Portable generator question revisited

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mark32

mark32

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Currently in NJ
I can't find the thread, but someone asked when using a manual transfer switch that does not switch the neutral, the generator's neutral and equipment ground should be isolated. Now, what happens when, under generator power, you have a line to ground short. The fault travels to the generator's frame and then what happens? As I see it there's no path into the generator itself in order to trip it's OCPD.
 
I don't agree on isolating the two on a portable generator.

First, as you said, it would prevent a fault from opening the OCPD.

Second, all I have seen have the frame bonded to the neutral from the factory and would require some dissasembly to change. I don't think that portable generators were tested and thus listed to be taken apart and re-wired in the field.

In practice, people buy a generator and a transfer switch and hook them up. The vast majority of residential switches don't switch the neutral and are connected to generators with the bonding jumper installed. Just what problems have occurred due to this practice?

I know of none. But I am here to learn.
 
In the OPs setup, there should be a N-G bond in the service entrance. If the TS does not switch the neutral (and it never switches the ground), then that bond is always in the circuit, so it's still the fault current path when the generator is feeding power.

The only reason I can think of to switch the neutral is to effectively switch which N-G bond is being used (service or generator).

If you do not switch the neutral and have bonds at both the service and the generator, you create parallel paths for both fault current (not so bad), and neutral current (bad).
 
Here is a great PDF from Cummins that shows why the bond should NOT be present if not switched, but I think it is more about fixed systems as opposed to portables.

http://www.cumminspower.com/www/literature/technicalpapers/PT-6006-GroundingAC-2-en.pdf

I guess that means we have to only install transfer switches that do switch the neutral?

Must be, because I don't think that opening up a portable generator and modifying it is the correct manner to deal with the issue. Especially, since once it's disconnected from the transfer switch it has to have the bonding jumper in place.
 
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zbang said:
In the OPs setup, there should be a N-G bond in the service entrance. If the TS does not switch the neutral (and it never switches the ground), then that bond is always in the circuit, so it's still the fault current path when the generator is feeding power.

The only reason I can think of to switch the neutral is to effectively switch which N-G bond is being used (service or generator).

If you do not switch the neutral and have bonds at both the service and the generator, you create parallel paths for both fault current (not so bad), and neutral current (bad).
Click to expand...

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Like zbang said, a neutral bond exists but enabling the generator's ocpd to open would depend on how the TS and the circuits it's feeding are wired. Correct me if I'm wrong, but one could either take the circuits (To be powered by the generator) completely out of the main panel and install them into the TS, or one could extend the intended circuit's ungrounded conductors into the TS and leave their grounds and neutrals in the main. One would then run a properly sized neutral and EGC over to the TS. In the latter case I can see the generator's breaker opening but not in the former case, thus, it would be beneficial to wire such a setup in the latter's manner. Yes?
 
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