Conversion of a solidly grounded system into an HRG system

The solidly grounded electrical system relied on overcurrent protective devices to interrupt the large current associated with a ground fault. Coordination of upstream and downstream devices could be difficult.  The HRG system can deliver very low ground-fault current—too low to operate a circuit breaker or fuse—supplemental ground-fault protection is required.

Use a residual-current-type device (RCM) to detect ground faults using a zero-sequence current transformer.  By installing such devices at each feeder, or even each load, fault location is automated, and selective circuit interruption is easily achieved, where necessary.

Some circuit elements, such as line-to-ground filter capacitors on the input of variable-speed drives, may be rated at line-to-neutral voltage. These elements should be upgraded to line-to-line voltage ratings to withstand the higher voltage present during a ground fault on an HRG system.

To avoid the situation in which a ground fault is not detectable, leaving the system in an unsafe condition, continuously monitor the NGR with an appropriate device. The NGR must be monitored for continuity and connection to system neutral.

In some circumstances, such as vermin nest building or incorrect system maintenance, an NGR can be shorted.  In this case, a ground fault will result in higher-than-expected current, which could have negative consequences like a higher level of equipment damage or an arc flash. Continuously monitor the NGR for a resistance reduction to alert operators to this condition.