Vehicle mounted generator?

[jasons24]

Let me start with the application.

I have a 220KW 480/277V wye generator mounted on a truck. From the connection point on the generator there is a 400A fused discfonnect. From the fused disconnect there is a 400A pin and sleeve receptacle. This generator supplies another piece of equipment that is also on a truck. Between the generator and the equipment is a 4 wire rubber cord. The generator housing is grounded to the frame that the generator sets on, the disconnect and the receptacle is also grounded to the same point. The equipment is not using a neutral connection at all.

My question is this: Does the neutral bussing (center tap of the generator) have to be grounded to the frame of the genset?

I have my own idea of the answer after much studying of the NEC and several grounding references. One generator manufacturer said that it does not need to be connected while another one said that it did?

I am needing some very good explanations as to why or why not and I will be in the discussion as it progresses.

Thanks in advance for your help.

Jason Skelton

 

[don_resqcapt19]

The code does not require that the neutral of the generator be bonded to anything for this application. See 250.20(B)(2).
Don

 

[jasons24]

I forgot an item that may be of importance, because the equipment being used is moving constantly(underground drilling machine) there is no grounding electrode other than the frame of the vehicle 250.34 (B)

The code does not require that the neutral of the generator be bonded to anything for this application. See 250.20(B)(2).
Don

 

[dbuckley]

Although you say "the generator housing is grounded to the frame that the generator sets on, the disconnect and the receptacle is also grounded to the same point" I respectfully disagree - they are bonded and thus equipotential, but electrically separate from the source of supply.

You can do this either way, the question is about the behavior you want under fault conditions. With no neutral connection, there is no ground reference to the bonded metalwork, so a phase to metal short will result in the entire bonded metallic collection of both trucks being at the same potential as a phase, as they are connected. But potential only exists in the context of difference, and there is no reference point. So no-one dies, and no fuses or breakers pop.

However, if a single fault becomes a double fault to metalwork, then you will get breakers popping, as fault current has a path.

Or you could have a ground fault detector between genset neutral and metalwork, that (shunt) operates the breaker, or just lights up an indicator, as in a typical "high resistance ground" installation, so no-one dies, maybe no breakers pop, but you get indication of a fault.

If you do connect genny neutral to the bonded metalwork, then everything is grounded in the conventional sense of the term as applied to a SDS system, so a single fault will pop breakers and stop the machine.

I think (and I'm on much more unsteady ground here in terms of code) you can also connect one of the phases to your metalwork, giving the corner ground thing like a delta transformer, but that would be in my opinion an unusual arrangement on a portable generator, based on the confusion that this circuit arrangement causes.