System Earthing

System Earthing

Earth faults :- 70 90% of all faults.

IF

EA

Earthing method determines :- Fault current IF Damage caused Steady state over voltages Transient over voltages Insulation requirements Quantities available to detect faults Type of Protection

System Earthing

Earthing Method

Solid / Low Z High Z

IF High LowOvervoltages in Low HighSound PhasesDamage High LowCost of Insulation Low HighLow Voltage Systems For SafetyMedium Voltage Systems To limit current

cost of insulationacceptable

High Voltage & To limit costEHV Systems of insulation

Solid or Direct Earthing Resistance Earthing Reactance Earthing Resonant or Petersen Coil

Earthing Insulated Earth

Methods of Earthing In Common Use

System Earthing

Solid

Lowest System Z0

IF High - Damage - Easy E/F Protn.No Arcing Grounds IF >> ICHARGE

Lowest Overvoltages

System Earthing

Reactance

Lower IFHigher Transient OvervoltagesCheaper than resistance at high voltsOvervoltages during E/Fs 0.8 1 x VØ/Ø Not often used except as tuned

reactor

System Earthing

Petersen CoilXE XCHARGING

Arcing faults self extinguishing - Good for transient faultsXE needs changing if XC altersOvervoltages during E/Fs VØ/Ø

Insulation importantRestricts use of auto-transformersDiscriminative E/F protection difficult

Tuned

System Earthing

Resistance

Reduced IF Reduced transient overvoltagesNot self extinguishing but E/F

easier to detect

System Earthing

Unearthed

InsulatedIF CapacitiveCan be self extinguishing if IF smallOvervoltages during E/Fs = VØ/Ø Arcing faults likely - high transient

overvoltagesInsulation important

System Earthing 660 V Solid - Safety

Insulated - Special cases where continuity of supply required

660 V 33 kV Resistance or reactance normally used

Solid - When IF is lowResistance - IF limited to IFLReactance - IF(E/F) limited to IF(3Ø) Petersen - Overhead lines. Lightning Coil

> 33 kV SolidOvervoltages more important (insulation)

Directly Coupled Resistance - Most commonGenerators Solid and - Not recommended

Reactance (High IF )

System Earthing

Generator - Transformer Units

IF ~ 200 300 A

IF ~ 10 15 A

Safety :- Power system neutral solidly earthed at transformer. Metallic tools and appliances solidly earthed. Sensitive protection by :-

RCD’s :- Residual current devices

ELCB’s :- Earth leakage circuit breakers

Low Voltage System Earthing

Earth Fault Hazard

ZF

ZE

VP

ZP

Unearthed Appliance

ZF = Fault impedanceZP = Human body impedanceZE = Environmental

impedanceVP = Case / earth potential

Earth Fault Hazard

ZF

ZE

VP

ZP

Unearthed Appliance

ZF = Fault impedanceZP = Human body impedanceZE = Environmental impedanceVP = Case / earth potential

IF

VH

RCD for High ZF

EFPP

/NH Z Z ZZ . E V

-: earth protective Without

Fuses for High IF

Protective Earth Conductor

Unearthed L.V. Winding

NormalConditions

H.V.

V

v

L.V.

Unearthed L.V. Winding

H.V.

xV

yv

L.V.

VF

VF = xV + (1 - y)v

Inter-winding fault ‘F’ causes dangerous rise in L.V. voltage

Breakdown Between HV and LV Windings

1730V

A2

B2C2

N

c2 b2

a2

254Vn

3000 / 440 V Transformer

Normal voltage conditions Neutrals earthed or unearthed

Breakdown Between HV and LV Windings

Voltage conditions with breakdown between HV and LV at point X on phaseLV neutral unearthed

1730V

850V

A2

B2C2

xH xn

c2 b2

a2xL

755V

254V

1009V

95V

Hand to Hand Resistance of Living Body - 50Hz AC (Freiburger 1933)

6000

5000

4000

3000

2000

1000

0 100 200 300 400 500 600Volts

Res

ista

nce

- Ohm

s

Very Dry Skin

Very Moist Skin

1mA Can be felt> 9mA Cannot let go15mA Threshold of cramp30mA Breathing difficult

Rise in blood pressure50mA Heart misses odd beat50 200mA Heavy shock

Unconsciousness> 200mA Reversible cardiac arrest

Current marksBurns

Effects of Body Current

Current at 50Hz Duration Physiological effects on humansto 60Hz r.m.s. of shockvalue mA

0-1 not Range up to threshold of perception.critical Electrocution not felt.

1-15 not Range up to threshold of cramp.Critical Independent release of hands from object gripped no longer possible. Possibly

powerful and sometimes painful effects on muscles of fingers and arms.

15-30 minutes Cramp-like contraction of arms. Difficulty in breathing. Rise in blood pressure. Limit of tolerability.

30-50 seconds Heart irregularities. Rise in blood pressure. Powerful cramp-effect. to minutes Unconsciousness. Ventricular fibrillation if long shock at upper limit

of range.

less than No ventricular fibrillation. Heavy shock.50 to a cardiac cyclefew hundred

above one Ventricular fibrillation. Beginning of electrocution in relation to heart phase notcardiac cycle important. (Disturbance of stimulus conducting system?) Unconsciousness.

