Separately Derived System-Generator

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infinity

infinity

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We have a large Generator (1250KW) that feeds a large switchboard. From the switchboard there are feeders that feed various ATS's through out the building. By design some of these ATS's switch the neutral conductor, some do not. Would this still be considered an SDS if some of the ATS's did not switch the neutral?
 
As I see it whether you call it an SDS or not you either have illegal redundant ground neutral bonds or you have some circuits downstream of the ATSs that interrupt the neutral that do not have a ground neutral bond at all.
Is it really necessary to interrupt the neutral at those ATSs?
 
The system was designed by someone who chose both neutral switching and non-switching transfer switches. The argument now is does the generator switchboard require a connection to the building GES via a GEC and a bonding jumper when some of the transfer switches have solid neutrals.
 
infinity said:
The system was designed by someone who chose both neutral switching and non-switching transfer switches. The argument now is does the generator switchboard require a connection to the building GES via a GEC and a bonding jumper when some of the transfer switches have solid neutrals.
Click to expand...
Having just one non-switched grounded-neutral ATS makes the generator non SDS, period.
 
Smart $ said:
Having just one non-switched grounded-neutral ATS makes the generator non SDS, period.
Click to expand...

That was my argument and the reason that a GEC and bonding jumper are not required at the generator switchboard but I was trying to find a concrete code answer to back it up. :D
 
From your description, the generator has the neutral bonded since some of the ATS's switch the neutral. If that is the case then any of the ATS's that don't switch the neutral have parallel paths that are creating objectionable currents. I am curious as to who signed off on this, was there an actual engineer involved?
 
The generator neutral is not factory bonded, it feeds a switchboard where they now want a bonding jumper is installed along with a GEC to the building GES.
 
I think you Code answer lies simply in 250.24(A)(5).
If any one of the ATS does not switch the neutral, then if you have a Generator N-G bond, which you would have for a SDS, then you violated 250.24(A)(5).
I agree with eliminating the switched neutrals and not having a N-G bond at the switchboard.
 
Smart $ said:
FWIW, switched neutrals are permitted in a non-SDS system. See 404.2(B) Exception.
Click to expand...

Good point ! I never thought of that in regard to an ATS, but there it is :D

so the short, to the point, answer is, leave the bond jumper out.
 
infinity said:
The system was designed by someone who chose both neutral switching and non-switching transfer switches. The argument now is does the generator switchboard require a connection to the building GES via a GEC and a bonding jumper when some of the transfer switches have solid neutrals.
Click to expand...

Following the others, with different phrasing:
It doesn't matter. The design/installation is screwed up. Installaing additonal GEC/bonding jumpers does not change that.

Install additional - system screwed up
Don't install additional - system screwed up

I suspect this is one of those where, "it is what it is" no one will pony up the money to fix. So the job is to justify - code interpretation; or come up with a low budget fix that allows management to say, "it is okay now".

Assuming 480V grounded Wye: With just the generator driving this, the system is 1500FLA and up in the 10KA - 15KA ASSC. That is absolutely classic for slow trpping CBs - maxium arcflash, maxium equipment damage. Once you start putting your name on the system changes, you are the engineer of record.

I wish I had some bright ideas for you - I don't.

Just curious: Is there an N-G bond at the generator?

edit to add: This is way above a design by code requirements.

ice
 
Smart $ said:
FWIW, switched neutrals are permitted in a non-SDS system. See 404.2(B) Exception.
Click to expand...

So in essence if the system can be setup as a non-SDS in a code compliant manner, meaning that several transfer switches that do in fact switch the neutrals would be permitted as long as there is one ATS that does not switch the neutral.
 
infinity said:
So in essence if the system can be setup as a non-SDS in a code compliant manner, meaning that several transfer switches that do in fact switch the neutrals would be permitted as long as there is one ATS that does not switch the neutral.
Click to expand...
That is correct. Not saying I recommend it... just saying such a setup is not a violation by itself.
 
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