UPS Short Circuit Contribution - For Arc Flash Study

[confused-engineer81]

My question is, do you include the UPS as an actual UPS or do you treat it as in bypass mode?

Or, put another way. Do you include the UPS 3 phase and single phase percent contribution and specify how long it lasts or do you just assume this will by bypassed by the static internal bypass? Have you seen this being an issue that would drastically change the incident energy values?

I prefer to assume UPS in bypass for simplicity when doing arc flash studies, but I would love your thoughts.

Also if you have whitepaper references that i could use to educate myself that would be amazing.

 

[zbang]

Generally, the UPS will electronically switch to the static bypass (if present) when it hits it's own overcurrent.

 

[wbdvt]

Typically if you look at the specs on a UPS it will shutdown very quickly on relatively low fault current. Usually fault current of 120% of full load and in several cycles to protect the expensive electronics. Since the bypass will provide the worst case short circuit current, I always model UPS as being bypassed.

 

[ron]

Agreed with zbang.
UPS will go to static bypass when a short occurs, so we just model in bypass.

 

[xptpcrewx]

My question is, do you include the UPS as an actual UPS or do you treat it as in bypass mode?

Or, put another way. Do you include the UPS 3 phase and single phase percent contribution and specify how long it lasts or do you just assume this will by bypassed by the static internal bypass? Have you seen this being an issue that would drastically change the incident energy values?

I prefer to assume UPS in bypass for simplicity when doing arc flash studies, but I would love your thoughts.

Also if you have whitepaper references that i could use to educate myself that would be amazing.

Anytime you perform a short-circuit or arc-flash hazard analysis, you should consider all possible switching states for the system. It’s not immediately apparent sometimes which switching configuration will give you worst case results. For one, the conditions that produce a worst case short-circuit may not be the same conditions that produce a worst-case incident energy exposure. Once you figure out all the applicable switching states of the system, you should model these switching configurations under minimum and maximum fault generation conditions. I realize this makes matters more complex, but this is what the ANSI and IEEE standards require.

Going back to your question. If supported, I usually model the UPS under 3 switching states:

(i) UPS/Backup mode operation,
(ii) Bypass mode operation, and
(iii) Momentary Closed-Transition operation (as a flash-over condition for the bypass breaker cubicle/cabinet only)

These are analyzed under minimum and maximum fault generation conditions so basically 6 arc-flash scenarios total...


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