Limiting Current = Limiting Damage
Resistance grounding is accomplished by inserting a resistor between the neutral terminal of a wye-connected transformer or generator and ground in place of the solid connection that is normally used. On low-voltage (600-V and less) systems, the high-resistance method, in which the resistance is selected to limit the current that can flow in a ground fault to a level that will not cause arcing or other damage at the point of the fault, is commonly used.
On 480/277-V systems, a 27.7-Ohm resistor is common, limiting the maximum ground fault current to 10A and the maximum power dissipation in a ground fault to approximately 700W. At these levels, a ground fault in a piece of equipment such as a motor can be allowed to persist without hazard. The result is a power system that can tolerate insulation failure on a single phase without blowing a fuse or tripping a circuit breaker.
Ground Detection is Mandatory
While the major advantage of resistance grounding is the ability to tolerate a single ground fault without interrupting the circuit, this advantage can only be maintained if ground faults are detected, located, and promptly cleared. If a ground fault is allowed to persist and a second ground fault develops on one of the other two phases, the result is a phase-to-phase short circuit through the ground path. Typically a high-current condition, this will damage both pieces of faulted equipment and result in simultaneous outages in two portions of the system.A resistance-grounded system must always be provided with a means of detecting the first ground fault condition. This is done by detecting the voltage developed across the grounding resistor as the fault current flows in it, or by sensitive relays that can detect the low levels of ground current flow on the service or individual feeders.
Once a ground fault is detected, it must be traced to the affected circuit or equipment and repaired as soon as practicable. The fault can be traced through the process of elimination by turning off individual circuits or pieces of equipment until the alarm clears, but this is rarely a practical or acceptable approach for the critical systems involved. More effective methods that do not interrupt service include use of a pulsing contactor and a clamp-on ammeter or the injection of a high-frequency signal in the ground connection.
Application Issues
The principal requirement for application of resistance grounding to a 480/277-V system is that there be no phase-to-neutral connected loads; it has to be a three-phase, three-wire system rather than a three-phase, four-wire system.When a ground fault occurs, a voltage is developed across the grounding resistor that displaces the neutral from its usual zero-voltage level. Because the National Electrical Code (NEC) considers a neutral conductor to always be at zero voltage with respect to ground, the use of resistance grounding on a system using a neutral conductor to serve 277-V loads such as lighting is prohibited.
If it is desired to have both a resistance-grounded system and 277-V lighting in a facility, a separate 480V - 480/277-V isolation transformer is required to supply the lighting. The secondary neutral of the isolation transformer would then be solidly grounded and the lighting circuits would not be part of the resistance-grounded system.
I have also run into situations where the ground fault detection relays used with high-resistance grounded systems can be affected by operation of variable-frequency drives (vfd's). The switching frequency of pulse width modulated vfd's may cause high-frequency currents to flow to ground through the capacitance inherent in motor windings.
These currents in turn develop high-frequency voltage signals across the grounding resistor, which may cause voltage relays to falsely indicate a ground fault on the system. This situation can be corrected by proper choice of voltage relay, the addition of filtering at either the vfd or the relay, or use of an isolation transformer with a grounded inter-winding shield ahead of the vfd. ES
