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 Post subject: ATS faults
PostPosted: Tue Feb 22, 2011 2:44 pm 

Joined: Thu Oct 14, 2010 8:04 am
Posts: 11
Hi,

I'm using easy power to model my arc flash scenarios. Came across a questions:

Is it reasonable to account for a fault in an Automatic Transfer Switch, with 1 utility source and 1 stand-by generator under the following condition: ATS is on Emergency Generator, Genset is running, ATS senses acceptable power from the utility source again and looks to switch over to normal...a fault happens during the switch over and the generator and utility source and the load are all faulted together...

I ask because Easypower can't model (to my knowledge) a fault that takes in both contributions to the ATS, just one at a time. If i choose to explain this event in my report easypower won't beable to print a equipment duty report for the ATS in this event.

Any help would be appreciated. Thank you everyone.

HarcFlash


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PostPosted: Tue Feb 22, 2011 7:40 pm 
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Joined: Wed May 07, 2008 5:00 pm
Posts: 862
Location: Rutland, VT
I would have to give this some more thought but it would be interesting to see what other responses are as I have not modeled that. I think that from an arc flash hazard, what are the chances that someone is "interacting with the equipment" at the precise moment that your scenario happens. I suppose it is possible that someone is right in front of it when this happens.

From a modeling point in EasyPower, you may be able to approach it this way:
1. Create a scenario
2. Change the ATS to a regular bus
3. In the Short Circuit Options, Arc Flash, change Calculate Arc Flash to use Integrated method.

Using the Integrated method will take into account the decreasing currents from multiple sources as their respective protective devices trip out.

I can't run this model right now, so if you try it, please post if it worked.

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Barry Donovan, P.E.
www.workplacesafetysolutions.com


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PostPosted: Wed Feb 23, 2011 10:41 am 
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Joined: Thu Jan 10, 2008 8:49 pm
Posts: 510
Location: New England
For a practical solution, forbid all live work when on generator and state that in your site policy.

Most ATS's are break-before-make, so electrically you can't have both sides joined. Your scenerio would be an arc flash, the creates a conductive plasma, that bridges the other source. I would rate that as statistically low probability.

I don't know Easypower that well, but the recommendation to treat it as a bus with two feeds, one from utility and one from generator, sounds right.


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PostPosted: Wed Feb 23, 2011 11:43 am 
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Joined: Mon Jan 18, 2010 11:35 am
Posts: 550
Location: Wisconsin
An interesting question, which I have never considered. I would not be surprised if statistically the most likely time a fault, inside of an ATS, will occur is while the contacts are transferring state.

Forbidding work 'while on generator' is a nice concept, but it is not always possible. There are many locations, like hospitals and water treatment facilities, that are designed to function entirely off of generator power for many days or even weeks.


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PostPosted: Wed Feb 23, 2011 11:49 am 
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Joined: Mon Jan 18, 2010 11:35 am
Posts: 550
Location: Wisconsin
oops, duplicate post


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PostPosted: Wed Feb 23, 2011 12:10 pm 

Joined: Thu Oct 14, 2010 8:04 am
Posts: 11
thank you for your help, seems to be very unlikely so i won't include it.


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PostPosted: Sat Apr 02, 2011 12:10 pm 

Joined: Wed Apr 21, 2010 9:02 pm
Posts: 2
Just consider worst case and move on

Hi,
We have dealt with this before. You can (as others have stated I guess) model the ATS as a bus fed by two disconnect switches, and run a scenario with both switches closed and both sources in service, print your label, and consider it done if the HRC is acceptable. It's the path of least liability. Also, you may want to consider that the expected failure rate of an ATS is much higher than that of a simple disconnect switch (per one of the IEEE colored books, don't remember which one at this time, and it makes sense anyway).

Some ATS units I have seen (e.g. a GE model we have) have terminals very close together for both sources' and load's terminations so there would likely be bridging across if an arc started close enough to these terminations. If results are too high you can put an upstream disconnect with fuses on the upstream source with the higher contribution or change upstream breaker settings or whatever (or on both sources if the expense is justifiable--lots of variables).


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PostPosted: Sat Apr 02, 2011 1:58 pm 

Joined: Mon Jan 26, 2009 5:00 pm
Posts: 46
Location: Midwest
I agree it's possible for an arc to parallel both sources if available. If this results in an unacceptable condition, than work should be prohibited unless one source is isolated and label for the worst single source scenario.


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