Grounding resistor fire

[z633]

34.5/4.8kV- 400A-10sec rated neutral ground resistor caught on fire after replacing a blown overhead line fuse. Sub main protected with 51G did not open, apparrently due to blown control circuit fuse feeding main breaker. Feeders protected with 51N. None opened. Initial call was for submersible well pump problem. After fire in grounding resistor was put out, the main was opened after replacing blown control fuse. Grounding resistor circuit then opened, and service was restored. Distribution service 4.8 kV 3-wire. All downstream equipment appears to now be functioning. My worry, with no ground reference and probably a phase line grounded somewhere in the system. "Wild leg delta" on 4.8kv system. Go to troubleshoot tomorrow. Only thought is to check phase lines referenced to ground. However I am worried as what to expect when checks are made. Any suggestions???

 

[RalphChristie]

Not sure if my answers will address your question, but:

There must have been some current flowing through the resistors, and to burn it either:

You've had a loose connection on the resistor grid,
You've exceeded the recommended time, (thus took to long to clear the fault)
Resistor grid were already damaged previously for some other reason.



Most probably the original fault is still on the system. With the grounding resistor disconnected nothing will happen, but for another ground-fault on another phase you'll have a L-L fault. Also, with your system now ungrounded, substantial overvoltages can develop.

Will your system be on or off when you are going to try to locate the fault? Due to the high voltage I would prefer the system to be off - by using a megger you can try to locate the fault by dividing you system in different segments.

Personally I do not recommend an ungrounded system. In my opinion you'll have to ground the system as fast as possible. However, before you put the system back on again, ensure the tripping is correct by testing it manually.

Regards
Ralph

[red]Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon[/red]

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[waross]

I'd use a voltage tester. Find the grounded leg and connect the voltage tester..
With a problem reported on a submersible pump, open the breaker on the pump first. If the leg is still grounded, then open distribution breakers until the ground goes away. Reclose the breakers. Then follow the system downstream opening breakers until the ground is located. Note. Measure the voltage of the grounded leg on the energised side of the breakers. If this doesn't find it, then go with the megger.