good grounding practices a brief introduction to the basics of electrical grounding for power...

GOOD GROUNDING GOOD GROUNDING PRACTICESPRACTICES

A Brief Introduction to the A Brief Introduction to the Basics of Electrical Grounding Basics of Electrical Grounding

for Power Systemsfor Power Systems

By:

Harry J. Tittel, E.E.

Vice President TEAMWORKnet, Inc.

TEAMWORKnet, Inc.6550 New Tampa HighwaySuite BLakeland, Florida 33815(863) 327-1080(863) 327-1091 fax

TEAMWORKnet, Inc.TEAMWORKnet, Inc. 22

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Introduction to GroundingIntroduction to Grounding

TABLE OF CONTENTS

1.0 Introduction to Grounding

2.0 Standard Industrial Grounding Methods and Types of Grounding

3.0 Grounding System and Design Considerations

4.0 Open Question and Answer Session


Introduction to GroundingIntroduction to Grounding The primary goal of the grounding system throughout any facility is The primary goal of the grounding system throughout any facility is

SAFETY. Secondary are effective lightning protection, diminishing SAFETY. Secondary are effective lightning protection, diminishing electromagnetic coupling (EMC), and the protection against electromagnetic coupling (EMC), and the protection against electromagnetic pulses (EMP).electromagnetic pulses (EMP).

Grounding is implemented to ensure rapid clearing of faults and to Grounding is implemented to ensure rapid clearing of faults and to prevent hazardous voltage, which in turn reduce the risks of fires prevent hazardous voltage, which in turn reduce the risks of fires and personnel injuries. Grounding serves the primary functions of and personnel injuries. Grounding serves the primary functions of referencing the AC systems and providing a means to ensure fault referencing the AC systems and providing a means to ensure fault clearing.clearing.

99.5% survival threshold – 99.5% survival threshold – 116 mA for one (1) second.116 mA for one (1) second. 367 mA for zero point one (0.1) second.367 mA for zero point one (0.1) second.


Introduction to GroundingIntroduction to Grounding A frequently quoted criteria is the establishment of a one (1) ohm A frequently quoted criteria is the establishment of a one (1) ohm

resistance to earth. A large number of equipment manufacturers resistance to earth. A large number of equipment manufacturers have this in their installation guides. The NEC requires only have this in their installation guides. The NEC requires only twenty-five (25) ohms of resistance for made electrodes, while twenty-five (25) ohms of resistance for made electrodes, while the ANSI/IEEE Standard 141 (Red Book) and ANSI/IEEE 142 (Green the ANSI/IEEE Standard 141 (Red Book) and ANSI/IEEE 142 (Green Book) specifies a ground resistance of one (1) to five (5) ohms. Book) specifies a ground resistance of one (1) to five (5) ohms.

External changes in the grounding system (environment) may External changes in the grounding system (environment) may effect the ultimate functionality of the entire electrical system.effect the ultimate functionality of the entire electrical system.

Frequency matters in very complex grounding systems. Leakage Frequency matters in very complex grounding systems. Leakage currents of equipment do not return to the earth; high frequency currents of equipment do not return to the earth; high frequency leakage currents return to the equipment which generated them, leakage currents return to the equipment which generated them, while power frequency leakage currents return to the derived while power frequency leakage currents return to the derived source. source.

The impedance of the system is viewed from the perspective of The impedance of the system is viewed from the perspective of power frequencies and immediate harmonics (i.e., 60Hz and its power frequencies and immediate harmonics (i.e., 60Hz and its associated harmonics).associated harmonics).


Introduction to GroundingIntroduction to Grounding Generally accepted electrical wiring practices are not good Generally accepted electrical wiring practices are not good

ground system wiring practices (i.e. no sharp bends or turns).ground system wiring practices (i.e. no sharp bends or turns). Grounding systems are not meant to last for ever. The best Grounding systems are not meant to last for ever. The best

grounding systems need to most attention paid to them as grounding systems need to most attention paid to them as they will corrode the quickest.they will corrode the quickest.


GROUNDING SYSTEMS

There are basically six (6) grounding systems in use. The six (6) systems are the equipment grounds, static grounds, systems grounds, maintenance grounds, electronic grounds and lightning grounds.