Current marks.

less than Ventricular fibrillation. Beginning of electrocution in relation to heart phase cardiac cycle Important Initiation of fibrillation only in the sensitive phase.

Above (Direct stimulatory effect on heart muscle?) Unconsciousness. Current marksfew hundred

over one Reversible cardiac arrest. Range of electrical defibrillation. Unconsciousness. cardiac cycle Current marks. Burns

Effects of Various Values of Body Current

Body Current / Time and Security

10,000

1,000

100

100.1 1.0 10 100 1000

Current (mA)

Time(mS)

Thresholdof

Perception

Thresholdof

Let Go

Let Go

Hold On

IEC Security Curve

Thresholdof

Fibrillation

Earthing Impedance Affects Touch & Step Potentials

Surface

True Earth

RETouch

VHVH

Step

E

RFIF

RG

IF

IF

True Earth RG

RG' = f(Distance)d

!

'R R R'R E V

GFE

GH

Don’t forget communications cables etc. entering S/S !

Interconnected Star (Zig-Zag) Earthing TransformerSingle Earthing Resistor

2I

I

I

3I

3I EarthFault

I IITransformer Insulatedfor Line Voltage

Resistor InsulatedFor System PhaseVoltage

3I2II

I II

Interconnected Star Earthing TransformerThree Earthing Resistors

2I

I

I

3I

3I EarthFault

I II

3I2II

I II

I IIResistors

3I

Note:- Resistors to be insulated for line voltage and to have 3 times the ohmic value of a single neutral resistor

Displacement of Neutral from Earth during an Earth Fault

Va

VbVc

ZE

N

Z

G

Z

Z

IF

Va

VbVc

G

NZ Z

Z . V Z VE

EaNEFGN

Earth Fault on System with Insulated Earth

Va

IF

Ic

IbVbVc

N

-jXc

G

-jXc -jXc

c

abjX-

V

c

acjX-

V

cacjX-

V

c

abjX-

V

Earth Fault on System with Resonant or Petersen Coil Earthing

Va

IF

VbVc

N

-jXc

a,G

-jXc -jXc

a, G a, G

-jXL

cacjX-

V

cabjX-

VL

aNjXV

cabb jX-

V -

Methods of Neutral Earthing (1)Aspect Solid Resistance Resistance & High value Low value Tuned Insulated

reactance reactor reactor reactor

Normal Suitable for Suitable for Suitable for phase Suitable for Suitable for If used for Suitable for lineinsulation phase voltage phase voltage voltage line voltage for phase voltage operation with voltage for long

continuously continuously continuously long periods continuously one line earthedfor long periodsinsulation mustbe suitable forline voltage

Over voltages:(a) Initiated by Not excessive Not excessive Not excessive provi- Can be very high Not excessive Not excessive if Arcing ground faults, ding all three phases e.g. neutral no mutual coup- can give very switching, etc are made or broken inversion ling between zero high voltages simultaneously & positive seq-

uence networks

(b) Travelling Negative In general, “ Full reflection at Full reflection at Full reflection at Full reflection waves reflection negative neutral neutral neutral at neutral

reflection atneutral

Protection:(a) Automatic No difficulty No difficulty No difficulty, normal Extremely diffi- No difficulty By using special Extremely segregation normal methods normal methods methods can be cult if more than normal methods technique can be difficult of faulty zone can be used can be used used one zone can be used done satisfac-

involved torily

(b) Travelling Diverters rated In general, In general, diverters Diverters rated In general, Diverter rated Diverters rated waves for phase volts diverters rated rated for line volts for line volts are diverters rated for for line volts are for line volts

are suitable for line voltage are essential essential line volts are essential are essentialare essential essential

Methods of Neutral Earthing (2)Aspect Solid Resistance Resistance & High value Low value Tuned Insulated

reactance reactor reactor reactor

Earth-faultCurrent(a) Value Highest value High value High value Negligible High value Negligible Capacitive if

small may beself exting- uished

(b) Duration Few seconds Few seconds Few seconds Long time Few seconds Few seconds or In general longcontinuous, timedepending onmethod of application

(c) Effect on Electromagnetic Electromagnetic Electromagnetic Electrostatic Electromagnetic If used for Electrostatic communica- interference interference interference interference interference may running contin- interference tion circuits may necessi- depending on depending on necessitate current uously with one

tate current degree of degree of limitation limitation line earthedlimitation limitation requires partic-ular consideration

Harmonic No limitation Partial limitations Partial limitation of Limits all Appreciably limits Appreciably limits -currents in of harmonic of harmonic harmonic currents harmonic all harmonic all harmonicneutral currents currents currents currents currents

Time rating of 30 sec. 30 sec. 30 sec. Continuous 30 sec. 30 sec. or -neutral apparatus continuous

General remarks Maximum In general use In general use where Confined mainly Cheaper than Best continuity Some applica-disturbance to a source neutral isto protection of resistor at very of supply. Can tions on shortsystem not available generator on high voltages be a danger to feeders, ingenerator trans- personnel general to beformer unit avoided