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Generator NGR (Neutral Grounding Resistor) Ratings

Generator NGR (Neutral Grounding Resistor) Ratings

Generator NGR (Neutral Grounding Resistor) Ratings

I have seen on my last couple projects Generator NGR's normally rated for 200A 10 seconds.

My question is … is it common to get a longer duration rating for these industrial NGR's?

i.e. instead of 10 seconds, get a rating of 60 seconds or even continuous to allow other generators to come online when an event occurs in another generator

RE: Generator NGR (Neutral Grounding Resistor) Ratings

It's not common to have a rating beyond 10 sec for low resistance grounding in my experience because it gets expensive and the resistors get bigger.

Even at 200 A, a lot of damage is being done, so trying to keep a generator on-line doesn't really make economic sense, especially if the fault is in the generator.

You can get longer ratings, of course. You can discuss with resistor manufacturers.

The other option of course is to use high resistance grounding and limit the fault current to much lower levels. But even then, most people trip generators right away and still buy 10 sec resistors.

RE: Generator NGR (Neutral Grounding Resistor) Ratings

This must be a low voltage generator? It would be very unusual to allow more than 5A or 10A of fault current to flow on a large set because of the risk to the hugely expensive stator core. High resistance earthing could require you to use a transformer with a delta primary and a secondary to match your system requirements, so they're frequently omitted to save costs, especially if the set is a standby unit.

RE: Generator NGR (Neutral Grounding Resistor) Ratings

As I understand you are interested to run the faulty generator for a minute or more until you can get the stand by generator take over.
As mentioned by others already, continuing the generator in service with a fault of 200A is ruled out.
But, I have seen a motorised circuit breaker / load break switch in the neutral circuit that is made to open (instead of tripping the generator on earth fault) as soon as the fault is sensed and then start the stand by set.
This way, the operator or control system gets time to start the stand by DG to maintain continuous power supply and at the same time the generator damage is prevented.
One thing to be mentioned here is that the power system including the generator is unearthed during this period and it is important to ensure that the duration doesn't exceed 1min (60s), unless the insulation of the system (including generator) is designed for the application.

RE: Generator NGR (Neutral Grounding Resistor) Ratings

Hi Sam,
What is the size (in MVA) and voltage rating of the generator(s)? Is there a dedicated unit transformer (ie one step-up transformer per generator)? If not, how many generators are directly connected to a common bus (ie no unit transformers).
FYI: I typically ask for 30s resistors, but normally at 50A or less (ie never at 200A). A 200A 30s resistor would be large and costly.

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

RE: Generator NGR (Neutral Grounding Resistor) Ratings


For medium-voltage generators in industrial settings, low resistance grounding is common, at least in the US. Also with multiple generators connected to common generator bus, it allows for easier coordination for ground faults. High-resistance grounding universally used for large generators with unit transformers.

Also, low resistance grounding is never used in the US at 600 V or less. (at least it shouldn't be).

RE: Generator NGR (Neutral Grounding Resistor) Ratings

Thanks dpc.

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