APC UPS Ground (No, not the shipping service!)

[Jacob S]

Hello Everyone,
I have a question I know someone here will be able to answer. I have an APC rackmount UPS (MN: SUA1000RM2U). Tonight, I decided to put my fluke 87 on its output to check its voltage voltage when I noticed a potential design flaw. When the UPS is running on normal power, I get 120v between hot and neutral and 120v between the hot and EGC, but when the UPS is running in battery mode, I only read 58v between the hot and EGC.

From my current knowledge of electrical, I can't figure out how this UPS could safely clear a fault between the hot and the EGC in battery mode.

Am I missing something, or is this a design flaw?

Thanks!
Jacob

 

[dbuckley]

The Real Deal with UPSs is how much current they can generate into a bolted fault whilst running off batteries. Some UPSs supply this data, others dont. It's normally stated at something like 500% for 5 cycles. So, your 10A UPS can supply 50A for 1/12th of a second before collapsing. Will the 10A MCB open in that time at that overload? I don't know, check your curves. But after 1/12th of a second, the UPS has gone into shutdown anyway...

If the mains is present, and a fault occurs, the UPS will go into bypass and let utility power clear the fault.

UPSs are quite some way from the infinite grid.

 

[Jacob S]

dbuckley,
Thanks! That makes perfect sense!
Jacob

 

[ELA]

This seems to imply that the UPS output is isolated from ground.
The 58 volts is probably not "real" if measured under load (or with a wiggy) it would most likely go to zero.

So ... could there even be a fault current to ground when running on Battery?

Seems to me that the output breaker is only intended for a hot to neutral fault since the output neutral is not bonded to ground.