UPS - Short circuit Current

[NickParker]

My client's spec for UPS states the below,

"The UPS shall be able to sustain overloads as follows:
150% load for 1 minute
125% load for 10 minutes
110% load for 1 hour."

While the overloading at 125% for 10 minutes and 150% for 1 minute, could be achieved by the venodors, but 100% overloading for 1 hour could not achieved, so vendors offer the next higher rating or the oversized UPS.

This raises the question that why would some one specify the overloading at 100% for 1 hour? I heard that this is to increase the short circuit current from the UPS?

Why to increase the short circuit current from the UPS?

 

[RRaghunath]

NickParker, what you wrote is 110% load for 1 hour and that corresponds to 10% overload (and not 100% overload). Isn't it!
10% overload for 1 hour is not abnormal.
Wonder why the vendor is proposing higher rated UPS?

 

[JezNZ]

It is not an abnormal specification, but yes it does usually result in de-rating (oversizing) the UPS.

As for short-circuit currents, this is a sticky issue that UPS normally normally requires to go into bypass to supply from the source to provide enough fault current to trip the downstream protection in a fault. In battery mode the UPS is often unable to supply enough fault current.

Every project I have worked on (not as the designer) has effectively ignored this issue.