Neutral Grounding Resistor Protection

[gordonl]

We use a large amount of low resistance grounded MV systems, the only protection currently is graded ground overcurrents.

A company has recently brought to my attention their NGR monitors which monitors the banks resistance with alarm for increased resistance. (Also incorporates overcurrent) I'm interested in the concept because an open NGR can have an expensive cost during a ground since the cable in my system isn't rated for ungrounded service.

My question is, how likely is an NGR to fail open, and how common is the use of NGR failure detection. (I'm trying to balance the cost versus risk of NGR failure)

I beleive these devices are requirements for mines in some jurisdictions, but I would think NGR failure would be far more common in those cases because the subs are generally portable.

Thank You in Advance,
Gord

 

[redtrumpet]

I assume you are talking about the Startco relay (SE-325), or possibly Patton & Cooke.

I don't have any data on how often the NGR fails open. However, I know of one major mining company that uses this relay on all of its above-ground substations at 13.8 kV and 2.4 kV at its major smelter location. Given that mining companies hate to spend money, I assume they have determined there is value in the NGR monitoring.

As you mentioned, CAN/CSA Std M421 requires the use of NGR monitoring in certain underground mining applications involving mobile and moveable equipment. However, the smelter location I refer to above is not subject to M421.

I have dealt with Startco extensively over the years (actually took a ground-fault course from them) and have found them to be extremely reputable and honest. If they provide you with NGR failure data, I am sure you can rely on it. One knowledgeable person to talk to is Blair Baldwin, 306-373-5505 or blair.baldwin@startco.ca.

Startco ground-fault products are used extensively by the Saskatchewan uranium and potash mining industries, but mostly in underground mining locations in accordance with M421. I believe they are also extensively used in Australia and western US (Nevada, Utah).

 

[zub]

if you are asking how to detect the open circuit during the normal conditions i don't know. But you can detect it when a ground fault occure by using a voltage transformer which connected parallel to the resistor. I'm sure you know this type of application but I wanted to say my opinion.

zub.

 

[zub]

sorry but i have to add a voltage relay connected to the PT of course....

sorry.

zub.