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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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    The bad news is that the 60909 method, which is usually gives results that are close enough for the public supply, does not  work at all for private generators or  especially for UPS/ inverter derived supplies.
    The reason is that that the it is not sensible to build a UPS or inverter capable of delivering many times its normal maximum load just so it can blow a fuse once or twice during its design life.. - I'd suggest that one that can deliver twice the nominal rating for more than a cycle is probably over-sized.

    The solution is either earth fault trips, and not worrying too much about selective tripping on  overloads, or to immediately divide the output of the UPS into several breakers, any one of which will happily instant trip at a current that does not overload the inverter.
    Either approach usually looks a bit odd compared to normal wiring.

    And yes, watch the neutral-earth bond thing. There may not be one, and any that is added externally may need to be switched in on UPS operation in the UK to meet ESQCR . Other places may permit multiple NE links on one system, in the UK we by and large do not, but some big American UPS makers do not realise this.
    Mike,

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  • 0

    The bad news is that the 60909 method, which is usually gives results that are close enough for the public supply, does not  work at all for private generators or  especially for UPS/ inverter derived supplies.
    The reason is that that the it is not sensible to build a UPS or inverter capable of delivering many times its normal maximum load just so it can blow a fuse once or twice during its design life.. - I'd suggest that one that can deliver twice the nominal rating for more than a cycle is probably over-sized.

    The solution is either earth fault trips, and not worrying too much about selective tripping on  overloads, or to immediately divide the output of the UPS into several breakers, any one of which will happily instant trip at a current that does not overload the inverter.
    Either approach usually looks a bit odd compared to normal wiring.

    And yes, watch the neutral-earth bond thing. There may not be one, and any that is added externally may need to be switched in on UPS operation in the UK to meet ESQCR . Other places may permit multiple NE links on one system, in the UK we by and large do not, but some big American UPS makers do not realise this.
    Mike,

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