Seperately Derived System

[HEYDOG]

I have a question about transfer of neutral in the transfer switch.
I had another electrician tell me about a recent install his company had done.
A 480 volt 75 Kva generator was relocated from a building to a new location. At the new location the service provided by utility is 120 / 208 volt. They installed the 480 volt generator and fed a step down transformer 120 / 208 volt secondary. From the transformer they installed a piece of greenfield they installed 3 hots, neutral and a equipment grounding conductor to a 3 pole transfer switch. The neutral and grounds are isolated in the transfer switch. The other side of the transfer switch is fed from a feeder with 3 hots, neutral, and equipment grounds. They have the feeder neutral and the secondary transformer neutral connected together in the transfer switch. The equipment grounds from transformer and feeder are connected together in transfer switch. I told them that because of the transformer being installed that it is a seperately derived system and that the neutrals should be switched because of parallel path in the equipment grounding conductors and the neutrals because back at the service equipment they are tied together. To top it off this is feeding I.T. equipment. What do you guys say?

 

[RUWired]

The neutral and grounds are isolated in the transfer switch.
They have the feeder neutral and the secondary transformer neutral connected together in the transfer switch.
The equipment grounds from transformer and feeder are connected together in transfer switch.

What did they do with neutral and grounds in the transformer? If they tied them together, they will have parallel neutral paths. if they seperated them, no parallel paths.
You did not mention an OCPD on the secondary of the transformer. Is there one installed and is there a bonding jumper in there?
Rick

 

[falconelectric]

hello.spanish please.

 

[mikehughes8]

The other side of the transfer switch is fed from a feeder with 3 hots, neutral, and equipment grounds.

I am reading this correctly. The utility supplied XFMR has a seperate equipment ground? The neutral is the equipment ground go from the utility XFMR to main disco.

 

[HEYDOG]

seperately derived system

seperately derived system

I am not concerned about the generator. I am wondering about the transformer that is fed by the generator. I have looked at the definition in article 100 of seperately derived system. I am interpreting that since the secondary is 120/208 and that the grounded conductor is not common to the primary (like a autotransformer) that it is a seperately derived system. Does this mean I have to switch the neutral (grounded conductor) in the transfer switch or do I have the option of wiring it like a generator either seperately derived or not seperately derived depending on the transfer switch ?

 

[RUWired]

Does this mean I have to switch the neutral (grounded conductor) in the transfer switch or do I have the option of wiring it like a generator either seperately derived or not seperately derived depending on the transfer switch ?

HEYDOG, you would be in good standing to talk to the AHJ on you job because there are not any articles writen on when a transformer is not a SDS. Like in 250.35(B), you have direction on sizing the equipment bonding jumper on the line side of the first OCPD. If there was a short article on the subject, i'd say wire it either way, but IMO you would be required to switch the neutral at the ATS as it stands.

Rick