fault current on output of a UPS

[mshields]

Fod an on line double converstion UPS with no tranformers involved what effect does the unit have on fault current.

i.e. If you had 50,000 AIC at the input of a 480 to 480Y/277V with no isolation transformer, what would be the fault current at the output terminals of the UPS?

thanks,

Mike

 

[mayanees]

UPS Rated output

UPS Rated output

I believe the output fault current equals the UPS rated output.
JM

 

[weressl]

Fod an on line double converstion UPS with no tranformers involved what effect does the unit have on fault current.

i.e. If you had 50,000 AIC at the input of a 480 to 480Y/277V with no isolation transformer, what would be the fault current at the output terminals of the UPS?

thanks,

Mike

It's an old problem. It depends.:grin:

480 input and 480Y/277V output(?) indicates a transformer to me somewhere.

UPS's commonly have a 150% overload capability for 5 sec or less htough the normal supply mode.

In case of high current, whether it is overload or short-circuit, the UPS will go on bypass and feeds raw, but acceptable power through the bypass. It is up to the characteristics of the bypass control of what will that limit be and if the bypass components have any current limiting characteristics. Mind you that when the bypass is activated, a static switch will transfer the short circuit load from the 'normal' ups circuit to the bypass and that the static switch has limited throughput, but when the bypass contactor takes over form the static switch, pretty much the bypass short circuit duty is available on the output.

The above is one of the reasons why fuses are the recommended as main and individual circuit protective devices on the load side of an UPS because they are much faster than CB's.

 

[ron]

Worst (highest) case is in bypass. Always use bypass if doing Short Circuit Calcs.
Least SCC is dependant on the manufacturer but generally 3-4 times FLA.

 

[ron]

Lazlo,
This may be a personal preference, but I could come up with several instances of fault current (lower than a fuse's CL range) in which CB's are faster than fuses.