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UPS input and output SC current

UPS input and output SC current

UPS input and output SC current

(OP)
Hi Guys,
Running a short circuit on UPS system, I noticed short circuit current in UPS output is much less than SC current in input.
Am I missing something or it is always the case?


RE: UPS input and output SC current

The UPS often has an internal bypass transformer, the impedance of which limits the fault current available at the output. The inverter won't sustain short-circuit current for very long before it shuts down to protect itself, normally initiating a trip to the bypass supply.

The supply cables to the UPS will act to drop the available fault current quite quickly. The transformer supplying the board must be a decent size or abnormally low impedance - it's unusual to see a fault level greater than 40kA from an 800A service, that is more like the fault level you'd expect on the LV terminals of a transformer of around 2MVA.

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