NEC 700.5 (D) and Auto Throw-Over scheme

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jcbabb

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Do any of the loyal readers here have any experience using switchgear utilizing an auto throw-over scheme instead of a separate automatic transfer switch (ATS)?

For instance, say we have a main-tie-main arrangement where the second main is a gen-set. In this case, the tie breaker along with the two main breakers are controlled with PLC (and are integral to the gear), and they take the place of a separate ATS. How can this arrangement be said to meet the requirements of NEC 700.5 (D), which requires separate transfer 'equipment' for emergency loads?

Conversely, what would separate transfer 'equipment' that satisfies 700.5 (D) look like in this situation?
 
I find it hard to be convinced that separate contactors that have the ability to all be closed at the same time but are intended to operate only in transfer mode under the control of a PLC actually give sufficient isolation between generator and grid.
 
View attachment throw-over.pdf

Consider the attached one-line of a piece of integrated switch gear. The main and tie breakers are all motorized; there are no contactors involved. Does this help clarify, or are you still concerned about the separation? I'm not positive that I understand your point.
 
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In addition the what GD states, IIRC, emergency transfer equipment must be listed. Even if there is an "out" to that under Code, I would imagine the transfer switching must be made mechanically impossible to connect to both grid and backup at the same time. Electrically-operated allows automation of the throw over, but cannot be the only means of isolation between the two sources.

JMO
 
Smart $ said:
In addition the what GD states, IIRC, emergency transfer equipment must be listed. Even if there is an "out" to that under Code, I would imagine the transfer switching must be made mechanically impossible to connect to both grid and backup at the same time. Electrically-operated allows automation of the throw over, but cannot be the only means of isolation between the two sources.

JMO
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So, you are saying that the PLC control would not be allowed as an emergency transfer device because it is not a mechanically held closure (and possibly not listed as emergency), right?

I do believe the gen-sets can be sync-op capable, but that was not the design intent. The intent is to have either utility or gen-sets online, not both.

Thanks for your input.

Edited to add: The only emergency loads in this project are a few egress lights, so we may just need to add unit equipment as needed.
 
jcbabb said:
So, you are saying that the PLC control would not be allowed as an emergency transfer device because it is not a mechanically held closure (and possibly not listed as emergency), right?...
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Just an opinion. NEC is not clear on the matter. Would probably have to resort to another NFPA standard and listing standards to verify one way or the other. The AHJ would have to make the call.
 
Is the system an Article 700 or 701 system? 700 requires that the transfer equipment be identified as "emergency transfer equipment" and 701 requires that the equipment be identified for "stand by use" While the code says identified and listed are not identical terms, most AHJs require listed equipment where the code says the equipment is to be "identified".

Now if the system is an Article 702 system, the code just requires that the transfer equipment be "suitable for the intended use".
 
jcbabb said:
This scenario is an Article 702 system, except for a handful of egress lights. The lights are the only Article 700 emergency load.
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So keep it simple and use "unit equipment" for the egress lights....then everything associated with the generator is under 702...much less restrictive.
 
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