Equipment grounds: An equipment ground is the physical connection to earth of non-current carrying metal parts. This type grounding is done so that all metal part of equipment that personnel can come into contact with are always at or near zero (0) volts with respect to ground. All metal parts must be interconnected and grounded by a conductor in such away as to ensure a path of lowest impedance for flow of ground fault current. Typical items (equipment) to be grounded are; electrical motor frames, outlet boxes, breaker panels, metal conduit, support structures, cable tray, to name a few.Static grounds: A static ground is a connection made between a piece of equipment and the earth for the purpose of draining off static electricity charges before a flash over potential is reached. This type grounding system is utilized in dry materials handling, flammable liquid pumps and delivery equipment, plastic piping, and explosive storage facilities.

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Standard Industrial System Grounding MethodsStandard Industrial System Grounding Methods

Methods of System Methods of System GroundingGrounding

CharacteristicsCharacteristics UngroundeUngroundedd

Solid Solid GroundGround

Low Resistance Low Resistance GroundGround

High Resistances High Resistances GroundGround

Susceptible to Transient Susceptible to Transient overvoltagesovervoltages

WORSTWORST GOODGOOD GOODGOOD BESTBEST

Under fault conditions (line-to-Under fault conditions (line-to-ground) increase of voltage stressground) increase of voltage stress

POORPOOR BESTBEST GOODGOOD POORPOOR

Arc Fault DamageArc Fault Damage WORSTWORST POORPOOR GOODGOOD BESTBEST

Personnel SafetyPersonnel Safety WORSTWORST POORPOOR GOODGOOD BESTBEST

ReliabilityReliability WORSTWORST GOODGOOD BETTERBETTER BESTBEST

Economics' (Maintenance costs)Economics' (Maintenance costs) WORSTWORST POORPOOR POORPOOR BESTBEST

Plant continues to operates under Plant continues to operates under single line-to-ground faultsingle line-to-ground fault

FAIRFAIR POORPOOR POORPOOR BESTBEST

Ease of locating ground faults (time)Ease of locating ground faults (time) WORSTWORST GOODGOOD BETTERBETTER BESTBEST

System coordinationSystem coordination NOT NOT POSSIBLEPOSSIBLE

GOODGOOD BETTERBETTER BESTBEST

Upgrade of ground systemUpgrade of ground system WORSTWORST GOODGOOD BETTERBETTER BESTBEST

Two voltage levels on same systemTwo voltage levels on same system NOT NOT POSSIBLEPOSSIBLE

POSSIBLEPOSSIBLE NOT POSSIBLENOT POSSIBLE NOT POSSIBLENOT POSSIBLE

Reduction in number of faultsReduction in number of faults WORSTWORST BETTERBETTER GOODGOOD BESTBEST

Initial fault current Into ground Initial fault current Into ground systemsystem

BESTBEST WORSTWORST GOODGOOD BETTERBETTER

Potential flashover to groundPotential flashover to ground POORPOOR WORSTWORST GOODGOOD BESTBEST

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TYPES OF GROUNDING SYSTEMSTYPES OF GROUNDING SYSTEMS Ungrounded System:Ungrounded System:

The ungrounded system is one that has no intentional The ungrounded system is one that has no intentional connection between the neutral or any phase and ground. connection between the neutral or any phase and ground. Please note that an ungrounded system is grounded Please note that an ungrounded system is grounded through the concept of capacitively coupling. The neutral through the concept of capacitively coupling. The neutral potential of an ungrounded system, with balanced loading potential of an ungrounded system, with balanced loading will be close to ground potential due to the capacitance will be close to ground potential due to the capacitance between each phase conductor and ground.between each phase conductor and ground.

Low ground fault current.Low ground fault current.

Very high voltages to ground potential on unfaulted phases.Very high voltages to ground potential on unfaulted phases.

Sustained faults lead to system line-to-line voltages on Sustained faults lead to system line-to-line voltages on unfaulted line.unfaulted line.

Insulation failure.Insulation failure.

Failure due to restrike ground faults.Failure due to restrike ground faults.

Continued operation of facility.Continued operation of facility.

