation
Determined by a materials conductivityMeasured in pS/m (picosiemens per meter) for petroleum products
Electrostatic charges accumulate when they are generated at a higher rate than they are dissipatedat a higher rate than they are dissipated
Function of the relaxation time constantTime required for a charge to dissipate to approximately 37% of its
i i l loriginal value
www.EngWorks.ca Grounding Fundamentals 163
Conductivity and Time Constants for Typical Materialsyp
Product Conductivity Relaxation Time (pS/m) (Seconds)
Benzene 0.005 >>100
Toluene 1 21
Gasoline 10 – 3000 0.006 - 1.8
Diesel 0.5 – 50 0.36 - 36
Fuel Oil 50 1000 0 018 0 36Fuel Oil 50 - 1000 0.018 – 0.36
Crude Oil > 1000 < 0.018
www.EngWorks.ca Grounding Fundamentals 164
Static DischargeFor an electrostatic charge to be a source of ignition,
four conditions must be present:pA means of generating an electrostatic chargeA means of accumulating an electrostatic charge capable
of producing an incendiary sparkA spark gapAn ignitable vapor air mixture in the spark gapAn ignitable vapor-air mixture in the spark gap
www.EngWorks.ca Grounding Fundamentals 165
Static Charge Generation
www.EngWorks.ca Grounding Fundamentals 166
Static DischargeSpark discharges occur between conductive objects
that are at different voltagesgBrush discharges can occur between a grounded
conductive object and a charged low conductivity j g ymaterial
Incendive discharge is a discharge that has enough energy to cause ignition
www.EngWorks.ca Grounding Fundamentals 167
Industrial Materials Prone to Static ElectricityyNonconductive glassNonconductive conveyor beltsNonconductive conveyor beltsRubberPlastic resinsPlastic resinsDry gasesPaperPaperPetroleum fluidsOil water mixturesOil water mixtures
www.EngWorks.ca Grounding Fundamentals 168
Sources of Static Electricity Dry materials handling
equipment Flammable liquid pumps and
handling equipmentMultiphase flow enhances
Charge Separation in a PipeMultiphase flow enhances
charge generation
Liquid filling operations Plastic piping systems Conveyor Belts
Liquid motion in tanks Liquid motion in tanks
www.EngWorks.ca Grounding Fundamentals 169
Sources of Static Electricity
www.EngWorks.ca Grounding Fundamentals 170
API 2003
Spark Promoters A spark promoter will provide
the necessary conditions for a spark gap to occur Loose floating conductive
objects Conductive downspoutsGage tapes, thermometers or
sample containers lowered into pa tank “tank gauging rod, high-level
sensor, or other conductive ,device that projects into the cargo space of a tank truck”
API 2003
www.EngWorks.ca Grounding Fundamentals 171
Static Sparks in Kanses
www.EngWorks.ca Grounding Fundamentals 172
Static Control Ignition hazards from static sparks can be eliminated
by controlling the generation or accumulation of static y g gcharges
Static removal involves recombining separated g pchargesUsually met by bonding all electrically conducting parts
www.EngWorks.ca Grounding Fundamentals 173
Methods of Static Control Piping Systems Keep fluid velocities low
Max 15 ft/sec
Filling Operations Filling Operations Eliminate splash filling and free fall of materials Reduce filling velocity to less than 3 ft/sec
Fluid Storage Non conductive material storage containers are not Non-conductive material storage containers are not
allowed for NFPA Class I, Class II and Class III materials
www.EngWorks.ca Grounding Fundamentals 174
Methods of Static Control Humidity Control 65% or higher will prevent static discharge
Antistatic treatmentsAdditi f b bl k t t i l Addition of carbon black to materials
Use bonding and grounding to prevent build-up of Use bonding and grounding to prevent build up of potential differences between conductive parts Small gauge conductors generally sufficient to prevent the
b ild p of staticbuild-up of static
www.EngWorks.ca Grounding Fundamentals 175
Static Grounding
Vehicle Connected to GroundVehicle Bonded Together
Vehicle Bonded together andTo Ground
www.EngWorks.ca Grounding Fundamentals 176
Static Grounding
Drum Container Storage Scheme
www.EngWorks.ca Grounding Fundamentals 177
Static Grounding
Bulk Fluid Transfer Operation
www.EngWorks.ca Grounding Fundamentals 178
Static Grounding
Bonding connections should be less than 10Ω for static control
www.EngWorks.ca Grounding Fundamentals 179
Bonding connections should be less than 10Ω for static control
Railcar Loading Bonding Scheme
www.EngWorks.ca Grounding Fundamentals 180
API RP 2003 Provides guidance on how to protect
against hydrocarbon ignition from static, lightning and stray current discharges
Discusses how static charges are accumulated and how they can be safely dissipated
Lightning protection for metallic tanks equipment and structures
Identification and mitigation of stray currents resulting from fault currents and cathodic protection applications
www.EngWorks.ca Grounding Fundamentals 181
NFPA 77 Applies to the identification,
assessment, and control of ,static electricity for purposes of preventing fires and explosions
Provides guidelines for t lli t ti l t i it icontrolling static electricity in
selected industrial applicationsapplications
www.EngWorks.ca Grounding Fundamentals 182
Lightning Protection
Section 6
Lightning Strikes
www.EngWorks.ca Grounding Fundamentals 184
Lightning No such thing as a standard lightning strike Highly complex phenomenong y p p Described by statistical means
+ + ++ +- -- -- -
Charge Separation in Cloud
Corresponding charge
High electric field causes ionization of air
++ +
+
+
+
+
Induced in groundCurrent flow in metallic pathways
www.EngWorks.ca Grounding Fundamentals 185
Lightning Strike Initiation
+ + ++ + ++ +- -- -- -
--- - Downward leader
+ + ++ +- -- -- -
--- - Upwards
+ + ++ +- -- -- -
--- - Charge flows
+
Downward leader
Upward leader+
++
--Upwardsleader meetsdownwards leader
-
--
--Charge flowsto ground through structure
++ +
+
+
+
+++ +
+
+
+
+++ -
-
+
-
--
www.EngWorks.ca Grounding Fundamentals 186
Lightning Discharge Direct effects of lightning
Heat energy and large h i l f
Lightning Stroke
Cumulative Frequency
98% 95% 80% 50% 5%mechanical forces
Direct ignition of flammable materials
First negative kA 4 20 90
Subsequent 4 6 12
Indirect effects of lightning Incendive sparks Electromagnetic pulse
kA 4.6 12
Typical Lightning Current Value
Electromagnetic pulse Earth current transients Bounded charges
Cur
rent
C
Time
www.EngWorks.ca Grounding Fundamentals 187
TimeTypical Lightning Discharge
Difference between Lightning and High Voltage ElectricityHigh Voltage Electricity
Factor Lightning High Voltage
Energy Level 25 kA typical, millions of volts
Usually much lower
Time of Exposure Brief, instantaneous
Prolonged
Pathway Flashover, orifice Deep, internal y , p,
Burns Superficial and minor
Deep with major injury j y
Cardiac Primary & secondary arrest, asystole
Fillibration
www.EngWorks.ca Grounding Fundamentals 188
asystole
Incidence of Lightning Lighting varies with Terrain Altitude Latitude Time of the year
Number of flashes per square kilometre per year
www.EngWorks.ca Grounding Fundamentals 189
Lightning Protection Lightning strikes cannot be stopped but their energy can be
diverted in a controlled manner Strike frequency goes up with the square of the height above the
average terrain Damage is caused by the lightning energy taking a random – high g y g g gy g g
impedance path to ground
3 components to a lightning protection system Air terminal or electrode the intercepts the surge Air terminal or electrode the intercepts the surge Low impedance conductor system to ground Ground electrode to dissipate the energy
If all equipment within an elevated potential area is bonded together, the potential for damage is minimized
www.EngWorks.ca Grounding Fundamentals 190
Inherent Grounding Inherent grounding
Metallic equipment, tanks and structures in direct contact with the ground do not require additional grounding if: The thickness of tanks, vessels and process equipment is greater than
5mm and are capable of withstanding a direct lightning strike without damagedamage
Indirect contact with the ground (self grounded) Sealed to prevent the escape of liquids, vapours or gas
M t t h i l f iliti i h tl d d d Most petrochemical facilities are inherently grounded and require no additional lightning protection
Equipment that may require special consideration Equipment that may require special consideration Open floating roof tanks Tank farms incorporating a containment liner
www.EngWorks.ca Grounding Fundamentals 191
Bounded Charge Dissipation Bounded Charges Occurs when a storm cell induces an electricalOccurs when a storm cell induces an electrical
charge on everything beneath it Consideration with open floating roof tanks
Floating Roof- - - - - - - - - - - - - - -
Floating RooftankTeflon seal isolates
roof from tank + + + + + + + + + + + + + + Bounded Charge+
++
+++
Flammable Product++++
++++
www.EngWorks.ca Grounding Fundamentals 192
+ + + + + + + + + + + + + + + + + + + + + + + + + + + + +
Bounded Charge Dissipation
Floating Rooftank
+ + + + + + + + + + + + + +Bounded Charge- -
Flammable Product
+ + + + + + + + + + + + + + Incendivedischarge toground
----
----
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
ground- -
www.EngWorks.ca Grounding Fundamentals 193
Protection against Lightning Floating Roof Lightning
ProtectionBoundedcharge
Floating roof cable connection Grounding Shunts (Not Recommended)
charge Dissipated withLightning strike
Cable connection tofloating roof
Groundingshunt
- - - - - - - -
floating roof
Flammable Product
www.EngWorks.ca Grounding Fundamentals 194
Methods of Lightning Protection Conventional air terminal Provides a low impedanceProvides a low impedance
path to groundLightning rods (sometimes
called Franklin rods)
+ + ++ +- -- -- -
--called Franklin rods)Conducting mastsOverhead wires
- -
R
+
RA
RARA = 0.84 x h0.6 x I0.74
Att ti di i t
++ +
+
++
RA = Attractive radius in metersh = height of lightning mast in metersI = Peak lightning current in kA
www.EngWorks.ca Grounding Fundamentals 195
Dissipative Array System (DAS) Claim of the technology is to
dissipate a charge before a lightning strike occurslightning strike occurs No scientific proof that this in fact
occurs Renamed the Charge Transfer Renamed the Charge Transfer
System (CTS) technology in recent years Still considered ineffective
www.EngWorks.ca Grounding Fundamentals 196
Early Stream Emission Air Terminals Consist of lightning rods
incorporating a device that triggers the early initiation of a lightning strike Effectiveness is also
questioned
www.EngWorks.ca Grounding Fundamentals 197
Lightning Surge Protection Transient Overvoltages can damage
electrical equipment Result in insulation breakdown and eventual
failure
Mitigated byg y Surge arrestors Equipment insulation standards
Lightning Strike EMFTravelling Wave
Line
Surge Voltage Wave
www.EngWorks.ca Grounding Fundamentals 198
Line
Surge Protection
Diminished Surge Voltage Wave
35kV 13 8kVTransient Voltage Surge
Surge Arrestor
70kV
35kV 13.8kV
25kV
Is
Transient Voltage Surge
Surge ArrestorIsSurge Voltage isInduced on secondarywinding by capacitive
Surge suppressorreduces surge voltage winding by capacitive
