Overlapping Neutral ATS

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ron

ron

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We had a project where the utility service switch had ground fault protection as it was 480/277V and high current. The 702 generator was specified as a SDS (with GFP) and the open transition transfer switch was 4 pole. This would allow the GFP to function properly.
The ATS showed up from ASCO with overlapping neutral contacts, to maintain the neutral connection momentarily (in parallel) while the phases transfered, then the neutral disconnects from the "other" source and remains only connected to the phases and neutral of the "preferred source"
http://www.ascopower.com/
What are your thoughts regarding the momentary parallel connection of the neutral and thus the upstream n-g bond for each SDS. Does this violate Article 250 since there are now parallel n-g bonds.

I think it is acceptable in the same way a closed transition switch momentarily violates the interlock requirements found in 700, 701 and 702.
 
ron said:
We had a project where the utility service switch had ground fault protection as it was 480/277V and high current. The 702 generator was specified as a SDS (with GFP) and the open transition transfer switch was 4 pole. This would allow the GFP to function properly.
The ATS showed up from ASCO with overlapping neutral contacts, to maintain the neutral connection momentarily (in parallel) while the phases transfered, then the neutral disconnects from the "other" source and remains only connected to the phases and neutral of the "preferred source"
http://www.ascopower.com/
What are your thoughts regarding the momentary parallel connection of the neutral and thus the upstream n-g bond for each SDS. Does this violate Article 250 since there are now parallel n-g bonds.

I think it is acceptable in the same way a closed transition switch momentarily violates the interlock requirements found in 700, 701 and 702.
Click to expand...


I think it was Iwire that posted this link but I'm not sure. It has some good reading on switched neutral and overlapping neutral. The website was pretty slow loading this morning.

Hope this helps,

GMc
 
Also, it looks like you have to register now. They must have just changed that recently.

GMc
 
GMc ,
Thanks.
I'm familiar with the article and understand the benefits of the solutions identified.
I've used overlapping neutral style 4 pole switches plenty of times, but this particular concept of it possibly being a violation of 250 is swimming around in my head recently so I was interested in other opinions about the concept possibly being a violation.
Most other ATS manufacturers don't offer the overlapping neutral option for an open transition switch, so maybe that says something.
 
Ron,

I will be interested to hear what others have to say. We have both overlapping and switched neutral here and I have been wondering why.

GMc
 
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Ron:

I think the short, temporary paralleling of Neutral and Ground is OK.

Is there any application where you could use this transfer switch and it would NOT cause a temporary paralleling of neutral and ground? I don't think so. So doesn't that mean ASCO and UL are both approving the the temporary double N-G bond?

Steve
 
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