SDS or not?

[jaggedben]

The EM transformer secondary conductors terminate at the transfer switch. Without an overcurrent device at the EM transfer switch (unlike the OSS transfer switch) aren’t I stuck having to add the SBJ at the EM transformer and do my grounding there?

You could read it that way. But is the bond at the service really on the 'load side' of the transfer switch? It's sort of off to the side.

I don't think the code section was written with this situation in mind.

I agree that the code requirements are clearer if you switch the neutral.

 

[rojay]

It’s definitely possible that my interpretation is off. That’s the benefit of getting other eyes on stuff- it helps to take my blinders off!

IF I have to treat the EM transformer as a SDS and ground the secondary neutral per all the 250.30(A) guidelines & IF the neutral conductors are solidly connected in the transfer switch, then in normal power mode I have a N-G connection downstream of the service ground. In EM power mode I would have a downstream N-G connection from the SDS…. I think.

I’ve been looking at this way too long!

 

[wwhitney]

If I have followed along correctly, the one line diagram below represents what you have. Each transformer's primary side is towards the generator.

There are 4 places in the diagram where neutral continuity may or may not be maintained between sources: each transfer switch, and each transformer. Maintaining continuity at all 4 locations is not allowed, as that will provide a neutral loop from Service 1 to Service 2. So you must break continuity at at least one of the 4 locations.

If you break continuity at just one of the 4 locations (say both transfer switches have a solid neutral, and one transformer has a primary-secondary neutral connection, one does not), then you have no SDSs. The generator's neutral and the transformer secondary neutrals will each be connected to either Service 1 or Service 2.

If you break continuity at exactly two locations (say at both transformers, which is the usual "SDS" configuration, although that distinction does not apply here), then you will have a section of neutral conductor that is isolated from both services. The system with this isolated neutral will be an SDS (the generator in this example). That section of neutral conductor will need a neutral-ground bond and a GEC.

Cheers, Wayne

 

[rojay]

If I have followed along correctly, the one line diagram below represents what you have. Each transformer's primary side is towards the generator.

Yes, almost. The only difference is that there is a single generator feeder. This feeder runs to a generator distribution panel where OCP is provided for the separate transformer primaries and the split is made. I think the common GDP is a violation of NEC 700.10(B)(5c) too? It seems that there needs to be separation between the EM & OS systems.

Another issue would be figuring out how the ground fault indication gets implemented into this mess. Regardless of whether or not the generator is seen as an SDS or not, I think 700.6(D) still applies here for the 277/480 volt, 1000 amp feeder.

 

[wwhitney]

Yes, almost. The only difference is that there is a single generator feeder. This feeder runs to a generator distribution panel where OCP is provided for the separate transformer primaries and the split is made.

Great. I omitted those details because they don't matter for the SDS question.

As to your other questions about the various Article 700 sections, those are beyond my expertise.

Cheers, Wayne