Ground Fault Coordination

[Power2all]

I have an SE 325 Startco NGR with a 51G Relay for ground fault protection. The let through current is 5A so I have pick up at 20% (1A). So what does this mean if I have a ground fault downstream? Are my only choices to dump the whole plant or keep operating with the fault and just alarm it? Any suggestions for achieving selective ground fault protection if I want to trip the breaker when GF is detected? At what point if any does the code (CEC or NEC) require tripping the breaker?

Thank you.

 

[dpc]

Are my only choices to dump the whole plant or keep operating with the fault and just alarm it?

With high resistance grounding, yes, those are your two choices. Normally, a ground fault is just alarmed and then some type of fault location technique is used to isolate the location of the ground fault.

 

[mgtrp]

On LV systems, it is common to design your system to operate continuously with a single ground fault. As you go up in voltage, this becomes less common.

On our MV systems, we sometimes use the guess and hope approach by setting the time delays on the various feeders slightly differently. The hope is that the faulted feeder is one of the first ones to trip, which is at least better than tripping everything.

 

[powr]

At what point if any does the code (CEC or NEC) require tripping the breaker?

The code does not require tripping the ground fault within a specified time frame for a high resistance ground fault. In fact one of the main advantages is that you can continue to operate on a ground fault (similar to ungrounded systems) but that the ground is easier to locate and the connection to ground virtually eliminates the over-voltage problem with arcing ground faults that plague ungrounded systems.

But as dpc said, it is still recommended that you find the fault quickly, because as soon as another ground fault occurs on a different phase, the resistance grounding will not limit the fault magnitude.

 

[7anoter4]

I think there are some problems functioning with one phase grounded in high resistant neutral grounding system. First of all in open installations as overhead
line one has to check what could be the personnel or equipment electrocution danger [or fire] mainly in a crowded place.
Second, the cable insulation has to be suitable. A cable of 100% insulation support only 1 minute, 133% 1 hour and 175% continuously.