Introduction

High Resistance Grounding is recommended for systems where power interruption resulting from single line-to-ground fault tripping is detrimental to the process

The maximum ground fault current allowed by the Neutral Grounding Resistor must exceed the total capacitance to ground charging current of the system.

The total capacitance to ground charging current of a system can be measured or estimated.

Measuring

Should only be done by qualified personnel. Power should be off before making any connection before the test. The system has to be ungrounded. All components and devices used should be rated properly for the system voltage. For the test the system will be fully energized.

One line will be bolted to ground using a fast-acting fuse rated for 10 A or less, a Circuit Breaker, a Rheostat and an Ammeter all in series.

The rheostat should be set at maximum resistance and the Breaker open.

The Breaker will then be closed and the Rheostat resistance will be reduced slowly to zero. At this point the current circulating to the ground and indicated in the Ammeter is the Capacitance to Ground Charging Current of the system.

To finish the test the Rheostat will be returned to maximum resistance and the Breaker will be opened.

Please note that as with any capacitor, and here we are looking at the electrical system as a large distributed capacitor, this current will change if the physical configuration of the system is altered (i.e. by adding feeders, motors and more importantly surge arresters)

Estimating

A quick estimate can be obtained by adding the charging current indicated in the following table according to the system kVA and the surge capacitors installed:

480 V systems

System kVA0.5 A / 1000 kVA
Surge capacitors0.5 A

2.4 kV systems

System kVA0.75 A / 1000 kVA
Surge capacitors1.0 A

4.16 kV systems

System kVA1.0 A / 1000 kVA
Surge capacitors1.5 A

For system voltages greater than 4.16 kV or when the estimated charging current is too close to 10 A a better estimate can be obtained by using the following tables according to the system voltage and the equipment installed.

480 V systems

Surge capacitors, 1 µF/phase0.31 A
Cables 350 to 500 MCM in conduit0.1 A / 1000 ft
Cables 2/0 to 3/0 MCM in conduit0.05 A / 1000 ft
Cables 2/0 to 3/0 MCM in trays0.02 A / 1000 ft
Cables #6 – 3/c with ground wires in water0.05 A / 1000 ft
Motors0.01 A / 1000 HP

2.4 kV systems

Surge capacitors, 0.5 µF/phase0.78 A per set
Cables (non-shielded) in conduit0.05 A / 1000 ft
Motors0.03 A / 1000 HP

4.16 kV systems

Surge Capacitors, 0.5 µF/phase1.36 A per set
Vulkene cable (shielded) #1 – 350 MCM0.23 A / 1000 ft
Vulkene cable (non-shielded) in conduit0.1 A / 1000 ft
Motors0.05 A / 1000 HP

13.8 kV systems

Surge capacitors, 0.5 µF/phase2.25 A per set
Surge capacitors, 0.25 µF/phase4.50 A per set
Cable 1000 MCM shielded1.15 A / 1000 ft
Cable 750 MCM shielded0.93 A / 1000 ft
Cable 350 MCM shielded0.71 A / 1000 ft
Cable 4/0 AWG shielded0.65 A / 1000 ft
Cable 2/0 AWG shielded0.55 A / 1000 ft
Motors0.15 A / 1000 HP

References

Westinghouse, “System Neutral Grounding and Ground Fault Protection,” publication PRSC-4B-1979, Westinghouse, 1979

General Electric Co., “Generator Neutral Grounding,” publication GET-1941, Schenectady, NY: General Electric

“Charging Current Data for Guesswork-free Design of High-Resistance Grounded Systems,” by D.S. Baker, IEEE Transactions on Industry Applications, Vol. IA-15, No. 2, March/April 1979.

Baldwin Bridger, Jr., High-Resistance Grounding, IEEE Transactions on Industry Applications, Vol. IA-19, No. 1, Jan/Feb 1983.