coupling effectg g
to below BIL of transformer
www.EngWorks.ca Grounding Fundamentals 199
Equipment Insulation Voltage Withstand Requirementsq Basic Impulse Level (BIL) is used to describe the
insulation class of electrical equipmentq p Based on the voltage rating of the equipment Based on specified crest value kVp Specified in the various
equipment standards
www.EngWorks.ca Grounding Fundamentals 200
Surge Voltage and Current Wave
Surge Arrestors
Surge arrestor must have a high i t d l ditiresistance under normal conditions
and a very low resistance under surge conditions
Metal oxide arrestor is the industry standard
Consist of a series connection of zinc Consist of a series connection of zinc oxide elements
www.EngWorks.ca Grounding Fundamentals 201
Surge Arrestors Class of Surge Arrestors (IEEE Std C62.11)
Station Class Intermediate Class Distribution Class – Heavy duty Distribution Class – Normal dutyy Secondary
www.EngWorks.ca Grounding Fundamentals 202
600V Secondary Surge Arrestor Distribution Class Surge Arrestor
Surge Arrestor Installation Considerations
Should be mounted as close as possible to the transformer bushings
Arrestor must be coordinated with the BIL of the equipment it is protecting
A dedicated “down lead” conductor to ground required for A dedicated down lead conductor to ground required for each arrestor
Down lead conductor should be mechanically and thermally y ycapable of handling the surge voltage to ground
Down lead should be as short as possible with no changes in directiondirection Minimum radius of 200mm No bends greater than 90º
www.EngWorks.ca Grounding Fundamentals 203
Lightning Arrestors – CEC Requirements
10-1000 Lightning Arrestors on Secondary Services1) Grounding conductor shall be as short (and straight) as possible2) The lightning arrestor grounding conductor may connected to the:
a) Grounded service conductorb) Common grounding conductorc) Service equipment grounding conductord) Separate grounding conductor
Common ground conductor
i i di d
Grounded service conductor
www.EngWorks.ca Grounding Fundamentals 204
Service equipment grounding conductor
Lightning References NFPA 780 “Standard for the
Installation of Lightning Protection g gSystems” provides detailed guidance on the design of lightning protection systems”
API 2003 “Protection against I iti A i i t f St tiIgnitions Arising out of Static, Lightning and Stray Currents”IEC 61024 “P t ti f St t IEC 61024 “Protection of Structures Against Lightning – Part 1”
www.EngWorks.ca Grounding Fundamentals 205
NFPA 780 Installation standard for
lightning protection systems for building structures and facilities handling flammable vapors gases and liquidsp g q
Does not apply to electric generating, transmission
d di t ib ti tand distribution systems
www.EngWorks.ca Grounding Fundamentals 206
Electronic Equipment Grounding
Section 7
History Grounding principles for communication systems
were developed to meet the operational h t i ti f th i tcharacteristics of the equipment Early telegraph systems used a two wire circuit path Later systems used the earth return as the signal pathLater systems used the earth return as the signal path
www.EngWorks.ca Grounding Fundamentals 208
Morse Landline Telegraph System
History Earth return offered several advantages Iron wire was used for telegraph conductors
The use of an “earth” ground doubled the distance a circuit could be run
Eliminated one wire from the circuit Problems endured: Quality of the signal was effected by weather
Leakage current to ground during wet weatherLeakage current to ground during wet weatherResistance of the return ground path varied with soil conditionsPresence of “foreign” voltages
www.EngWorks.ca Grounding Fundamentals 209
History Early development of the
telephone system also relied th lid DC fon the solid DC reference
ground as the return path lines were particularly noisy, p y y,
picking up electrical noise from power lines, adjacent telephone lines, telegraph lines, streetcars, g pand machinery
The grounded system was later replaced with a systemlater replaced with a system employing two wires per telephone line eliminating most of the noisemost of the noise
www.EngWorks.ca Grounding Fundamentals 210
Electronic Equipment Grounding Terms
Signal Common Grounding referred to as the “DC Signal Common” Zero reference system for data lines Very sensitive to transient voltages
DC Power Supply Reference Ground Bus DC Power Supply Reference Ground Bus -ve terminal on a DC power Supply
Equipment Ground Bus Used for equipment chassis bonding Often referred to as the safety ground bus
Variety of other terms used (depending on manufacturer) Variety of other terms used (depending on manufacturer) AC Safety Ground, Computer Reference Ground, DC Signal Common,
Earth Common, DC Ground Bus
www.EngWorks.ca Grounding Fundamentals 211
Electronic Systems Grounding Most electronic computer systems employ a DC
reference ground Required for logic circuits
Problems occur when the DC reference ground is tied to the AC safety groundtied to the AC safety ground With the logic circuits referenced to the equipment chassis
ground, any small amount of chassis potential caused by current flow in the grounding of the device could cause reference error in the equipment.
www.EngWorks.ca Grounding Fundamentals 212
Noise in Sensitive Circuits Errors result when the noise is greater than the
actual signalg Results in parity check errors
signal is ignored if check fails
30-50VL iLogicSignal
Noise does not impact signal
3-5VLogicSignal
Noise cause parity check errors
www.EngWorks.ca Grounding Fundamentals 213
Circuit Noise Sensitivity Measurement Signal to Noise Ratio Measure of the interference in a communications circuit Measured in dB
Bit Error RateSNR = 10log dBS
N
Measure of the number of bits received to those in error
10 610-6
10-7
10 8Erro
r Rat
e
10-8
10-9
Bit
E
www.EngWorks.ca Grounding Fundamentals 214
0 10 20 30 40 50 60SNR (dB)
Categories of Noise Traverse Mode Noise A disturbance that appears between two active conductors pp
in an electrical system Measurable between two line conductors or from line to
t lneutral Originates from within the power system
A
V
A
B
C
N
G
www.EngWorks.ca Grounding Fundamentals 215
Categories of Noise Common Mode Noise Appears simultaneously in each active conductorpp y The term "common" refers to the fact that identical noise
appears on both the active and neutral wires Generally involves the ground conductor
A
V
A
B
CV
N
G
www.EngWorks.ca Grounding Fundamentals 216
Typical Problems Associated with Electronic Systems Groundingy gElectronic Equipment Symptoms Electrical Condition
Temporary or chronic data hang-upsSlow data transfers, multiple retries
Different signal reference levelsI d d t blI/O Damage Induced currents on cable
Intermittent lock-upsCorrupted SignalsI/O damage
Transient voltages and currents
I/O damageRandom data errorsSlow transfer in analog circuits
Stray currents and common mode noise in equipment gro nding cond ctorgrounding conductor
www.EngWorks.ca Grounding Fundamentals 217
Electronic Equipment Grounding Computers require a “quiet” ground where no voltage
transients or electromagnetic noise occursg Stabilize input voltage levels Act as a zero voltage reference point for circuits
Led to the practice of installing an “Isolated Ground” system specifically for electronic equipment
This practice was in direct conflict with the CEC which requires that all grounding systems be i t t dinterconnected CEC is concerned with safety – not with performance
www.EngWorks.ca Grounding Fundamentals 218
Principles of Noise Mitigation For noise to be a problem Requires a noise source of sufficient magnitudeq g Some means of communicating the unwanted noise to the
electronic circuitGalvanic couplingElectrostatic / Capacitive couplingMagnetic or Inductive couplingg p g
Solving the problem involves either reducing the amplitude of the noise voltage or effectively isolating the circuit from the noise source
www.EngWorks.ca Grounding Fundamentals 219
Source of Electrical Noise Motor starts High current in-rush is impressed on the communications g p
circuit Fluorescent lighting High frequency noise associated with the ballast operation
Switching power supplies or VFD systems High frequency noise associated with switching power
suppliesHi h lt d t li ht i t ik d High voltage surges due to lightning strikes and electrical faults
www.EngWorks.ca Grounding Fundamentals 220
Galvanic Coupling Occurs when two circuits
share a common conductor Examples: Telephone circuits
that used the same common t th DC t li ireturn path as DC tram lines in
the early days Easily solved by separating Easily solved by separating
the circuits by using separate return conductors
www.EngWorks.ca Grounding Fundamentals 221
Electrostatic/Capacitive Coupling Form of coupling that is
proportional to the p pcapacitance between the noise source and the signal
R1
I1wires Function of:
C1 RL
I1
I2I3
Distance from the noise source to the signal wires
Length of the signal wires E
C2R2
Noise source dVg g Strength of the noise voltage Frequency of the noise voltage
ENoise source dT
www.EngWorks.ca Grounding Fundamentals 222
Electrostatic/Capacitive Coupling Mitigation Shielding of the signal wiresg g Separating the source from the noise Reducing the amplitude of the noise voltage Reducing the frequency of the noise voltage Twisting of the signal wires
www.EngWorks.ca Grounding Fundamentals 223
Shielding Conductor shield provides a lower impedance path for the
noise current to flow
R1 C3
I2Copper braid (85% coverage) provides a noise reduction ratio of
RL
I3
I4I5
noise reduction ratio of 100:1Aluminum Mylar tape with drain wiren provides a
R2C4
I1
noise reduction ratio of 6000:1
E
C2
Noise sourcedVdT
C3 and C4 are 1/100 C1 and C2
C1
www.EngWorks.ca Grounding Fundamentals 224
dT
Shield Grounding Shielding on instrumentation and communication
circuits eliminates the electrostatic induction into wires carrying low signal voltages
Shielding method may beg y Braided copper wire Metalized foil, with a copper drain wire Metal conduit (if steel conduit, this also serves as a
magnetic shield)Shields must be grounded Shields must be grounded One end only for frequencies up to 1 Mhz Two or more locations for frequencies > 1 Mhz Two or more locations for frequencies > 1 Mhz
www.EngWorks.ca Grounding Fundamentals 225
Magnetic or Inductive Coupling Depends of the rate of
change and the mutual inductance between the source of noise and the signal wiresg
Influenced by:Magnitude of the noise
currentcurrentFrequency of the noise
currentA l d b h i lArea enclosed by the signal
wiresDistance between the noise
d th isources and the wires
www.EngWorks.ca Grounding Fundamentals 226
Magnetic or Inductive Coupling Mitigation Twist the signal conductorsg
This results in lower noise due to the smaller area for each loop. This means less magnetic flux to cut through the loop and consequently a lower induced noise voltageconsequently a lower induced noise voltage
Noise voltage that is induced in each loop tends to cancel out the noise voltages from the next sequential loop
