Grounding branch panels

[daveinca]

Can you tell me the Code for grounding branch panels? This is a data center with 480v-in 120-208 out PDUs which I assume have proper ground-neutral bonding, being a "power source". We are running 60 amp circuits from 2 PDUs to 2 125 amp branch panels, then to individual 20 amp 120v Smart switches which will do a fractional-cycle speed transfer from one input to the other in case of failure. I maintain that the ground and neutral should not be bonded, but I am meeting a lot of "all the other (non-UPS/PDU) panels are bonded" and "they do it that way at the other place". Besides quoting Code, what problems could occur from this incorrect bonding?

 

[websparky]

Re: Grounding branch panels

The 2002 NEC Handbook has this:

These requirements provide for a low-impedance path to ground so that line-to-ground faults on circuits supplied by the transformer result in sufficient current flow to operate the overcurrent devices. These requirements also apply to generators or systems that are derived from converter windings, although these systems do not have the same wide use as separately derived systems that are derived from transformers.

 

[daveinca]

Re: Grounding branch panels

Thanks for the prompt response. Does this mean that ground-neutral bonding at a branch or sub panel is forbidden by Code?

 

[websparky]

Re: Grounding branch panels

Yes!

this connection shall be made at any point on the separately derived system from the source to the first system disconnecting means or overcurrent device, or it shall be made at the source of a separately derived system that has no disconnecting means or overcurrent devices. The point of connection shall be the same as the grounding electrode conductor as required in 250.30(A)(2).