Two gensets, three transfer switches

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Cold Fusion

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way north
I have seen what happens when both the utility and the generator thinks its supplying the load. Well, I waw it once the "discussion" had. The switchgear was the loser in this discussion. Arc F'd. But not arc flash. A big mess. ...
That is an example of poor design and poorly selected equipment, and maybe even poor commissioning. An 150kva/208V gen can only put out 2ka - 3ka SSC. An 208V, 800A, 5%Z utility can deliver maybe 16ka. I would hope that the designer could deal with that pretty easily.

Normally one uses a second check relay in addition to the dead buss relay. So it takes two failures to close into a live buss out of sync. But it can happen. The equipment has to handle it with no extraneous energy escape. There may be a damaged CB, but usually that is considered acceptable after two failures.

cf
 
A couple issues.

A couple issues.

I have had problems with the AHJ and natural gas generators which were to be used on life safety systems since the gas can shut down in disaster situations making it useless. Also "Listed" is the key word. Whatever you intend to do and however you would like to do it make sure the equipment is "Listed" for that purpose. If you are backing up any fire pumps or smoke control systems be especially cautious because these can have extra requirements with the electrical, fire and building inspectors. Sometimes a pre meeting to discuss your plan saves a load of headaches during inspections.
 

steve66

Senior Member
Normally one uses a second check relay in addition to the dead buss relay. So it takes two failures to close into a live buss out of sync. But it can happen. The equipment has to handle it with no extraneous energy escape. There may be a damaged CB, but usually that is considered acceptable after two failures.

cf[/quote]

So were up to two motorized breakers with aux contacts tied to a PLC, a second check relay, a UPS, and we haven't even talked about the two circuits to detect normal and emergency voltage (6 circuits if its 3 phase). And we haven't talked about programming the PLC either.

There might be a cost advantage for material, but the labor to install all that has to be a killer.

And I have to think the total reliability of this has got to be pretty low. Not a major problem for residential, but I wouldn't consider this for anyone who is really counting having the genset pick up in 10 seconds.
 

steve66

Senior Member
That is an example of poor design and poorly selected equipment, and maybe even poor commissioning. An 150kva/208V gen can only put out 2ka - 3ka SSC. An 208V, 800A, 5%Z utility can deliver maybe 16ka. I would hope that the designer could deal with that pretty easily.


cf
Circuit breakers might be able to save themselves, the gear, and the wiring, but I'm not sure I would expect the genset to survive a 16KA fault, especially if it were running at the time.
 
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