Inductive coupling is reduced by ratios varying from 14:1 for a four-inch lay to 141:1 for one-inch lay
www.EngWorks.ca Grounding Fundamentals 227
Magnetic or Inductive Coupling Mitigation Enclose the signal wires with a magnetic shieldg g
The magnetic flux generated from the noise currents induces small eddy currents in the magnetic shield which then create an opposing magnetic flux Ø1 to the original flux Ø2opposing magnetic flux Ø1 to the original flux Ø2
Galvanized steel conduit is an effective magnetic shield
Placing parallel (untwisted) wires into a steel conduit will provide a noiseprovide a noise reduction of approximately 22:1
www.EngWorks.ca Grounding Fundamentals 228
Physical Segregation Separate the noise sources from the noise sensitive
equipmentq p Cable spacing based on susceptibility levels defined by
IEEE 518 Level 1 – High: Analog signals less than 50V and digital
signals less than 15V Level 2 – Medium: analog signals greater than 50V Level 2 – Medium: analog signals greater than 50V Level 3 – Low: Switching signals greater than 50V Level 4 – Power: Voltages 0 – 1000V; Currents 20–800Ag ;
www.EngWorks.ca Grounding Fundamentals 229
Physical Segregation
Level Level Separation1 2 2 30
Level Level Separation1 2 2 – 30mm1 3 3 – 160mm1 4 4 670mm
1 2 2 – 30mm1 3 3 – 110mm
1 4 4 – 670mm 1 4 4 – 460mmCables Contained in Separate Trays One Cables in Conduit and the other
In Tray
Level Level Separation1 2 2 – 30mm
y
1 2 2 30mm1 3 3 – 80mm1 4 4 – 310mm
www.EngWorks.ca Grounding Fundamentals 230
Both Cables in Conduits
Electrical Segregation
Y Y Y
Shielded IsolationTransformer UPS
M M
SensitiveLoads
Worst Case
MMY
MM ~=
=~
Worst CaseSensitive Loads are subject
to voltage fluctuations causedby motor loads Better
www.EngWorks.ca Grounding Fundamentals 231
Best
Separately Derived AC Power Distribution System using an Isolation Transformery g Isolates power to the control system from the rest of
the AC distribution systemy Provides good line regulation and transient filtering
Transformers should be of a shielded designg Provide superior noise isolation using the same concepts
used for shielded cables Input power to the transformers should be sourced
from the highest line voltage availableK f t t f h ld b id d if th K factor transformers should be considered if the control system load employs a large number of switching power suppliesswitching power supplies
www.EngWorks.ca Grounding Fundamentals 232
Isolation Transformer Grounding
www.EngWorks.ca Grounding Fundamentals 233
Separately Derived AC Power Distribution System using an UPSy g Provides a continuous power supply to the control
system in the event of a power interruptiony p p Protects the control system from power system
surgesg Isolation transformers cannot prevent surge events from
being transmitted to the load without additional surge protectionprotection
Provides a “conditioned” AC power supply to the control systemcontrol system Completely disconnects the control system power supply
from the source providing superior isolation from power system transients and noise
www.EngWorks.ca Grounding Fundamentals 234
1 Phase UPS
www.EngWorks.ca Grounding Fundamentals 235
3 phase UPS
www.EngWorks.ca Grounding Fundamentals 236
UPS Grounding
UPS configuration with common source for UPS and bypass circuityp Does not meet the definition of a separately derived circuit Common mode noise attenuation may be a problem
www.EngWorks.ca Grounding Fundamentals 237
UPS Grounding
Addition of a bypass transformer meets the definition of a separately derived sourcedefinition of a separately derived source Improved common mode noise attenuation Neutrals in UPS and bypass transformer are connected Power distribution center must be within 15m of the UPS
www.EngWorks.ca Grounding Fundamentals 238
UPS Grounding
Best configuration for common mode noise attenuation No restriction on distances Allows more flexibility in UPS voltages
www.EngWorks.ca Grounding Fundamentals 239
Multiple UPS Grounding Scheme
www.EngWorks.ca Grounding Fundamentals 240
Ground Loops Occurs when there is more than one ground connection path
between two pieces of equipment ground current may take more than one path to return to the grounding ground current may take more than one path to return to the grounding
electrode form the equivalent of a loop antenna which very efficiently picks up
interference currents Conductor lead resistance transform the currents into voltage
fluctuations Consequences
Ground reference in the system is no longer a stable potential Signals ride on the noise Noise becomes part of the program signal
Example Audible 60hz noise in your stereo system
www.EngWorks.ca Grounding Fundamentals 241
Ground Loop
U it A
+
-Input
+
-Output
CommunicationCable
InternalConnection
Unit A Unit B
PowerGround
PowerGround
1A Current Flowing
Low Resistance 0.1Ω0.2V0.1V
1A Current Flowing
Stray Current in Ground Causes Current to Flowin communication conductors
www.EngWorks.ca Grounding Fundamentals 242
Ground Loop – Motor Start
+Input
+Output
CommunicationCable
InternalConnection
Unit A Unit B
--
PowerGround
PowerGround
Connection
FromFromElectricalSources
MMotor Start
www.EngWorks.ca Grounding Fundamentals 243
Motor frame bondedTo ground
Ground Loop Mitigation
Add one or more separate groundsN t CEC d li t Not CEC code compliant
+
-Input
+
-Output
CommunicationCable
InternalConnection
Unit A Unit B
PowerGround
C t Fl i Mi i i d Separate
High Resistance
Current Flowing Minimized pInstrumentationGround
www.EngWorks.ca Grounding Fundamentals 244Motor frame bondedTo ground
Ground Loop Mitigation Interrupting the continuity of the grounding conductor Shielded communication cables
Interrupt ground
++
CommunicationCable
Unit A Unit Bpath here
-Input
-Output
PowerGround
PowerGround
www.EngWorks.ca Grounding Fundamentals 245
Ground Loop Mitigation• Control the path of the ground current
• Use an insulated ground receptacle
CommunicationUnit A Unit B
+
-Input
+
-Output
Cable
PowerGround
PowerGround
IsolatedInstrument ground
Single point groundInsulated ground conductor
www.EngWorks.ca Grounding Fundamentals 246
Isolated Ground Receptacle Helps to limit electrical noise introduced into a circuit via the
grounding conductor Establishes a dedicated ground path connected to ground at
one point only
www.EngWorks.ca Grounding Fundamentals 247
Conventional Receptacle Isolated Ground Receptacle
Isolated Ground Receptacle
B h Ci it
NEMA IG#5-15R2Isolated Ground Receptacle
Branch CircuitPanelboard
JunctionBox
Power Transformer
Metal Device Box
Conduit or CableInsulatedIsolatedIsolatedGround
wireSystemGround
Bare BondingConductor or Conduit
www.EngWorks.ca Grounding Fundamentals 248
Concept of a Single Point Ground System
Poor or faulty grounds are the most common causes of control system faultsy
The best way of insuring the performance and reliability of a control system is to employ a single y y p y gpoint ground network system Consists of an organized system of ground wiring that
t i t i i l d di t d i t th l t dterminates in a single, dedicated point on the plant ground grid
Provides a clean reference for control signalsProvides a clean reference for control signals
www.EngWorks.ca Grounding Fundamentals 249
Single Point Ground System
www.EngWorks.ca Grounding Fundamentals 250
Single Point Ground System Multiple Enclosures
www.EngWorks.ca Grounding Fundamentals 251
Instrument Tri-Ground System
GroundC d t
Main Transformer
LightningArrestor
NGR
Conductor
System GroundConnection to
system ground may be temporarilydisconnected to
AC Ground
isolate ground loop
InstrumentTri Ground
www.EngWorks.ca Grounding Fundamentals 252
Tri Ground
Intrinsically Safe Circuit Grounding
H d A Non Hazardous Area
I i i C l
Hazardous Area Non Hazardous Area
Field Device
IntrinsicSafe
Barrier
ControlSystem
Interface
AssociatedApparatus
IS Apparatus
InterconnectingWiring System
pp
www.EngWorks.ca Grounding Fundamentals 253
Wiring System
Intrinsic Safety – Simple Field Devices
Th l
Non Hazardous Hazardous Location
ControllerSimple
Thermocouple
Device
Controller
InternalFault
Controller
ExplosionInternalFault
ControllerFault
ISBarrier
Device isConsideredSafe under FaultC di i
www.EngWorks.ca Grounding Fundamentals 254
IS GroundConditions
Intrinsic Safe Barrier Circuit
Protects Zener from Destruction Limits the output
t
Safe Area HazardousArea
Limits input current current
FieldCurrentLimiting
Control
Fuse
Z FieldDevice
LimitingResistorSystem
Interface
ZenerDiodes
IS GroundLimits the output
lt
www.EngWorks.ca Grounding Fundamentals 255
voltage
Intrinsic Safety - Grounding
Incorrect Ground Scheme Correct Ground Scheme
www.EngWorks.ca Grounding Fundamentals 256
Intrinsic Safety - Grounding Extremely important for the safe operation of an IS
wiring systemsM t b i ibl id tifi d d ibl Must be visibly identified, secure and accessible
Must be capable of carrying the maximum fault currentcurrent #12 AWG minimum conductor size
Total resistance must not exceed 1Ωf Must be insulated from ground in all places except at
the point of connection to the ground electrode Duplicate ground conductors requiredup ca e g ou d co duc o s equ ed
Aluminium must not be used as a ground conductor material Potential for electrolytic corrosion Potential for electrolytic corrosion
www.EngWorks.ca Grounding Fundamentals 257
IEEE Standard 1100(Emerald Book)( )
Recommended engineering principles and practices for power and grounding p p g gsensitive electronic equipment Provides consensus in an area
where conflicting information haswhere conflicting information has prevailed
Excellent reference that describes the many challenges associated withthe many challenges associated with grounding electronic equipmentPower related noise controlSignal related noise controlSignal related noise control
www.EngWorks.ca Grounding Fundamentals 258
Station Electrode Design
Section 8
Ground Grid Design
www.EngWorks.ca Grounding Fundamentals 260
Ground Grid Design Fundamentals In the event of a fault or transient phenomena
(lightning or switching transients) the ground grid ( g g g ) g gmust Ensure personnel safety Protect equipment against damage
Design Considerations Grid must be able to withstand the maximum ground
current without damage Limit the ground potential rise between two points to a safe Limit the ground potential rise between two points to a safe
value
www.EngWorks.ca Grounding Fundamentals 261
CEC Code Requirements Section 10 “Grounding and Bonding” addresses
grounding electrodes for facilities operating at less g g p gthan 750V to ground Requirements are minimal
Section 36 “High Voltage Installations” addresses the grounding of facilities operating at more than 750V to
dground Requirements are in addition to those defined in Section 10
More substantial in nature and therefore require a deeper More substantial in nature and therefore require a deeper understanding of ground electrode theory
www.EngWorks.ca Grounding Fundamentals 262
CEC Section 10 Requirements CEC Section 10 does not specify a minimum
ground resistance for a grounding electrode but g g gspecifies the acceptable methods of obtaining a grounding electrode NEC specifies a ground resistance of 25Ω or less
