Generator

bennie · May 22, 2003

Ed, will you again post the drawing of the generator that is on a system with GFP devices?

Thank you: Bennie

Ed MacLaren · May 23, 2003

Re: Generator

Is this the drawing you were referring to?

It shows that a ground fault in the critical loads feeder can shut down other loads.

Ed

bennie · May 23, 2003

Re: Generator

Thanks Ed, that is the one. I have printed it for my records.

bennie · May 23, 2003

Re: Generator

I have a question... Both power sources must be active for the GFP to trip. Other than a few minutes during utility power restoration, how often are both systems operating at the same time?

brian john · May 23, 2003

Re: Generator

Bennie:

Both systems can be on line simultaneously under several conditions, during weekly/monthly periods of exercise (hopefully). After an utility power outage and the ATS has not retransferred to the utility source, typical time delays are from one to thirty minutes

bennie · May 23, 2003

Re: Generator

What are the odds that a fault will develop during the time of simultaneous operation?

Can anyone name an event, that has a chance of one in a million of occurring, having so much attention for prevention?

I think someone has been fantasizing about why a neutral should be switched in a transfer switch.

Ed MacLaren · May 23, 2003

Re: Generator

I think someone has been fantasizing about why a neutral should be switched in a transfer switch.

Here is why the neutral must be switched when the generator winding center-tap (neutral) is bonded to the frame.

The continuous (unswitched) neutral is bonded to enclosures at two points, which permits unbalanced load current (red arrows) into the equipment grounding conductors.

Ed

bennie · May 23, 2003

Re: Generator

Remove the green wire between the generator and transfer switch. Connect generator with PVC. Bonding can be done at both places if parallel current can be avoided.

The green wire can not be a fault path, there is nothing to clear the fault if one developed.

bennie · May 23, 2003

Re: Generator

As an afterthought...When parallel current is no problem on a service, and the entire distribution system, how come it's a problem with a back up generator?

[ May 23, 2003, 07:11 PM: Message edited by: bennie ]

Ed MacLaren · May 23, 2003

Re: Generator

When parallel current is no problem on a service, and the entire distribution system, how come it's a problem with a back up generator?

It would be a problem in a service, feeder, or branch circuit, and sometimes is, when the grounded (neutral) conductor is bonded to the non-current-carrying metal enclosures at more than one point.

BTW, I added a breaker to the generator.

P.S. I'm almost certain we had this same discussion a year ago. I must be losing it.

Ed

[ May 23, 2003, 10:09 PM: Message edited by: Ed MacLaren ]

bennie · May 23, 2003

Re: Generator

We got started on this...I don't remember where it went.

The green wire is still not necessary. A ground fault will use the neutral.

brian john · May 23, 2003

Re: Generator

Bennie:

I assume you have some electrical experience, and understand the ramifications of currents on a ground conductor (other than fault current), from a PQ stance, operation of OC devices, GF protection, step voltage, should I go own or are you having fun pulling my leg.

MAKE AN ASSUMPTION:

You are responsible for a data center dealing with millions of dollars a minute or an hour, are you wiling to risk the exposure of a possible fault when the site is connected to genset and utility maybe for peak shaving for 8 or more hours, or maybe for one second, time is irrelevant. Murphy won’t risk this and I ain't about too.

Beside it is a code violation for major reasons

AND THAT IS WHY WE HAVE SEPERAATLY DERIVED SYSTEMS.
Defined and installed per the NEC.

hurk27 · May 23, 2003

Re: Generator

I still don't like switching of the neutral. This just provides one more mechanical point to fail and I would not want the neutral to ever fail from a generator. so I like the non-SDS generator hook up instead of the SDS type.
If you loose the neutral across a transfer switch there is no backup for it like the water pipe on a service.
Maybe this is why most gen-packs that we install come with a two-pole transfer-switch and is wired non-SDS. Look at any Generiac gen-set this is the way it comes from the factory. It even has the isolated neutral terminal in the TS.

[ May 23, 2003, 09:40 PM: Message edited by: hurk27 ]

bennie · May 23, 2003

Re: Generator

Brian: Where in the NEC does it state "both ground/neutral and an equipment ground conductor are required for a generator"?

Where does it state that an equipment ground conductor is required from the service panel to the utility transformer?

[ May 23, 2003, 09:40 PM: Message edited by: bennie ]

brian john · May 23, 2003

Re: Generator

Bennie Don't ask such questions read the book please. NEC and scores of others..

Hurk:

Proper maintenance should be performed on all electrical mechanical systems, losing the neutral would be just as bad as an arcing ground fault from a phase conductor, that burns the ATS to scrap. If one can't be confident of the equipment under their supervision they should contract with someone that is. For systems with ground fault protection an ATS with an overlapping neutral properly maintained and TESTED is the way to go IMHO

bennie · May 23, 2003

Re: Generator

Brian: Thanks for the advice, I went to Home Depot and bought a book on house wiring. I may have a career after all.

hurk27 · May 23, 2003

Re: Generator

I have to agree with you Bennie as the purpose of keeping the neutral and grounds isolated is because of the voltage drop between supply and load will impose a voltage upon the grounding.
but if the frame of a generator is bonded to the neutral and no grounding is installed in-between the generator and the transfer-switch there would be no drop-voltage on the frame of the generator as it is connected at the source of the power. and since the grounding done at the service would have the same voltage drop for reference. there would be no touch potential on the grounding there.

roger · May 23, 2003

Re: Generator

Brian,

I assume you have some electrical experience

you weren't kidding when you said you were new here.

You are responsible for a data center dealing with millions of dollars a minute or an hour, are you wiling to risk the exposure of a possible fault when the site is connected to genset and utility maybe for peak shaving for 8 or more hours, or maybe for one second, time is irrelevant. Murphy won’t risk this and I ain't about too.

Well this would be stupid without some serious safegarding thru say a UPS.

Roger

hurk27 · May 23, 2003

Re: Generator

Brian

Proper maintenance should be performed on all electrical mechanical systems, losing the neutral would be just as bad as an arcing ground fault from a phase conductor, that burns the ATS to scrap. If one can't be confident of the equipment under their supervision they should contract with someone that is. For systems with ground fault protection an ATS with an overlapping neutral properly maintained and TESTED is the way to go IMHO

First of all how many home owners do you think will properly maintain a back-up generator???
yes they might change the oil ,filters and do small stuff. but to say that statment is wishfull thinking.

many companys wouldnt on a regular basses check the condition of a transfer switch. and I have been called on a few failures of them. luckly it wasn't the neutral that failed.

russellroberts · May 23, 2003

Re: Generator

Bennie and Ed exposed again!

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