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#59243 - 11/30/05 06:50 PM Grounding/Bonding Generator
bot540 Offline
Member

Registered: 09/14/04
Posts: 177
Loc: Vernon Hills, IL
Is it common practice to drive a ground rod and bond it to the case for a stand-by generator? Does it matter if the generator is considered a seperately derived system? For it to qualify as seperately derived the neutral has to be switched in the transfer along with the ungrounded conductors right?
Another thing I don't understand is that this transfer switch I received from Owens Cummings has the neutral lugs bolted,bonded to the case. Three of them for the service,load,and generator. If my understanding is correct, since this isn't considered a seperately derived system because the neutral isn't swiched shouldn't this neutral bar be isolated from the ground? If this is a violation where would I find it in article 250?
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#59244 - 11/30/05 07:41 PM Re: Grounding/Bonding Generator
JoeTestingEngr Offline
Member

Registered: 11/04/05
Posts: 786
Loc: Chicago, Il.
That doesn't sound too good to me. It would make all the conduit between the ATS/MTS and the main panel a return current carrying conductor. I deal with alot of emergency generator callouts. We deal with two ground concepts, earth ground and structure ground. If the generator is placed anywhere someone could possibly touch it and our structure, I'll bond to structure. Otherwise, I bond to the panel where we hook up the generator. The generator neutral is referenced to the ground that the load is normally referenced to. I wouldn't use that ATS/MTS unless I could isolate the neutral lugs from the case.
Joe

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#59245 - 11/30/05 08:27 PM Re: Grounding/Bonding Generator
Bob Offline
Member

Registered: 02/05/02
Posts: 182
Loc: Mobile, AL, USA
Is it common practice to drive a ground rod and bond it to the case for a stand-by generator? It depends on your next question.

Does it matter if the generator is considered a seperately derived system? Yes
If it is a SDS then the neutral and frame are bonded and a ground rod is installed and bonded to the frame.

For it to qualify as seperately derived the neutral has to be switched in the transfer along with the ungrounded conductors right?
That is correct

If my understanding is correct, since this isn't considered a seperately derived system because the neutral isn't swiched shouldn't this neutral bar be isolated from the ground? Yes

If the neutral is NOT switched the system neutral is connected directly to the generator neutral treminal.

The system EGC is connected to the generator frame and remove the bonding jumper from neutral to frame. You may install a ground rod and bond it to the gen frame. It is not required.

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#59246 - 11/30/05 10:33 PM Re: Grounding/Bonding Generator
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9045
Loc: Estero,Fl,usa
The objective is to only have one bonding jumper in the system at a time. If the neutral is bonded in the generator you have to switch out the service bonding jumper with the transfer equipment. (a separately derived system)
The other method is to keep the neutral unbonded in the generator and solidly connect the neutral in the transfer equipment, only switching the phases.
I am not sure why you would ever create a neutral bond in transfer equipment.

BTW I think most portable generators do bond the neutral but Honda usually doesn't.
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#59247 - 12/02/05 12:06 PM Re: Grounding/Bonding Generator
JJM Offline
Member

Registered: 10/31/05
Posts: 102
Is the ATS you're referring to the Onan RSZ series? Although the RSZ is essentially a GE ZTX, Onan/Cummins doesn't offer a switched neutral (or aux LV contacts for that matter) like GE does. This is why I went for the "GE version" of the switch:



I posted some pics of my genny install in a previous post:

http://electrical-contractor.net/ubb/Forum1/HTML/002476.html

I just didn't want to deal with an inspector or the POCO claiming the generator was still "interconnected" with an unswitched neutral, so I switched the neutral to avoid the possibility of such dispute.

Since this ATS (and the Onan RSZ) is not service entrance rated, a service disconnect will be needed. This is where I bonded the neutral. At the 60A generator disco, the neutral is also bonded (without this, when the ATS switches to the generator, there would be no neutral bond). But keep in mind the main building ground point is still at the 200A disco.

If I'm not mistaken, some generator manufacturers, like Generac, specify a seperate ground rod for the generator. I don't have my code book in front me, but I believe not having a single building ground point violates code. My concern would be paralell paths, particularly if EMT, IMC, or RMC or even metal flex is used for conduits. Accordingly, I would never use a seperate genny ground.

I believe a system can still be defined as seperately derived, even if it uses a common ground.

Joe

[This message has been edited by JJM (edited 12-02-2005).]

[This message has been edited by JJM (edited 12-02-2005).]

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#59248 - 12/02/05 12:15 PM Re: Grounding/Bonding Generator
SJT Offline
Member

Registered: 07/24/02
Posts: 242
Loc: PATCHOGUE, N.Y.
Hello Bob,
I see your input about the Generator set up, and would the ground rod be just for a ground to direct a lighting hit? We have some Generators that are remotely away from the Bldg. they serve. I wonder if a ground rod would be good for lightning, when connected to the frame and the Equipment ground?

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