G f10-700 Grounding Electrodes shall consist ofa) Manufactured grounding electrodesb) Fi ld bl d di l t db) Field assembled grounding electrodesc) In-situ grounding electrodes
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Manufactured Ground Electrode
Must be certified to CSA C22.2 No.41 “Grounding and Bonding Equipment”g q p Rods must be driven to their full length and separated by a
minimum of 3m Connected by a bonding conductor sized by Table 17
R d El t d
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Rod ElectrodePlate Electrode
Field Assembled Ground Electrode
Min 6m bare copper conductor buried or encased in concrete
conductor must be encased within the bottomencased within the bottom 50 mm of a concrete foundation footing, with the footingfooting
in direct contact with the earth, at not less than 600 ,mm below finished grade
Field Assembled Ground Electrode
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Field Assembled Ground Electrode
In-situ Ground Electrode Copper water pipe Metal reinforcement of concrete slabs, concrete pilings,
and concrete foundations Iron pilings, when they are in significant contact with earth
600 mm or more below finished grade600 mm or more below finished grade
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In-situ Grounding Electrode
10-700 Grounding Electrodes(5) Where local conditions such as rock or permafrost
prevent a rod or grounding plate from being p g g p ginstalled at the required burial depth, a lesser depth shall be permitted
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Horizontal Ground Rod Installation
Other Section 10 Requirements Lightning rod systems must be connected to ground
using a separate grounding electrode that is not used g p g gas the grounding electrode for any other system
Where a facility incorporates more than one ground y p gelectrode for lightning, communication or other systems Must be separated by a minimum of 2m Bonded together by a minimum #6AWG conductor
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CEC Section 36 Requirements36-302 Station ground electrodeEvery outdoor station shall be grounded by means of a station
ground electrode that shall meet the requirements of Ruleground electrode that shall meet the requirements of Rule 36-304 and shall
a) consist of a minimum of four driven ground rods not less than 3 ma) consist of a minimum of four driven ground rods not less than 3 m long and 19.0 mm in diameter spaced at least the rod length apart and, where practicable, located adjacent to the equipment to be grounded;
b) have the ground rods interconnected by ground grid conductors not less than No. 2/0 AWG bare copper buried to a maximum depth of 600 mm below the rough station grade and a minimum depth of 150 mm below the finished station grade; anddepth of 150 mm below the finished station grade; and
c) have the station ground grid conductors in Item (b) connected to all non-current-carrying metal parts of equipment and structuresall non current carrying metal parts of equipment and structures
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Distribution Utility Standard Ground Electrode Designg
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Distribution Utility Standard Ground Electrode Designg
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CEC Requirements for Station Ground Resistance36-304 Station ground resistance (see Appendix B)1) The maximum permissible resistance of the station1) The maximum permissible resistance of the station
ground electrode shall be determined by the maximum available ground fault current injected g jinto the ground by the station ground electrode or by the maximum fault current in the station, and the
d i t h ll b h th t d ll ilground resistance shall be such that under all soil conditions that exist in practice (e.g., wet, dry, and frozen conditions)frozen conditions) …..
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CEC Section 36 – Ground Potential Rise
36-304 Station ground resistancethe maximum ground fault current conditions shall….the maximum ground fault current conditions shall
limit the potential rise of all parts of the station ground grid to 5000Vg g
2) In addition to subrule (1), the touch and step voltage at the edge, within, and around the station grounding electrode…..shall not exceed the tolerable values specified in Table 52
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CEC Section 36 – Tolerable Touch and Step Voltagesp g
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IEEE Standard 80
IEEE Standard 80 Defines the safe limits
for touch and step potentialspotentials Provides guidance on
the design of groundthe design of ground systems for outdoor substationsP i il d b tiliti Primarily used by utilities for grounding on high voltage substationsg
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Ground Potential Rise (GPR)
Ground potential rise is a function of the current magnitude injected into the earth and the soil g jresistivity Measured with respect to a remote point
May vary from a few meters to several hundred meters awayMay vary from a few meters to several hundred meters away
5000V criteria specified in the CEC is based on the maximum GPR communication circuits are d i d t h dldesigned to handle
GPR = IG X RGPR IG X RgIG = Maximum Grid CurrentRg = Grid Resistance
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Ground Potential Rise
3 i t i i ith
N∆
G
∆ N
A3 wire transmission with noMetallic return path
N∆ ∆ G BC
NGR EGGroundGeneratorTransformer IG
E th
EG
GPR
GroundFault
G
EarthRg Ground Path
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System Ground Fault Return Path
Non Metallic conduitWith no bonding conductorWith no bonding conductor
Y
0.1Ω Ig=5kA
500V Ground Return Path
Lack of bonding conductor forces ground fault return path through the earth creating personnel hazard
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path through the earth creating personnel hazard
System Ground Fault Return Path
Metallic conduitwith bonding conductorwith bonding conductor
Low ImpedanceGround Return Path
YIg=5kA
High ImpedanceGround Return PathGround Return Path
Bonding conductor provides low impedance path to source: Stray current is minimized with improved
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source: Stray current is minimized with improved safety
Current GF Path with Local Source
YY
Multiple low impedance ground pathslimit the ground potential rise withinth t ti
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the station
Current GF Path with Remote Source
Overhead ground wire currentpath
YY
Multiple high impedance ground current paths back to source
Stray Current Paths
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Multiple high impedance ground current paths back to source
Ground Potential Rise (GPR) Grid system must limit the ground potential rise
(GPR) between two points to a safe value GPR can cause hazardous voltage in the form of Step &
Touch PotentialsMay occur in location remote to the actual fault
Safe values of GPR, Touch and Step Voltages are determined by the human tolerance to shock currentsFunction of current magnitude, duration and frequencyg , q y
GPR = IG X RgIG= Maximum Grid CurrentRg = Grid Resistance
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Step Voltage
I
Potential Rise aboveremote earth during short
R1
IF
RF
ESTEPcircuit
IF
R2RKESTEP
R0RF
RKK
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R1 R2 R0
Touch Voltage
IFETOUCH
Potential Rise aboveremote earth during shortcircuit
R1
RFRK
ETOUCH
circuit
IF
RK
1
RF/2
TOUCH
RK
R0
RRF/2
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R1 R0
Touch and Step Potential IEEE 80
ρ = resistivity of earth beneath surfaceρs = surface material resistivity (Ω . m)
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ρs surface material resistivity (Ω m)hs = thickness of surface material in m
Relationship between GPR, Touch and Step Potentialsp
YY
EtEs
Emesh GPR
Remote Earth
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Mesh Voltage Defined as the maximum
surface voltage potential g pdifference between a grid conductor and and a point between two grid conductors
Th ti l i Theoretical maximum touch voltage found within a ground grid
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Touch and Step Potential Two ways of making a grounding system safe1 Minimize the touch and step voltages that may1. Minimize the touch and step voltages that may
appear at any point within the substation and around its perimeterp
2. Increase the tolerable touch and step voltages by placing a high resistivity material over rough grade Asphalt Crushed rock
Both methods are typically used together
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Ground Surface Potential Gradients
10kA High ground resistanceincreases step potential
GroundRod
R1
Re10 VoltsInfinite Earth
R = R1+Re1 = 3 ohm
Infinite Earth
nd S
urfa
cePo
tent
ial
30kV
8kV Step Potential
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Gn P
0 Distance from Rod
8kV Step Potential
Ground Surface Potential Gradients
Multiple ground rods reduce the10kA
Multiple ground rods reduce the ground surface potentiallow resistance surface reducesstep potentialstep potential
4kAR1
Re10 VoltsInfinite Earth
R1+Re1 = 3 ohms R2
Re26kA
0 ohms R2+Re2 = 2 ohms
Infinite Earth
nd S
urfa
cePo
tent
ial
12kV
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Gn P
0Infinite Earth (0V)
Symmetrical Grid Current The current that causes the ground potential rise in a
grid is from a remote sourceg Only a portion of the current is responsible for the
ground potential riseg p Multiple return paths include Overhead ground conductors Cable shields
The current flowing into the ground that is responsible for the GPR is adjusted by a split factor to incorporate the effect of the multiple paths
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Symmetrical Grid Current
Ig = Sf x If
Ig = RMS Symmetrical Grid CurrentS = Split factor (Current Division Factor)Sf = Split factor (Current Division Factor)If = RMS value of the symmetrical ground fault current
Sf may be estimated using computer programs or by graphical analysisby graphical analysis
Typically ranges between 10 – 70% of If
Refer to IEEE 80 for more information Refer to IEEE 80 for more information
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Split Factor
Split factor accounts for the multiple current paths thatwill occur in a fault situation Overhead ground wire current
path
will occur in a fault situation
YY IfIf
Ig = Sf x If
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Split Factor Graphical EstimateIEEE 80 Annex C
Curve most likelyCurve most likelyto be used for a single circuitcustomer owned28%substation
The symmetrical grid current (I ) would becurrent (Ig ) would be approximately 28% of the total fault current for a substation with a2.5Ω grid resistance
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2.5Ω Substation
Grid Current for Design
IG = Cp x Df x Ig
IG = Maximum Grid CurrentCp = Estimated growth factor during station life span