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TYPES OF GROUNDING SYSTEMSTYPES OF GROUNDING SYSTEMS Solidly Grounded System:Solidly Grounded System:

The solidly grounded system is one that has the neutral The solidly grounded system is one that has the neutral connected to ground without an intentional impedance. connected to ground without an intentional impedance. In contrast to the ungrounded system the solidly In contrast to the ungrounded system the solidly grounded system will result in a large magnitude of grounded system will result in a large magnitude of current to flow (Aids in coordination), but has no current to flow (Aids in coordination), but has no increase in voltage on unfaulted phases.increase in voltage on unfaulted phases.

Low initial cost to install and implement, but stray currents Low initial cost to install and implement, but stray currents then become a possible consequence.then become a possible consequence.Common in low voltage distribution systems, such as Common in low voltage distribution systems, such as overhead lines.overhead lines.typically feeds to transformer primary with high side fuse typically feeds to transformer primary with high side fuse protection.protection.Not the preferred grounding scheme for industrial or Not the preferred grounding scheme for industrial or commercial facilities due to high magnitude fault currents.commercial facilities due to high magnitude fault currents.


TYPES OF GROUNDING SYSTEMSTYPES OF GROUNDING SYSTEMS Low Resistance Grounded System:Low Resistance Grounded System:

The low resistance grounded system is one that has the The low resistance grounded system is one that has the neutral connected to ground through a small resistance neutral connected to ground through a small resistance that limits the fault current. The size of the grounding that limits the fault current. The size of the grounding resistor is selected to detect and clear the faulted resistor is selected to detect and clear the faulted circuit..circuit..

The resistor can limit ground currents to a desired level The resistor can limit ground currents to a desired level based on coordination requirement or relay limitations.based on coordination requirement or relay limitations.Limits transient overvoltages during ground faults.Limits transient overvoltages during ground faults.Low resistance grounding is not recommended for low Low resistance grounding is not recommended for low voltage systems due to the limited ground fault current. voltage systems due to the limited ground fault current. This reduced fault current can be insufficient to positively This reduced fault current can be insufficient to positively operate fuses and/or series trip units.operate fuses and/or series trip units.Ground fault current typically in the 100 – 600 Amp range.Ground fault current typically in the 100 – 600 Amp range.


TYPES OF GROUNDING SYSTEMSTYPES OF GROUNDING SYSTEMS High Resistance Grounded System:High Resistance Grounded System:

The high resistance grounded system is one that has the neutral The high resistance grounded system is one that has the neutral connected to ground through a resistive impedance whose connected to ground through a resistive impedance whose resistance is selected to allow a ground fault current through the resistance is selected to allow a ground fault current through the resistor equal to or slightly more that the capacitive charging resistor equal to or slightly more that the capacitive charging current of the system. current of the system.

The resistor can limit ground currents to a desired level based on The resistor can limit ground currents to a desired level based on coordination requirement or relay limitations.coordination requirement or relay limitations.

Limits transient overvoltages during ground faults.Limits transient overvoltages during ground faults.

Physically large resistor banks.Physically large resistor banks.

Very low ground fault current, typically under 10 Amps.Very low ground fault current, typically under 10 Amps.

Special relaying methods utilized to detect and remove ground Special relaying methods utilized to detect and remove ground faults.faults.

High resistance grounding is typically applied to situations where it High resistance grounding is typically applied to situations where it is essential to prevent unplanned outages.is essential to prevent unplanned outages.

Recent trend has been to utilize high resistance grounding methods Recent trend has been to utilize high resistance grounding methods on 600 volt systems and lower.on 600 volt systems and lower.


GROUNDING SYSTEMSGROUNDING SYSTEMS System grounds: A system ground refers to the point in an electrical System grounds: A system ground refers to the point in an electrical

circuit that is connected to earth. This connection point is typically at circuit that is connected to earth. This connection point is typically at the electrical neutral. The sole purpose of the system ground is to the electrical neutral. The sole purpose of the system ground is to protect equipment. This type ground also provides a low impedance protect equipment. This type ground also provides a low impedance path for fault currents improving ground fault coordination. This path for fault currents improving ground fault coordination. This ensures longer insulation life of motors, transformers and other system ensures longer insulation life of motors, transformers and other system components.components.

