System Grounding

[VTer]

We have primary utility service and a 25kV-480Y/277V outdoor located pad-mounted transformer. The natural X0 bushing is bonded to ground at the transformer. In addition, 480/277V 4W switchgear lineup in the building also has the system bonding jumper between the neutral bus and ground bus. The condition that concerns me is that we have the green equipment grounding conductors connected between the switchgear ground and transformer ground. The equipment grounding conductor will carry portion of the normal neutral current. Is this allowable by NEC in this scenario or is it a code violation? Attached is a simple grounding diagram of the existing setup for your reference

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[dpc]

Unless something has changed in the NEC or its interpretation recently, it has always been my understanding that any two-winding transformer is considered a separately-derived source by the NEC, including grounded wye - grounded wye.

 

[wroggent]

jghrist,

his drawing shows delta-wye.

dpc,

I believe jghrist is correct about the wye-wye - as long as the two neutrals are solidly connected.

From 2014 NEC:

"Exhibit 250.12 A 208Y/120-volt, 3-phase, 4-wire system that has a direct electrical connection of the grounded circuit conductor (neutral) to the generator and is therefore not considered a separately derived system."

"Exhibit 250.13 A 208Y/120-volt, 3-phase, 4-wire system that does not have a direct electrical connection of the grounded circuit conductor (neutral) to the generator and is therefore considered a separately derived system."

Article 100 Definitions "Separately Derived System. An electrical source, other than a service, having no direct connection(s) to circuit conductors of any other electrical source other than those established by grounding and bonding connections. Examples of separately derived systems include generators, batteries, converter wingdings, transformers, and solar photovoltaic systems, provided they have no direct electrical connection to another source. The earth, metal enclosures, metal raceways, and equipment grounding conductors may provide incidental connection between systems. This definition clarifies that those systems can still be considered to be separately derived systems as long as the separately derived systems have no direct electrical connection to service-derived systems. The grounded circuit conductors are not intended to be directly connected."

 

[dpc]

Yep, I've read the definition. My experience has been that these (wye-wye) are treated as separately-derived systems. That could be incorrect, or just the local interpretation. There are some grey areas concerning how the neutrals are electrically connected and if the neutrals are bonded or if the equipment grounding conductors are tied together. At this point, I can't even remember the original question and it was a delta-wye anyway, so I guess it's a moot point. Your two examples are for generators, BTW - not really the same situation.