Cp = 1 for zero growthp f gDf = Decrement factor for the duration of the faultIg = RMS value of the symmetrical ground fault currentg f y g f
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Decrement FactorDecrement factor accounts for the total asymmetric fault current flowing between the grounding system and the surrounding earth
Fault Duration tf Df
Sec CyclesSec Cycles0.008 0.5 1.650 1 6 1 250.1 6 1.250.25 15 1.10.5 30 1.0
S b i T i N k S S ( 30 )
0.5 30 1.0
296www.EngWorks.ca Industrial Power System Protection and Control 296
SubtransientNetwork (0-5 cycles)
Transient Network (5-30 cycles)
Steady State Network (>30 cycles)
Design Information Provided by Utility
Current values to be used in the design of the station
d idground grid
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Ground Resistance The “WildCard” in the grounding design
Ground grid resistance varies with soil Ground grid resistance varies with soil conditions and may change over time
Ch i t t bl Changing water tableResistivity of the ground will change in drying or drought
conditionsconditions Chemical content of soilPresence of salts decrease resistivityPresence of salts decrease resistivity
Frozen ground or permafrost conditionsConsideration in all Canadian grounding situationsg g
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Soil Resistivity (Ω . cm) = 100 x (Ω . M)
MediumResistivity (Ω . cm)
Minimum Average MaximumSurface Soil, Loam 100 5,000Clay, Shale, Gumbo 300 4,000 20,000Sand and Gravel 5 000 10 000Sand and Gravel 5,000 10,000Limestone 500 400,000Granite, basalt 1,000,000Low Hills, Rich Soil 3,000Medium hills, Medium Soil 20,000St Hill R k S il 50 000Steep Hills, Rocky Soil 50,000Sandy, dry coastal country 30,000 50,000 500,000Freshwater Lake 10,000 20,000 20,000,000Sea water 2,000 10,000 20,000
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Effects of Moisture on Resistivity
Moisture Resistivity Ω-cmMoisture Content
Resistivity Ω cmTop Soil Sandy Loam
0 > 109 > 109
2.5 250,000 150,0005 165,000 43,00010 53,000 18,50015 19,000 10,50020 12,000 6,30030 6,400 4,200
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Effects of Temperature on Resistivity for Sandy Loamy
Temperature ºC
Resistivity Ω cmºC Ω-cm
20 7,200
10 9,900
0 (Water) 13,800
0 (Ice) 30,000
-5 79,000
-15 330,000
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Seasonal Variation in Earth Resistance
19mm Rod in Stony Clay SoilCurve 1 – 1m below surfaceC 2 3 b l fCurve 2 – 3m below surface
Moisture and temperature is more stable at greater depthsp g pbelow the surface
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Effect of Chemicals on Earth Resistivity for Sandy Loamy
Effect of Temperature on Resistivity of Soil with Salt (20%
Effect of Salt on Resistivity of Soil (Moisture 15% Temp 17ºC) Resistivity of Soil with Salt (20%
Moisture 5% Salt)
Temperature ºC
Resistivity (Ω-cm)
( p )
Added Salt % by Weight of
Moisture
Resistivity (Ω-cm) C (Ω-cm)
20 11010 142
Moisture
0 10,7000.1 1,800 10 142
0 190-5 312
0.1 1,8001.0 4605 190 5 312
-13 1,4405 19010 13020 100
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Electrode Resistance
Rg (rod) = ρ(Ω.cm)335 cm
Ω
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Applies to 3m ground rod and is accurate within 15%
Multiple Ground Rod Resistance Resistance of a grounding system of 2-24 rods
placed on rod length apart will provide a grounding p g p p g gresistance divided by the number of rods multiplied by the factor F taken from Table 14 IEEE Std 142
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Alternate Formulas for Ground Rod Resistance
Contact resistance of one ground rodρ 4Lρ
R = 2πL X Ln 4La - 1
ρ = Soil resistivity in Ω-cm Ground Rod Separationρ Soil resistivity in Ω cmL = rod length in cma = rod diameter in cm
D = 2.2 X L
Contact resistance of multiple ground rods
Rn = Rn X 2 – e-0.17(n - 1)
n n
n = number of ground rods
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Smoking Ground Rod Current loading capacity of a ground rod is a factor Current passing through an electrode will have a direct p g g
impact on the temperature and moisture conditions immediately surrounding the ground rodM t b h k d Must be checked
I =34,800 X d X L
I =√ ρ X t
I = Current loading per foot of rod lengthg p gd = rod diameter in metersL = Length in metersρ = ohm metert = seconds (3 0 seconds is the value recommended by IEEE)
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t = seconds (3.0 seconds is the value recommended by IEEE)
Ground Rod Resistance to EarthGround Rod Resistance to Earth Tutorial
Ground Rod Resistance to Earth Tutorial
1. Determine the resistance to earth for a ground rod system consisting of Qty 4 – 10 foot long 5/8” y g y g(16mm) ground rods spaced at 10’ intervals and interconnected connected together and placed in clay soil
2. Calculate the resistance to earth for a ground rod t i ti f Qt 4 20 f t 5/8” (16 )system consisting of Qty 4 – 20 foot 5/8” (16mm)
ground rods interconnected together and placed in clay soilclay soil
3. Calculate the current loading capacity of the system
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Ground Rod Resistance to EarthGround Rod Resistance to Earth Tutorial
Answers
4 - 10’ Ground Rods
Rg (rod) =ρ(Ω.cm)335 cm
Ω Average ρ for clay soil = 4000 Ω·cm 335 cm
Rg (rod) = 4000 Ω.cm335 cm
= 11.94 Ω
For 4 ground rods
11 94ΩRg (4 rods) =
11.94Ω4
X 1.36 = 4Ω
IEEE 142Table 14
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4 – 20’ Ground RodsContact resistance of one ground rod
ρ = Soil resistivity in Ω-cmρ 4L
4000 4(610)
L = rod length in cma = rod diameter in cm
ρR = 2πL X Ln 4L
a - 1
4000R = 2π(610) X Ln 4(610)
1.6 - 1 = 6.649Ω
Contact resistance of multiple ground rods
Rn = Rn X 2 – e-0.17(n - 1) n = number of ground rods
n n
Rn = 6.6494 X 2 – e-0.17(4 - 1) = 2.32Ω
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Current Carrying Capacity of Ground Rod Systemy
I =34,800 X d X L
I = Current loading of the ground rod systemd = rod diameter in metersL = Length in meters
√ ρ X tL Length in metersρ = ohm metert = seconds (3.0 seconds typical value)
34 800 X 0 016 X 3 048 X 44 – 10ft Ground Rods I =
34,800 X 0.016 X 3.048 X 4
√ 40 X 3= 620 Amps
4 – 20ft Ground Rods I =34,800 X 0.016 X 6.1 X 4
√= 1240 Amps
√ 40 X 3
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Primary and Auxiliary Ground Electrodes
Primary Ground Electrode Installed specifically for grounding purposesp y g g p p
Ground rods Interconnecting wire mesh
A ili G d El d Auxiliary Ground Electrode Installed for purposes other than grounding
T picall ha e limited c rrent carr ing capacit Typically have limited current carrying capacity Examples
Steel building pilesg pSteel reinforced concrete foundationsRebar grounding
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UFER Ground First used by the US Army to ground a series of bomb storage
vaults in the vicinity of Flagstaff, Arizona Dry desert conditions made for a very poor ground electrode system Herbert Ufer developed an alternate electrode system based on using
the steel rebar used to reinforce concrete
Concrete is inherently alkaline and hydroscopic (absorbent) in nature The high pH provides a supply of ions to conduct current The high pH provides a supply of ions to conduct current
soil around concrete becomes “doped” by the concrete has an effective resistance of 3000 Ω-cm
Bases for the development of concrete encased electrodes
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Concrete Encased Electrodes Can be used as part of an effective low resistance grounding
system Will typically lower the overall resistance of the ground
Very cost effective! Adds very little cost to the installation Adds very little cost to the installation Reduces the amount of buried conductor required for an installation Aids in reducing the amount of construction re-work
B i d d d t l it t di d i t hi Buried ground conductors are a popular item to dig up during trenching operations
The 2006 NEC requires that rebar encased in concrete be incorporated into the system ground No equivalent CEC requirement
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Methods of Connecting Rebar to Building Steel
Option 1 – Connect structural steel and rebar using ground wire Requires electrical trade to be on site during pouring of Requires electrical trade to be on-site during pouring of
foundations
C Wi
Bolted Connection to Steel
GroundWell
Copper Wire
GroundingCompression
To groundgrid
pConnection orCadweld
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Methods of Connecting Rebar to Building Steel
Option 2 - Tie the vertical rebar to anchor bolts and the steel columns are grounded through the bolts g gand nuts
Rebar welded To anchor boltsGround
Well
To ground grid
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Rebar Grounding Installation Considerations
Rebar must be bare or zinc coated
Surge Current Conductivity of Rebar in Concrete
Minimum length – 6m Minimum diameter 13mm
I t ll d i i i f
Rebar Diameter (in.)
Surge Ampere per Foot
0 375 3400 Installed in a minimum of 50mm of concrete Preferably located near the
0.375 3400
0.5 4500ybottom of the foundation
Concrete must be in direct contact with earth
0.625 5500contact with earth
0.75 6400
1 0 81501.0 8150
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Ground Conductors For a given application, ground rods are more
effective than ground grid conductorsg g Ground rods will penetrate the frost level injecting current
into unfrozen ground Basic Requirements for the selection of a Ground
Grid conductorH ffi i t d ti it Have sufficient conductivity
Resist fusing and mechanical deterioration under fault conditionsconditions
Be mechanically reliable and rugged Resist corrosion
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Ground Grid Conductor Options Copper Highest conductivityg y Highest cost Subject to theft Commercial hard drawn specification most often used
Copper-clad steel Good option where theft is a problem
Aluminum Not recommended as is subject to corrosion
Steel Poor conductivity limits use
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Sizing of Grid Conductors Based on the design ground fault current and the
fault duration time
Akcmil = I · Kf √tc
Akcmil = area of conductor in kcmilI = Fault current in kAtc = current duration in seconds (IEEE recommends 3.0 seconds)Kf = constant based on the material (Refer to table 2 IEEE 80)
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Material Constants(Excerpt from Table 2 IEEE 80)( p )
Material Conductivity KfCopper, Annealed Soft Drawn 100% 7.00C C i l H d D 97% 7 06Copper, Commercial Hard Drawn 97% 7.06Copper Clad Steel Wire 40% 10.45Aluminum 6201 Alloy 52.5% 12.47Steel 1020 10.8% 15.95Stainless Steel 304 2.4% 30.05
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Grid Conductor Sizing Example Fault current is estimated at 6kA Commercial hard drawn copper selected as the grid Commercial hard drawn copper selected as the grid
conductor What size of grid conductor is appropriate to handle What size of grid conductor is appropriate to handle
the maximum fault current for 3.0 seconds?