Maintenance grounds: This type ground is utilized for safe work Maintenance grounds: This type ground is utilized for safe work practices, and is a temporary ground.practices, and is a temporary ground.

Electronic and computer grounds: Grounding for electronic equipment Electronic and computer grounds: Grounding for electronic equipment is a special case in which the equipment ground and the system ground is a special case in which the equipment ground and the system ground are combined and applied in unity. Electronic equipment grounding are combined and applied in unity. Electronic equipment grounding systems must not only provide a means of stabilizing input voltage systems must not only provide a means of stabilizing input voltage levels, but also act as the zero (0) voltage reference point. Grounding levels, but also act as the zero (0) voltage reference point. Grounding systems for the modern electronics installation must be able to provide systems for the modern electronics installation must be able to provide effective grounding and bonding functions well into the high frequency effective grounding and bonding functions well into the high frequency megahertz range.megahertz range.

Lightning protection: Lightning protection grounding requirements are Lightning protection: Lightning protection grounding requirements are dependent upon the structure, equipment to be protected, and the dependent upon the structure, equipment to be protected, and the level of lightning protection required of desired.level of lightning protection required of desired.


GROUNDING SYSTEMSGROUNDING SYSTEMS► Several factures should be considered in the initial design of Several factures should be considered in the initial design of

the grounding system.the grounding system. The area available for installation of the grounding system. This The area available for installation of the grounding system. This

could lead to the requirement and utilization of chemical rods, or could lead to the requirement and utilization of chemical rods, or wells.wells.

Water table and seasonal changes to it.Water table and seasonal changes to it. Soil condition and resistivity, Please see chart of typical results. Soil condition and resistivity, Please see chart of typical results.

Also elevation above sea level and hard rocky soil are concerns Also elevation above sea level and hard rocky soil are concerns that would need to be addressed.that would need to be addressed.

Available fault currents (i.e., three (3) phase, line-to-ground, and Available fault currents (i.e., three (3) phase, line-to-ground, and line-to-line-to ground, etc.).line-to-line-to ground, etc.).

NEC and ANSI/IEEE requirements. Also include here the NEC and ANSI/IEEE requirements. Also include here the requirements of the process equipment to be installed.requirements of the process equipment to be installed.

Consideration to the number of lightning strikes and thunder storm Consideration to the number of lightning strikes and thunder storm days per year.days per year.

Utility ties and/or service entrance voltage levels.Utility ties and/or service entrance voltage levels. Utilization of area were ground system is to be installed, (i.e., do Utilization of area were ground system is to be installed, (i.e., do

not install under paved parking lot).not install under paved parking lot).


GROUNDING SYSTEMSGROUNDING SYSTEMSSOIL RESISTIVITIES(Approximate Ohm-Meters)

Description1,2 Median Min. Max.Topsoil's, loams 26 1 50Inorganic clays of high plasticity 33 10 55Fills-ashes, cinders, brine wastes 38 6 70Silty or clayey fine sands with slight plasticity 55 30 80Porous limestone, chalk 65 30 100Clayey sands, poorly graded sand-clay mixtures 125 50 200Fine sandy or silty clays, silty clays, lean clays 140 80 200Clay-sand-gravel mixtures 145 40 250Marls3 155 10 300Decomposed granites, gneisses4, etc. 300 100 500Clayey gravel, poorly graded gravel 300 200 400Silty sands, poorly graded sand-silt mixtures 300 100 500Sands, sandstone 510 20 1,000Gravel, gravel-sand mixtures 800 600 1,000Slates, schists5, gneiss, igneous rocks, shales, granites, basalts 1,500 1,000 2,000Quartzite's, crystalline limestone, marble, crystalline rocks 5,500 1,000 10,000

Notes: 1. Low resistivity soils are highly influenced by the presence of moisture.2. Low resistivity soils are more corrosive than high resistivity soils.3. Crumbly soil composed mostly of clay with a high limestone content.4. Metamorphic rock formed by recrystallization of granite, separated into bands.5. Metamorphic rock much coarser than gneiss.This chart compiled from data published in:IEEE Standard 142-1991, Recommended Grounding PracticesBritish Standard Code of Practice, CP-1013: 1965, EarthingMegger: A Simple Guide to Earth TestingBiddle: Getting Down to Earth


GROUNDING SYSTEMSGROUNDING SYSTEMS1. Parity sized grounding conductors.

