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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

    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.

    That would imply that the UPS is capable of delivering infinite current (limited only by the downstream wiring) - I think you're going to be a bit disappointed on that score. Constant voltage perhaps, but with the equivalent of a large internal resistance I would have thought (or more likely the UPS's own protection systems would step in).

    Where the UPS can reliably switch to bypass on overload - then it might be a little simpler - mains like when mains is present, but if mains is absent then switching to by-pass effectively provides automatic disconnection. Whether all that happens fast enough to satisfy the requirements for ADS (let alone any hopes of discrimination) would still need to be established though. RCDs (or equivalents) can make the problem a lot easier though.

       - Andy.

Reply
  • 0

    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.

    That would imply that the UPS is capable of delivering infinite current (limited only by the downstream wiring) - I think you're going to be a bit disappointed on that score. Constant voltage perhaps, but with the equivalent of a large internal resistance I would have thought (or more likely the UPS's own protection systems would step in).

    Where the UPS can reliably switch to bypass on overload - then it might be a little simpler - mains like when mains is present, but if mains is absent then switching to by-pass effectively provides automatic disconnection. Whether all that happens fast enough to satisfy the requirements for ADS (let alone any hopes of discrimination) would still need to be established though. RCDs (or equivalents) can make the problem a lot easier though.

       - Andy.

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  • 0 in reply to AJJewsbury

    I split the two scenarios up to try to make it clear that I wasn't claiming the UPS can't deliver infinite current but is limited to it's rated current.

    Up to that limit, if it acts as a constant voltage source then that would imply zero impedance contribution to the circuit

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