Grounding Transformer Secondaries

[SK13]

A 480vac, 3 phase circuit, consisting of three ungrounded conductors and an equipment grounding conductor, is run to supply a machine in an industrial plant.
At the machine, the circuit is tapped onto to supply a small single phase transformer with a 120vac secondary. This transformer supplies a duplex receptacle for limited loads (e.g. radio, fan.) The transfomer is a two winding transformer, so there is no direct electrical connection between the primary and secondary (it is not an autotransformer.) The secondary circuit is protected by a fuse in one conductor.

I contend that this secondary circuit fits the definition of a "seperately derived system" and that the conductor that is not fused needs to be grounded.
Further, if the transformer is 1000VA, or less, the secondary can be considered to be a class 1 control circuit and it is sufficient to bond the grounded conductor to the metal frame or enclosure of the transformer. The equipment grounding conductor of the secondary circuit is also connectecd to the transformer frame or enclosure.
However, if the transformer is greater than 1000VA a seperate grounding electrode conductor must be run to a suitable grounding electrode (e.g. nearby metal water pipe, effectively grounded building steel, etc.) The equipment grounding conductor of the transformer's supply circuit may not be used to ground the grounded conductor of the transformer secondary.

Is this a correct interpretation of the code rules?

 

[don_resqcapt19]

You are correct.

The equipment grounding conductor of the transformer's supply circuit may not be used to ground the grounded conductor of the transformer secondary.

If you install the primary EGC in a manner that meets all of the rules for the EGC and the grounding electrode conductor, then there is no reason that this conductor cannot do both jobs.
Don

 

[dbuckley]

Surely this would depend on if the transformer is part of the machine supply, in which case I fully agree it's a SDS with all the baggage that entails, or the transformer and the receps are part of the machine, in which case the NEC SDS rules don't apply...?

 

[jim dungar]

Surely this would depend on if the transformer is part of the machine supply, in which case I fully agree it's a SDS with all the baggage that entails, or the transformer and the receps are part of the machine, in which case the NEC SDS rules don't apply...?

What standards are you using for your machine? UL508A and NFPA79 are two of the most popluar standards used for machine control panels and they both contain langauge similar to that in the NEC.

 

[SK13]

One of the requirements for the installation of a grounding electrode conductor is that it be continuous from the grounded conductor to the grounding electrode. (250.30(A5) & 250.64(C), 2005NEC).
I don't expect that many equipment grounding conductors going to a piece of equipment in an industrial facility, far removed from the service entrance are contiuous.

 

[benaround]

SK13,

I don't think it's a SDS, Def. of Premises Wiring(System) and Def. of SDS.