A = I · K √tAkcmil = I · Kf √tc
Akcmil = 6 · 7.06 √3.0Akcmil 6 7.06 √3.0
Akcmil = 73.37kcmil → #1 AWG
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Grid Conductor Sizing Table
kcmil AWGCurrent Carrying Capacity (kA)
HD Copper Copper Clad Steel 1020pp ppSteel Wire
500 - 40 27.62 18.1250 20 4 13 81 9 05250 - 20.4 13.81 9.05212 4/0 17.34 11.71 7.67133 2/0 10.88 7.35 4.8183.7 #1 6.84 4.62 366.4 #2 5.43 3.67 2.4
Based on a 3.0 Second Fault Duration
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Grounding Connections All grounding
connections must be selected to withstand the short circuit forces and heating effects associated with an extended groung faultextended groung fault
Resist the effects of corrosioncorrosion
High pullout resistanceresistance
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Time-current curves for ground grid conductors and connectors
Typical Connections found within a Grid Designg
Conductor to Ground Rod
Conductor toEquipment to
G id C d tSubstation
F t G id
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Conductor to Conductor
Grid ConductorFence to Grid Conductor
Cadweld Connections Thermite welding process is used to fuse the connection Suitable for high current applications Will meet the requirements of IEEE 837 ““IEEE Standard
for Qualifying Permanent Connections Used in Substation Grounding”Grounding
Cadweld Connection
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Grounding Connections Compression Connections Acceptable alternative to Cadweld connections in p
substation applications
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Compression Tool
Corrosion Considerations A basic corrosion cell consists of the following: Anode – An electrode losing metalg Cathode – An electrode gaining metal Electrolyte – chemicals in solution in contact with the
anode and cathode Connecting conductor
V
0.78V+-
COPPERIRON
V
COPPERIRONAnode Cathode
Earth
G l i C ll
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Galvanic Cell
Corrosion Considerations When iron or steel is connected to copper with a low
impedance conductor, it corrodesp , Rate of corrosion is dependent on the current flow Each ampere-year of current flow will result in 20lbs of
steel being lost
Soil CorrosivityμA – 5AOhm-Cm Corrosivity
<2000 Very HighA +-
μ 5
2-5000 High5-10000 Moderate10 25000 Mild
IRON COPPERIRONAnode Cathode
Earth 10-25000 Mild
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Earth
Corrosion Mitigation Use a UFER ground system whenever possible Do not use an UFER ground system in conjunction with a g y j
copper ground rod and grid system in the same vicinityWill result in deterioration of the concrete rebar
I i i h id i i l i i In situation where a grid system is in close proximity to large amounts of steel and corrosive soil conditions existconditions exist Consider using galvanized ground rods and insulated
ground conductorsgConductor insulation should have a high resistance to chemical
degradation
Consider installation of a cathodic protection system Consider installation of a cathodic protection system
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Other CEC Requirements36-312 Grounding of metallic fence enclosures of
outdoor stations(1) The fence shall be located at least 1 m inside the perimeter of the station ground electrode area.p g(2) The station ground electrode shall be connected to the fence by a tap conductor at each end post, corner post, and gate post, and at intermediate posts at intervals not exceeding 12 m by a conductor of not l th N 2/0 AWGless than No. 2/0 AWG copper
www.EngWorks.ca Grounding Fundamentals 333
Grounding of Fence Enclosures
Grounding Detail Gate Grounding Detail Fence
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Other CEC Requirements36-304 Station ground resistance(4) After completion of construction, the resistance of the ( ) p ,
station ground electrode at each station shall be measured and changes shall be made if necessary to verify and ensure that the maximum permissible resistance of Subruleensure that the maximum permissible resistance of Subrule (1) is not exceededExceptionp Station phase to phase voltage is less than 7500V Ground surface has a 150mm layer of crushed rock or asphalt
(5) d f i l h ll t d t l t 1(5) ground surface covering layer shall extend at least 1 m beyond the station grounding electrode area on all sides.
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Measurement of Ground Resistance
Current IA is passed through auxiliary probe A IA
Voltage between L and P is measured
R is then calculated based on
VA
1 – 10A
R is then calculated based on IA and VLP
Several ground resistance t t k dmeasurements are taken and
the results are averaged Moisture and temperature
L P A
IEEE Std 81 addresses Ground data should also be recorded Resistance testing
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IEEE Standard 81
Describe the techniques d t dused to measure ground
resistance and impedance Factors that impact earth Factors that impact earth
resistivity choice of instruments and
techniques purpose of the measurement accuracy required Potential sources of error
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Design of a Station Ground Grid Typical plant ground grid should have a resistance
of 10Ω or less For satisfactory lightning protection, grounding
network resistance must be less than 5Ω IEEE 80 Guidelines IEEE 80 Guidelines 1Ω or less for transmission substation 5Ω or less for distribution substation5Ω or less for distribution substation
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Options Available for the Design of a Station Ground Grid 1. Design the GPR for the station to be less than the
Touch Potential Specified in the CECp Suitable for situations where the ground fault current is of
limited value Station resistance is determined by selecting the
appropriate number of ground electrodes using the formulas provided in IEEE 142 Green Bookformulas provided in IEEE 142 Green Book
Ground electrodes are then connected together using 4/0 AWG grid conductors
Results in a very conservative design
www.EngWorks.ca Grounding Fundamentals 339
Options Available for the Design of a Station Ground Grid 2. Use the publication “Simplified Rules for Customer
Owned Substations” (CEA Report 249 D541) Publication is referenced as a referenced in CEC Section
36-304(3) and appendix B Outlines a procedure for design of a pre-approved stationOutlines a procedure for design of a pre approved station
electrode designMethod 1 based on simple calculations, tables and curves
– Takes into account frost penetration in winter– Takes into account frost penetration in winterMethod 2 based on design curves
– Valid only for fault durations less than 0.5 seconds and where frost penetration is negligiblep g g
Examples are provided Was developed primarily to simplify the ground grid design
process prior to the availability of computer aided groundprocess prior to the availability of computer aided ground grid design programs
www.EngWorks.ca Grounding Fundamentals 340
Options Available for the Design of a Station Ground Grid 3. Use the procedure outlined in
IEEE 80 Calculation intensive but will result
in an appropriate and safe design Suitable for the design of Utility andSuitable for the design of Utility and
Large scale customer owned substations where ground fault current levels are highcurrent levels are high
Most computer aided ground grid design programs use the methods outlined in IEEE 80outlined in IEEE 80
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Station Ground Grid Design
GPR < Touch Voltage MethodMethod
Relationship between GPR, Touch and Step Potentialsp
YPremise of the methodIs to reduce the GPR
Y To less than the tolerableTouch voltage
EtEs
Emesh GPR
Remote Earth
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GPR < Touch Voltage PremisePremise of the method is to reduce the GPR to less than the tolerable touch voltage as defined in Table 52 of the CEC
YY
Etouch maxEstep max
Emesh GPR
Remote EarthGPR = IG X RgIG= Maximum Grid Current
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GRg = Grid Resistance
Step 1 Ground Fault Current1. Determine the maximum ground fault current (IG)
that might be injected into the station ground g j gelectrode Reference CEC 36-304
Specifies the maximum ground fault current or: The maximum fault current for the station
Information is typically provided by the UtilityInformation is typically provided by the Utility
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Step 2 Ground Resistivity2. Determine the resistivity of the ground in the area
of the substation Should be determined by test Often done as part of the geotechnical survey for civil works Ground resistivity under all conditions (wet, dry, frozen earth)
Determine the type of surface layer to be used in the vicinity of the substationvicinity of the substation 150mm of crushed stone generally used
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Step 3 Tolerable Touch Voltage
3 Determine the tolerable
Touch Voltage Criteria
3. Determine the tolerable touch voltage from CEC table 52
Typical Value Used basedon Table 52 Note 2
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Step 4 Required Ground Resistance
4 Calculate the required ground resistance for the4. Calculate the required ground resistance for the station ground electrode The use of 150mm of crushed stone over the surface of
the substation will help raise the Etouch and the overall required ground resistance and simplify the design
R = Etouch/IGR = Grid ResistanceIG = Maximum Grid CurrentEtouch = Tolerable touch voltage from CEC table 51
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Step 5 Number of Ground Rods5. Estimate the number of ground rods required to
obtain the specified ground resistancep g Estimate the ground resistance of one ground rod Divide out the resistance of one ground rod by the
number of rods required to achieve the target station ground resistance
IEEE 141
Rg (rod) =ρ(Ω.cm)33
Ω
IEEE 141
Rg (rod) 335 cm
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Valid for 5/8” 10 foot ground rods
Step 6 Ground Rod Layout6. Space out the ground
rods in a symmetrical tt th h t th
Substation fencepattern throughout the substation Minimum spacing of one
Min. 1 rod length
p gground rod distance apart
Ground rods on the peripheral are more 1m Minperipheral are more effective than ground rods in the interior of the substation
Grid design should extend beyond the fence of the substation a minimum of 1 meter
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Step 7 Interconnect Ground Rods7. Interconnect the ground rods using a minimum
2/0AWG bare copper ground wire Conductors shall be buried to a minimum depth of
150mm below finished grade to a maximum of 600mm below the rough station grade
G t D t il d d i→ Go to Detailed design
2/0AWG CU Wire
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Station Ground Grid Design
IEEE 80 Design ProcedureProcedure
IEEE 80 Ground Grid Design Appropriate for both Utility and Large customer
owned substation facilities Large generation facilities should reference IEEE 665
“IEEE Guide for Generator Station Grounding”Generating station typically cover a much larger physical area and
have numerous large buried structures and foundationsWorkers generally work indoors and are not in direct contact with
fthe earth or layer of crushed gravel
Defines the safety criteria which establishes the basis for design and then provides a procedure forbasis for design and then provides a procedure for the design of a practical grounding system
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Step 1 Field Data Obtain a field map of the location
and determine the area that may be used for the installation of a ground grid
Conduct a soil resistivity test Conduct a soil resistivity test Determine the soil resistivity profile of
the area in concern Select and determine the resistivity of Select and determine the resistivity of
the surface layer material to be used in the design of the substation
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Step 2 Conductor Size Select the ground grid conductor material
