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 Post subject: 275kVA UPS
PostPosted: Thu Mar 10, 2011 9:25 am 

Joined: Wed Nov 12, 2008 10:26 am
Posts: 46
Location: CA
Two questions:
1. Should a 275kVA, 480v UPS be considered as a source of arc flash?
2. If you've modeled a UPS with SKM software, were you able to get the needed the necessary component data from the UPS vendor?


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PostPosted: Sat Mar 12, 2011 1:47 pm 

Joined: Mon Jan 26, 2009 5:00 pm
Posts: 46
Location: Midwest
The UPS itself has limited fault clearing capability. This should be available from the manufacturer. Generally, they can handle an overload condition for up to a few minutes.

For a fault, 300-500% rated current is typical, but this is going to cause static switch and automatic bypass transfer sequence within subcycle timeframe.

When I model a UPS in SKM, the built in UPS model doesn't seem useful other than for the limited fault current the modules can produce. I take this out to the full two seconds as one scenario.

I run additional scenarios for utility and generator sources, ignoring the UPS and just modelling the system in bypass.

This can become cumbersome for a system with multiple, redundant UPS modules, multiple generators.


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PostPosted: Tue Mar 15, 2011 9:37 am 

Joined: Wed Nov 12, 2008 10:26 am
Posts: 46
Location: CA
Ex twidget,
Thanks for your reply.
Do you think an arc flash could occur at the output of this UPS? Or, are its internal current limits going to keep the arc from forming?

The UPS is an EATON 9395. I've asked the EATON UPS rep to contact EATON's Arc Flash experts and ask for their analysis. EATON has a great AF website.
http://www.eaton.com/Electrical/USA/MarketSolutions/Safety/index.htm


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PostPosted: Thu Mar 17, 2011 11:41 am 

Joined: Mon Jan 26, 2009 5:00 pm
Posts: 46
Location: Midwest
The UPS may or may not be able to sustain the arc flash. The inverters will have a high impedance that limit the amount of current that the unit can produce. This is independent of the batteries or utility source that is supplying it.

it's possible that a very large UPS can generate enough fault current to sustain an arcing fault without going to bypass. SKM allows you to model the UPS like a utility source, with fault current as a percentage of full load output. 300-500% is typical here.

Once the UPS detects a transient exceeding an overload level, it will fire the static switch to clear the fault, then go to bypass. Once this happens, utility or generator power is bypassing the UPS.

That's why I run it both ways, UPS only scenarios, and bypass only scenarios. Both seem plausible.


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