2. Grounding symmetry in all parallel feeders.

3. Zones of equipment with localized transformers to isolate the equipment and control leakage current.

4. Limiting the quantity of devices grounded by any single conductor.

5. Utilizing specialty transformers to limit ground interference.

6. Faraday cage design concepts.

7. Use different networks throughout the facility as opposed to a single ended data network.

8. Reference grids in all computer, data processing and information technology rooms.

9. Perimeter ground ring bonded to the service entrance.

10. Intentional continuity of structural steel.

11. Bonding of all communication cables to structural steel.

12. Architectural steel treatment for lightning protection.

13. Ufer ground treatment per NEC for all main vertical steel footers.

14. Grounding grid below moisture barrier.

15. Bonding horizontal steel pans to structural steel.


GROUNDING SYSTEMSGROUNDING SYSTEMS► Several factures can degrade initially good grounding Several factures can degrade initially good grounding

systems. These factors indicate the importance of continuous systems. These factors indicate the importance of continuous periodic testing (Typically once per calendar year unless periodic testing (Typically once per calendar year unless problems arise). A change (lower) in the water table across problems arise). A change (lower) in the water table across the USA would lead to a degrade in the grounding system. the USA would lead to a degrade in the grounding system. Another consideration in the ground system would be in Another consideration in the ground system would be in facility growth and the addition of non-metallic piping and facility growth and the addition of non-metallic piping and conduit which do not provide low resistance ground conduit which do not provide low resistance ground connections. Along with the these concerns are the increase connections. Along with the these concerns are the increase load and associated increase in available fault currents. The load and associated increase in available fault currents. The better the ground system, the more attention should be paid better the ground system, the more attention should be paid to corroded electrodes. All these could result in the need for a to corroded electrodes. All these could result in the need for a decrease in the grounding resistance.decrease in the grounding resistance. Testing: Periodic testing should be done to assure grounding Testing: Periodic testing should be done to assure grounding

system effectiveness.system effectiveness.


GROUNDING SYSTEMSGROUNDING SYSTEMS


Florida Electrical Resources Earth Ground Test Report A Division of TEAMWORKnet, Inc. Project : 302xxxx Test Page 1

FLORIDA ELECTRICAL RESOURCES 6550 New Tampa Hwy., Suite B Lakeland, Florida 33815 (800) 727-4337 (863) 327-1080 Fax (863) 327-1091 A Division of TEAMWORKnet, Inc.

Earth Ground Test Data Analysis & Problem Classification Engineer Bill Engel, P.E. & Harry Tittel, E.E. Test Method Clamp on Ground Tester Client / Facility Client Name Photograph No. 0 Amperage Reading 0 mA Location Saw Mill Area Test Results 46.2 Notes: Ground Tested High. Perform Date January 8, 2001 Maintenance on Ground or Time 9:30 AM Replace. Ambient Conditions 83 °F, Humid, Dry Ground

Test Point Digital Image Test Point Location Representation

GROUND CABLE

Saw Mill


Florida Electrical Resources Earth Ground Test Report A Division of TEAMWORKnet, Inc. Project : 302xxxx Test Page 2

FLORIDA ELECTRICAL RESOURCES 6550 New Tampa Hwy., Suite B Lakeland, Florida 33815 (800) 727-4337 (863) 327-1080 Fax (863) 327-1091 A Division of TEAMWORKnet, Inc.

Earth Ground Test Data Analysis & Problem Classification Engineer Harry Tittel, E.E. Test Method Clamp on Ground Tester Client / Facility Client Name Photograph No. 7 Amperage Reading 34 mA Location Stacker, Sorter, C-N-S Area Test Results 1.5 Debarker Area Notes: Ground Tested Within Date January 8, 2001 Parameters. Time 11:35 AM Ambient Conditions 93 °F, Humid, Dry Ground

Test Point Digital Image Test Point Location Representation

GROUND

Stacker Sorter C-N-S

Oil

Log Deck

Debarker


Open question and Answer session.

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