and calculate an appropriate conductor size
Ak il = I · Kf √tAkcmil I Kf √tc
Akcmil = area of conductor in kcmilI = Maximum 3 phase fault current in kAtc = current duration in seconds (IEEE recommends 3.0 seconds)Kf = constant based on the material (Refer to table 2 IEEE 80)
Maximum 3 phase fault current value is provided by the Utility
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Step 3 Touch and Step Criteria Calculate the tolerable touch and
step voltage criteria for the stationp g
i i i f h b h fρ = resistivity of earth beneath surfaceρs = surface material resistivity (Ω . m)hs = thickness of surface material in m
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Step 4 Initial Design Preliminary design should incorporate a
conductor loop surrounding the available area with cross conductors to provide convenient access for equipment grounds
Recommend ground rodsRecommend ground rods be placed around the perimeter of the grid
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Step 5 Grid Resistance Estimate the resistance of the initial design
ρRg = +√20ALT
111 +
11+h√20/A
Rg = Substation resistance in Ωρ = Soil resistivity in Ω . mA = Area occupied by the grid in m2A Area occupied by the grid in mh = Depth of the grid in mLT = Total length of conductors and rods in m
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Step 6 Grid Current Determine the maximum ground fault
current expected within the substation This value can usually be obtained from the
Utility If not, calculate the fault current using the
following:
IG = Cp x Df x IgG p f g
IG = Maximum Grid CurrentCp = Estimated growth factor during station life span
C = 1 for zero growthCp = 1 for zero growthDf = Decrement factor for the duration of the faultIg = RMS value of the symmetrical ground fault current
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Step 7 GPR < Touch Voltage? Determine if the GPR is less than the
acceptable touch voltage for the station
GPR = IG X Rg < Etouch
I = Maximum Grid CurrentIG = Maximum Grid CurrentRg = Grid ResistanceEtouch = Etouch50 or Etouch70
If YES → Grid Design is completeGo to Detailed Design
If NO → go to step 8
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Step 8A Calculate Mesh Voltage Calculate the MESH voltage for the grid
design
ρ · Km· Ki · IGEm = LMLM
EM = Mesh Voltageρ = Soil resistivity in Ω·mKm = Geometrical correction factor for grids of varying dimensionKi= Correction factor for grid geometry IG = Maximum grid currentL = Effective length of grid conductors and ground rods in mLM = Effective length of grid conductors and ground rods in m
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Km = Geometrical correction factor
K =1
lD2 D + 2 · h 2 h Kii+ l
816 · h · d
Km = 2 · π
· ln + 8 · D · d-
4 · d +Kh
· π(2 · n – 1)ln
For grids with ground rods along the perimeter and thoughout the grid area: K = 1For grids with ground rods along the perimeter and thoughout the grid area: Kii = 1For grids with no ground rods: Refer to IEEE 80 Formula 82 for calculation of Kii
Kh = 1 +√ ho
ho = 1m (grid reference depth)h
D = Spacing between parallel conductors in mh = Depth of ground grid conductors in md = Diameter of grid conductor in m
√ o
d Diameter of grid conductor in m n = Effective number of parallel conductors in a given grid
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Km = Geometrical correction factorn = na · nb · nc · nd
2 L LC = is the total length of the conductor in
nb = 1 for square grids1 f d t l id
Lp
2 · LCna =
C g fthe horizontal grid in m
Lp = is the peripheral length of the grid in m
nc = 1 for square and rectangular gridsnd = 1 for square, rectangular and L – shaped grids
Otherwise0 7 A
nb =4 · √A
Lp nc = ALx · Ly Lx · Ly
0.7 ·A
nd =Dm
√Lx2 · Ly
2
Ki= Correction factor for grid geometryKi = 0 644 + 0 148 · n
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Ki 0.644 + 0.148 n
LM = Length of Grid Conductors and Ground Rods
For grids with no ground rods or very few rods scattered throughout the grid – but none on the corners or on the perimeter
LM = LC + LRLC = is the total length of the conductor in the horizontal grid in mC g f gLR = is the total length of all ground rods
For grids with ground rods in the corners, as well as along the perimeter and throughout the gridthroughout the grid
LM = LC + 1.55 + 1.22Lr
√Lx2 · Ly
2 LR√ x y
Lr = is the length of each ground rod in metersLx = is the maximum length of the grid in the x direction in mL i th i l th f th id i th di ti i
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Ly = is the maximum length of the grid in the y direction in m
Step 8B Calculate Step Voltage
ρ · Ks· Ki · IGE
Es = Step Voltageρ = Soil resistivityKs = Spacing factor for step voltageEs = LS
s p g f f p gKi= Correction factor for grid geometryIG = Maximum grid currentLS = Effective buried conductor length m
LC = is the total length of the conductor in the horizontal grid in m
LR = is the total length of all ground rods LS = 0.75 · LC + 0.85 · LR
1 1 1 1 2
D = Spacing between parallel conductors in m
h = Depth of ground grid
2 · hKS = 1
π1
+ D + h1
+ D1
1 – 0.5n-2 conductors in md = Diameter of grid conductor
in m n = Effective number of parallel
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n = Effective number of parallel conductors in a given grid
Step 9 Em < Etouch?
If YES go to step 10If NO → Modify design
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Step 10 Es < Estep?
If YES go to Detailed DesignIf NO → Modify design
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Detailed Design of the StationDetailed Design of the Station Ground Grid
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Station Ground Grid Detailed Design1. Connect all non current carrying metal equipment to
the station ground gridg g Two 2/0 AWG connections for electrical equipment
apparatus Overhead transmission ground wires Metal structures
Pedestals Pedestals Security fence Substation building steel
Underground metal pipes and other metallic structures passing through the station Intervals not exceeding 12m Intervals not exceeding 12m
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Station Ground Grid Detailed Design2. Connect the neutral ground resistor to the station
ground grid using a conductor sized to maximum current of the resistor Provide for the inspection of grounding connections
3 Connect the lightning arrestor to the station ground3. Connect the lightning arrestor to the station ground grid using a minimum 4/0 AWG conductor Must be short, straight and direct as possible, g p
4. Interconnect adjacent substations and the reinforced steel of adjacent plant structures using 4/0 AWG i t id d t4/0 AWG intergrid conductors
5. Connect isolated instrument ground to the station ground grid at one location onlyground grid at one location only
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Intergrid Conductors
Reinforced Steel of Building or Plant
Substation Fence
Buried Substation Ground Electrode
Intergridconductors
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Step by Step Instructions for the Design of a Station Ground Electrode6. Verify the following design parameters GPR Touch and Step voltages Current loading capacity of the ground rod
7 T t th i t f th t ti d l t d7. Test the resistance of the station ground electrode Procedures outlined in IEEE 81 Tests are not required if the phase to phase voltage is less than
7500V and a 150mm surface layer is installed– See CEC 36-306
8. Modify the design as requiredy g q Install additional ground rods or use longer ground rods to
reduce the station resistance Modify the resistance of the soil using chemical salts Modify the resistance of the soil using chemical salts
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Grid Design Documentation Soil resistivity data Dates the measurements were made Temperature of the air and soil Wetness of the earth Measurement methodology Material used for earth surface covering layer
Value assumed for the maximum fault current injected into the station ground grid
I di t if l l l t d bt i d f th Indicate if value was calculated or obtained from another source
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Grid Design Documentation Indicate what soil model was used and how it was
obtained Worst case scenario
Indicate the maximum GPR, touch and step voltages predicted for the systempredicted for the system Fault duration assumed
Incorporate the results of ground resistance testing Incorporate the results of ground resistance testing of the grid
Incorporate scale drawings of the ground rod p g gplacement and ground conductor interconnections Show all fences, underground pipes, intergrid conductor
connectionsconnections
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Grounding Tutorial
Section 9
Substation Ground System Modular Substation incorporating
5 kV Switchgear and MCCs 600 V Switchgear and MCCs 600 V Switchgear and MCCs UPS power distribution system DCS Control System
Grounding system consists of: Grounding system consists of: Power Distribution System Ground
5kV Low resistance ground system 600V High resistance ground system 600V High resistance ground system
Equipment Ground Instrumentation ground Lightning surge arrestor ground
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Substation Single Line
Surge Arrestor
Overhead lineWith no Neutral
YLRG NGR
Surge Arrestor
M
5kV
MM M
Y600V
HRG NGR
MM
M M
~=
=~
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UPSPP
Substation Layout
600V MCC DCS
5 kVSwgr5kV MCC600V Swgr UPS
5kV
LRGNGR
HRG NGR
600VXFMR
5kVXFMR
Surge
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SurgeArrestor
Tutorial Objectives Use the HRG and LRG NGRs sizing from the previous tutorial
5A NGR on the 600V System 125A NGR on the 5kV System
Design a station ground grid system based on the following informationinformation Maximum ground fault current = 0.4kA (Provided by utility) Assume a 150mm crushed rock surface layer over a clay subsurface
layer with a resistivity of 6000 (Ω cm) under normal conditionslayer with a resistivity of 6000 (Ω . cm) under normal conditions
Verify the GPR, touch and step potentials for the design
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Grounding Tutorial
Answers
GPR < Touch Voltage MethodStep 1 – Determine the Maximum Ground Fault
Current 3phase fault current given as 5kA Maximum ground fault current injected into the grid is
given as 0.4kA Step 2 – Determine the resistivity of the soil in the
i i it f th b t tivicinity of the substation Resistivity of Clay given as 6000 (Ω . cm) or 60 Ω·m 150mm Surface layer of Crushed stone specified 150mm Surface layer of Crushed stone specified
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Step 3 Touch Potential
Step 3 - Ground grid must be capable of handling 0.4kA and limit the touch and step potentials as p pspecified in CEC table 52
U l l thUse lower value as the Ground grid voltage Rise Criteria
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Step 4 Required Ground Resistance Target grid resistance is based on limiting the station
GPR to less than the maximum touch voltageg
R = Etouch/IGtouch GR = Grid ResistanceIG = Maximum Grid CurrentEtouch = Tolerable touch voltage from CEC table 51
M i i d i t f d id 885V 2 21Ω
Etouch Tolerable touch voltage from CEC table 51
Maximum required resistance of ground grid =0.4kA
= 2.21Ω
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Step 5 Number of ground rods required
Resistance of one ground rod
Rg (rod) =ρ(Ω.cm)335 cm
Ω
6000Ω.cmRg (rod) =
6000Ω.cm335 cm
= 17.91Ω
Estimate 16 ground rods to start Estimate 16 ground rods to startRg= Rn X Fn
2 14Ω
17.91Ω= Rn X 1.9216
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= 2.14ΩRn
Step 6/7 Station Ground Grid3.3m
600V MCC DCS
3.3m Ground rods with
Inspection well
5 kVSwgr5kV MCC600V Swgr UPS
5A NGR
5kV
5A NGR
3 d d
Bare 2/0 AWG
600VXFMR
5kVXFMR
125ANGR
SurgeArrestor
3m ground rods(16 in total)
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IEEE 80 Design Calculation Step 1 Field Data Maximum 3 phase fault current 3 p
given as 5kA Maximum ground fault current
i j t d i t th id i iinjected into the grid is given as 0.4kA
Resistivity of Clay given as 6000 (Ω . Resistivity of Clay given as 6000 (Ωcm) or 60 Ω·m
