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UPS Fault Level Calculation

Semaj

Does anyone know if there is a standardised way of calculating fault levels at the output of a large UPS like IEC60909 does for transformer and generator supplied faults?

I imagine things will change between manufacturers but a good rule of thumb would be very useful as it seems that no matter the installation, downstream of the UPS the breaker settings have been poorly thought out and my concern is that these downstream circuits will not actually acheive ADS due to the fault level being too low to trip the downstream breaker because the trip settings are too high.

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  • 0

    Thanks for the great replies. 

    I never considered looking at the UPS as just an inverter output and as a constant voltage source. I was thinking more about the energy limiting effect of draining the capacitors in the DC stage and then the inverter adding some unknown impedance to that and generally overcomplicating things. 

    So it seems then by considering the UPS as a constant voltage source we have two scenarios to consider. 

    1) small / downstream final circuits where the fault current is going to be limited by the circuit impedance to well below the UPS max output

    2) large circuits / submains where the fault current required by the OPD is greater than the UPS output  

    In the first scenario we would calculate the fault loop impedance going back to the UPS but would consider the UPS itself to have zero impedance. The worst case lowest fault current will actually be in bypass because of the contribution of the upstream impedances (although made more complicated because the source voltage might be higher than the UPS output voltage setting!) 

    In the second scenario the fault will draw more current than the UPS can deliver and it will switch to bypass and we are back to just calculating the loop impedance like usual back to the transformer.

    We therefore really only need to consider the protection settings for when the system is on bypass.

    Does this make sense or have I missed something?

  • 0 in reply to Semaj

    What if it cant switch to by-pass ... ie. the mains has actually failed & its running as a 'true' UPS powered by its own batteries?

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  • 0 in reply to Semaj

    What if it cant switch to by-pass ... ie. the mains has actually failed & its running as a 'true' UPS powered by its own batteries?

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