NGR or residual current device

[deltawhy]

Hello,
I have a question regarding a 300kVA 208Y/120V generator we are installing in one of our buildings. Do you think it is necessary to install either a NGR or ground fault protection into the genset? If so, which method is best at limiting ground fault damage? Based on cost it is probably better to go with a ground fault trip unit on the breaker, but I may be mistaken.

Thanks

 

[crthompson]

The following link has some good information about ground fault protection... I hope this helps...

http://www.postglover.com/Literature/HR002-05_Hi_Resis_Grd_Conversion_Gde.pdf

 

[ScottyUK]

Do you have any loads connected line-neutral? Pretty much every national wiring regulations require the neutral to be grounded if there are line-neutral loads, which would rule out the NGR. Earth fault protection can be made to discriminate on time and current, while an NGR can be sized to allow continuous operation with a single earth fault.

What are you trying to achieve other than prevent damage? How important is your load?

 

[dpc]

As Scotty said, you can't put a grounding resistor in the neutral and expect to serve 120 V single phase loads. That is an NEC violations as well as a really bad idea.

The application of ground fault trip may depend on the purpose of the generator. If this is for standby or emergency power, you might want to think twice about installing ground fault trip. 300 kVA is about 832 A at 208V, so I don't think there is an NEC requirement for ground fault, assuming you are in NEC-land.

 

[deltawhy]

Thanks for the info.
I'm in Canada so its the CEC, but essentially the same thing I would assume, anything over 2000A at 208Y/120V requires ground fault protection. I just bring this up because the last backup system I designed fed only transformers. Would you start thinking about NGR at the current levels that require ground fault protection? I'm just trying to get a feel for where you guys usually start using them.

Thanks