3 pole vs 4 pole transfer switches and SDS's

[mshields]

I've encountered a system that has 2 transfer switches and I am being asked to specify a third. One of the existing ATS's is a 4 pole and one is a 3 pole. I don't know if there is a neutral to ground bond at the generator. I do know that given the one 3 pole ATS, there should not be. So I'll determine that.

So how many poles should the 3rd switch that I'm specifying be?

I believe the answer is that there is no reason to spend the extra money on a 4th pole when there is one switch that already has an unswitched neutral. It seems to me that the thing to do is to:

a) assure that there is no neutral to ground bond at the generator.
b) provide a 3 pole ATS

I'll have to live with the 4 pole switch that's already there which while incorrect ought not to cause any problems.

Do you agree with my analysis?

 

[augie47]

Are both the existing ATS 3 phase ? (It may be the 3 pole switch is single phase and still switching the neutral)

Off hand, it doesn't seem you can mix the two types. A switched neutral would require a N_G bond (and GES) at the generator which would be a violation on an un-switched neutral ATS.

 

[mh06670]

Red flags: The 4P ATS must have a N-G Bond at the generator to be separately derived systems and proper ground fault protection. Otherwise, the fault current will not travel through the ground and back to the source. The fault would travel back through the service entrance, through the solid neutral and back to the generator. The ground fault detector would not sense any difference in current and the generator breaker would not trip.


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[Russs57]

A question in I may? In SDS systems with neutrals bonded at generators and 4 pole ATS’s, would a 3 pole ATS be allowed when ATS only serves 3 phase motors?

 

[mshields]

An important piece I did not share with you

An important piece I did not share with you

It's a 208V system, so NO GROUND FAULT

How does that change your analysis?

 

[kwired]

A question in I may? In SDS systems with neutrals bonded at generators and 4 pole ATS’s, would a 3 pole ATS be allowed when ATS only serves 3 phase motors?

If you used a 4 pole there is nothing to connect to the fourth pole, other then a grounded conductor that otherwise isn't utilized. IMO 3 pole is fine if there is no neutral load.