Separatly Derived System

[laketime]

I have a customer who had his house wired for a 50 amp back up generator...portable system with a generator inlet receptacle and a generator/main interlock. The generator he bought is configured as a separately derived system; the neutral and ground on the generator are bonded to the frame of the generator. He tried to use it and the GFCI on the generator trips. The manufacturer says the generator inlet has to be supplied from a "switching neutral breaker". No one seems to be able to tell me how this works. I found an Eaton product but the rep couldn't answer the few questions I had. Can the neutral just be pulled off the frame to fix this problem or does anyone have ideas or seen this before? :?

 

[ron]

You need to review the generator installation and operations manual. Those that indicate how to un-bond the neutral from the frame, cane be used in the fashion that you have currently.

If you can't legitimately un-bond the neutral, then you need to switch the neutral at the interlock so that it is a SDS

 

[GoldDigger]

To amplify that explanation a little, the NEC specifies that when power is supplied from a service there must be ONE bond between POCO neutral and the EGC and GES network. (Except for trivial exceptions for adjacent service boxes with non metallic connections between them.)

So there will be a grounded conductor to EGC connection upstream of the transfer switch on the POCO side.
If there is also a grounded conductor to EGC connection at the generator, then the EGC will form a parallel path for neutral current to the generator and will cause a GF device at the generator to trip.

If you cannot remove the bond at the generator, then you have to isolate the grounded conductor from the EGC at the main box, and the way to do that is to interrupt the neutral conductor as well as the hot conductors in the transfer switch so that the generator cannot see the main bonding jumper on the POCO side.

There is one other important option in a neutral-breaking transfer switch: Because of the possibility of equipment damage to line-to-neutral devices if the neutral is opened first, there can be a designed delay between opening the hot conductors and a similar delay between closing the neutral conductor and closing the hot conductors. The result is a slightly longer delay between opening the hot on one side and closing it on the other when transferring.

 

[qcroanoke]

Can the neutral just be pulled off the frame to fix this problem.

Yes. Just remove the bond between neutral and ground.
Might not be to code but it will fix it.

 

[Smart $]

Yes. Just remove the bond between neutral and ground.
Might not be to code but it will fix it.

Doing so is to Code. It is only debatable if the manufacturer's instructions specifically say the consumer cannot... and even then, I'd argue the conflict with 250.6(B).