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 kVA | 0.5 A / 1000 kVA |
| Surge capacitors | 0.5 A |
2.4 kV systems
| System kVA | 0.75 A / 1000 kVA |
| Surge capacitors | 1.0 A |
4.16 kV systems
| System kVA | 1.0 A / 1000 kVA |
| Surge capacitors | 1.5 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/phase | 0.31 A |
| Cables 350 to 500 MCM in conduit | 0.1 A / 1000 ft |
| Cables 2/0 to 3/0 MCM in conduit | 0.05 A / 1000 ft |
| Cables 2/0 to 3/0 MCM in trays | 0.02 A / 1000 ft |
| Cables #6 – 3/c with ground wires in water | 0.05 A / 1000 ft |
| Motors | 0.01 A / 1000 HP |
2.4 kV systems
| Surge capacitors, 0.5 µF/phase | 0.78 A per set |
| Cables (non-shielded) in conduit | 0.05 A / 1000 ft |
| Motors | 0.03 A / 1000 HP |
4.16 kV systems
| Surge Capacitors, 0.5 µF/phase | 1.36 A per set |
| Vulkene cable (shielded) #1 – 350 MCM | 0.23 A / 1000 ft |
| Vulkene cable (non-shielded) in conduit | 0.1 A / 1000 ft |
| Motors | 0.05 A / 1000 HP |
13.8 kV systems
| Surge capacitors, 0.5 µF/phase | 2.25 A per set |
| Surge capacitors, 0.25 µF/phase | 4.50 A per set |
| Cable 1000 MCM shielded | 1.15 A / 1000 ft |
| Cable 750 MCM shielded | 0.93 A / 1000 ft |
| Cable 350 MCM shielded | 0.71 A / 1000 ft |
| Cable 4/0 AWG shielded | 0.65 A / 1000 ft |
| Cable 2/0 AWG shielded | 0.55 A / 1000 ft |
| Motors | 0.15 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.