150mm Surface layer of Crushed t ifi dstone specified
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Step 2 Conductor Size Select the ground grid conductor material
and calculate an appropriate conductor size
Akcmil = I · Kf √tc
Akcmil = area of conductor in kcmilI = Maximum 3 phase fault current in kAtc = current duration in seconds (IEEE recommends 3.0 seconds)K = constant based on the material (Refer to table 2 IEEE 80)Kf = constant based on the material (Refer to table 2 IEEE 80)
Akcmil = 5 · 7.06√3 = 61.14 kcmil
61.14 kcmil → Minumum #2 AWG
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2/0 AWG Selected for Mechanical Strength
Step 3 Touch and Step Criteria
ρ = resistivity of earth beneath surfaceρ resistivity of earth beneath surfaceρs = surface material resistivity (Ω . m)hs = thickness of surface material in m
60
CS = 1 -3000
601 -0.09
2(.15) + 0.09= 0.773
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( )
Step 3 Touch and Step Criteria
ETouch =116 + 0.174(0.773)(3000)
= 735V√0.5
EStep =116 + 0.696(0.773)(3000)
= 2446V√0.5√0.5
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Step 4 Initial Design Preliminary design should incorporate a
conductor loop surrounding the available area with cross conductors to provide convenient access for equipment grounds
14m
13 3 d d 39
10m
13 - 3m ground rods = 39m5x14m + 5x10m = 120m #2/0 AWGArea = 10m X 14m =140m2
Grid depth = 450mmp
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Step 5 Grid Resistance
ρRg = +√20ALT
111 +
11+h√20/A√20ALT 1+h√20/A
Rg = Substation resistance in Ωρ = Soil resistivity in Ω . mA = Area occupied by the grid in m2
h = Depth of the grid in mLT = Total length of conductors and rods in m
60Rg = +√20(140)159
11 1 +1
1+0.45√20/140
Rg = 2.48Ω
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Step 6 Grid Current Maximum ground fault current
injected into the grid is given as 0.4kA
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Step 7 GPR < Touch Voltage? Determine if the GPR is less than the
acceptable touch voltage for the station
GPR = IG X Rg < Etouch
I = Maximum Grid CurrentIG = Maximum Grid CurrentRg = Grid ResistanceEtouch = Etouch50 or Etouch70
GPR = 400A X 2.48Ω = 992V
GPR 992V > E 734V
Go to step 8 and Calculate Mesh
GPR = 992V > Etouch = 734V
Voltage
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Step 8A Calculate Mesh Voltage Calculate the MESH voltage for the grid
design
ρ · Km· Ki · IGEm = LM
= 60 · Km· Ki · 400
LMM
EM = Mesh Voltageρ = Soil resistivity in Ω·m
M
ρ yKm = Geometrical correction factor for grids of varying dimensionKi= Correction factor for grid geometry IG = Maximum grid currentL = Effective length of grid conductors and ground rods in mLM = Effective length of grid conductors and ground rods in m
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Km = Geometrical correction factor
K =1
lD2 D + 2 · h 2 h Kii+ l
816 · h · d
Km = 2 · π
· ln + 8 · D · d-
4 · d +Kh
· π(2 · n – 1)ln
For grids with ground rods along the perimeter and thoughout the grid area: K = 1For grids with ground rods along the perimeter and thoughout the grid area: Kii = 1
Kh = 1 +√ ho
ho = 1m (grid reference depth) = h 1 +√ 1
0.45 = 1.2
D = Spacing between parallel conductors in m = 3.5mh = Depth of ground grid conductors in m = 0.45md = Diameter of grid conductor in m = 0.0093m
√ o √
d Diameter of grid conductor in m 0.0093mn = Effective number of parallel conductors in a given grid
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Km = Geometrical correction factorn = na · nb · nc · nd
2 L
LC = is the total length of the conductor in the horizontal grid in m = 120m
Lp = is the peripheral length of the grid in m 2 120
Lp
2 · LCna =
Lp
p= 48m
A = Area of Grid = 140m2= 482 · 120
= 5
1 0148
nc = 1 for square and rectangular gridsnd = 1 for square, rectangular and L – shaped grids
nb =4 · √A
p =4 · √140
= 1.01
K C ti f t f id t
n = 5 · 1.01 · 1 · 1 = 5.05
Ki= Correction factor for grid geometry
Ki = 0.644 + 0.148 · n = 0.644 + 0.148 · 5.05 = 1.39
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LM = Length of Grid Conductors and Ground Rods
For grids with ground rods in the corners, as well as along the perimeter and throughout the grid
LLM = LC + 1.55 + 1.22
Lr
√Lx2 · Ly
2 • LR
Lr = is the length of each ground rod in metersLx = is the maximum length of the grid in the x direction in mLy = is the maximum length of the grid in the y direction in mL i th l th f d t i th id iLC = is the length of conductors in the grid in mLR = is the length of rods in the grid in m
3LM = 120 + 1.55 + 1.22
3√102 · 142 • 39 = 181.46
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Mesh Voltage
16 · 0.45 · 0.0065Km =
1
2 · π· ln
3.52
+ 8 · 3.5 · 0.0065
3.5 + 2 · 0.45 2- 4 · 0.0065
0.45
1+
1 2· π(2 · 5.05 – 1)ln
8
K 0 70 1.2 π(2 5.05 1)Km = 0.70 Ki = 1.39L = 121 57
ρ · Km· Ki · IG 60 · 0.7· 1.39 · 400
LM = 121.57
ρ m i GEm = LM
= 181.46= 128.7V
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Step 8B Calculate Step Voltage
ρ · Ks· Ki · IGE
Es = Step Voltageρ = Soil resistivity = 60 Ω·mKi= Correction factor for grid geometry = 1.39Es = LS
i f f g g yIG = Maximum grid current = 400ALS = Effective buried conductor length mKs = Spacing factor for step voltage
LC = is the total length of the conductor in the horizontal grid in m = 120
LS = 0.75 · LC + 0.85 · LR
C is the total length of the conducto in the ho i ontal g id in m 0LR = is the total length of all ground rods = 39m
LS = 0 75 · 120 + 0 85 · 39 = 123 15LS 0.75 120 + 0.85 39 123.15
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Step 8B Calculate Step Voltage
2 · hKS = 1
π1
+ D + h1
+ D1
1 – 0.5n-2
D = Spacing between parallel conductors in m = 3.5h = Depth of ground grid conductors in m = 0.45
1 1 1 1 5 05 2
d = Diameter of grid conductor in m = 0.0065n = Effective number of parallel conductors in a given grid = 5.05
2 · 0.45KS = 1
π1
+ 3.5 + 0.45 + 3.51
1 – 0.55.05-2 = 0.513
ρ · Ks· Ki · IGEs = LS
= 60 · 0.513 · 1.39 · 400
123.15 = 139V
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Step 9 Em < Etouch?
Emesh = 128.7V < 734V = Etouch
YES go to step 10
Emesh 128.7V 734V Etouch
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Step 10 Es < Estep?
Es = 139V < 2446V = Estep
YES go to Detailed Design
Es 139V 2446V Estep
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Detailed Design3.3m 2/0 AWG Insulated Gnd Wire
600V MCC DCS
3.3m Ground rods with
Inspection wellIsolated Inst. Gnd BusEqpt Gnd Bus
5 kVSwgr5kV MCC600V Swgr UPS
5A NGR
IntergridGrounding Conductors
5kV
5A NGR
3 d d
Bare 2/0 AWG
600VXFMR
5kVXFMR
125ANGR
SurgeArrestor
3m ground rods(16 in total)
2/0AWG toXo Terminal
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Xo TerminalGnd Conductor as short as possible
Substation Ground System Instrumentation Ground Designed to insure that all components of the Designed to insure that all components of the
control system operate at the same potentialEliminate potential ground loopsEliminate potential ground loopsIsolate system noise on the ground systemAllow ground to be accessible for disconnect to assist in
isolating ground loops
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Control System Grounding Scheme
SWGR UPS UPSPNLPNL
Main Transformer
InstrumentPanel Bonding
N
LightningA t
NGR
GroundConductor
Bonding ConductorPanel BondingConductors
Arrestor
System Ground
CP CP CPIsolated Ground Bus
Insulated Instrument Ground conductors
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Computer Analysis GPR < TouchSummer Conditions
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Computer Analysis GPR < Touch
Summer Conditions
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Computer Analysis GPR < Touch
Winter Conditions
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Computer Analysis IEEE 80
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Computer Analysis IEEE 80
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Computer Analysis Optimized
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Computer Analysis Optimized
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Summary and Wrap up
Section 10
Learning Objectives Review1. To understand why we ground Protect life from the danger of shock Protect life from the danger of shock Limit the voltage on a circuit Facilitate operation of protective devices Facilitate operation of protective devices
Low Impedancepath to source G1 L
Fuse
AccidentalGround
path to sourceallows fuse tooperate
G1 L
Neutral
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Learning Objectives Review2. To describe the difference between grounding and
bondingg System grounding refers to the intentional connection of a
phase or neutral conductor to earth for the purpose of t lli th lt t d ithi di t bl li itcontrolling the voltage to ground, within predictable limits
Bonding or equipment grounding refers to the interconnection and connection to earth of all normallyinterconnection and connection to earth of all normally non-current carrying metal parts Insures that all metal parts remain at ground potentialReduces the shock hazard to personnelReduces the shock hazard to personnelProvides a low impedance return path for ground currents
– Allows the circuit protection device to operateMinimize the fire and explosion hazard
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Learning Objectives Review3. To apply the safety requirements as defined by the
Canadian Electrical Code and the IEEE as they yrelate to grounding CEC Section 10 Defines when a system should be grounded and when equipment
should be bonded Describes the acceptable methods for grounding and bonding
and stipulates the size of grounding and bonding conductors Defines what an acceptable grounding electrode shall be
CEC Section 36CEC Section 36 Describes the grounding and bonding requirements for high
voltage substationsGPR < 5000\v– GPR < 5000\v
– Touch and Step potential as per table 52
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Learning Objectives Review3. To apply the safety requirements as defined by the
Canadian Electrical Code and the IEEE as they yrelate to grounding IEEE 142 (Green Book) Recommended Practice for the Grounding of Industrial and
Commercial Power Systems
IEEE 1100 (Emerald Book)IEEE 1100 (Emerald Book) Recommended Practice for Powering and Grounding Electronic
Equipment
IEEE 80 IEEE 80 Guide for Safety in AC Substation Grounding Primarily concerned with outdoor AC substations
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Learning Objectives4. To select the appropriate systems grounding
scheme for an industrial facilityyCondition Un-
groundedSolid
GroundLow
ResistanceHigh
Resistance
Immunity to transientImmunity to transient overvoltages Worst Good Good Best
Arc Fault Damage Protection Worst Poor Better Best
Safety to Personnel Worst Better Good Best
Service Reliability Worst Good Better Best
Continued operation after initial ground fault Better Poor Poor Best
Ground fault locating Not G d B tt B t
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gPossible Good Better Best
Learning Objectives5. To implement a static electricity control and
lightning protection systemg g p y Static Control
Bond together and to ground
Lightning ProtectionLightning strikes cannot be stopped but their energy can be
diverted in a controlled mannerRequires a low impedance path to ground
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Learning Objectives6. To avoid the problems typically associated with the
grounding of sensitive electronic systemsg g y Ground loops - use the single point grounding concept Methods of Noise Mitigation Physical Separation Electrical Segregation Harmonic Filtering Harmonic Filtering Shielding or screening of noise sources
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Learning Objectives7. To design a ground grid for a high voltage industrial
substation Limit the ground potential rise between two points to a safe
value Limit the touch and step potentials to a safe value Must be able to withstand the maximum ground current
without damagewithout damage Important part of a safe and reliable electrical systems
design
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