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UPS fault protection

What does a UPS use for ADS Trying to get my head round if there is a power failure and a earth fault on the    Load side of the UPS how can we be sure the circuit will disconnect in the required time?
  • If there is a UPS you presumably do not want it to disconnect, after all the U is for uninterruptible ?

    Many smaller UPS make this assumption, and only have death or glory short circuit protection where it keeps going as long as it can until it dies.

    Larger UPS supplying a site installation rather than just critical kit will have an NE bond and RCD or earth fault relays.

    M
  • In most cases with a modest size UPS there is no protection against a live to earth fault whilst running in battery mode. Any inadvertant connection between the nominaly live (bown wire) of the UPS output and earth will tend to tie the brown wire to earth and raise the bue wire to line voltage WRT earth.

    The load will still see normal line voltage and there is very little, if any, risk of dangerous electric shock. Touching the blue wire could be fatal, but it should be insulated and protected against touch to the same extent as the brown wire.


    On a mains system (or a large generator) we go to a lot of trouble to promptly clear live to earth faults to protect human life. On a grid supplied system, during a live to earth fault, any person touching a class one appliance and in contact with true earth will probably be exposed to a shock at about half line voltage. Potentialy fatal hence the need  for prompt dissconection.


    On a modest size UPS system IN MAINS MODE then similar circumstances apply and any live to earth fault should promptly operate the OCPD.

    IN BATTERY MODE the UPS output is generally floating and a fault between either live conductor and earth is unlikely to be detected, or to have any serious consequences AT THE TIME. The OCPD would operate on mains restoration if the fault remained.


    Most small and medium size UPS systems do not have any bond between the neutral output connection and earth whilst in battery mode. Adding any such connection is unwise for several reasons.
  • broadgage:

    In most cases with a modest size UPS there is no protection against a live to earth fault whilst running in battery mode. Any inadvertant connection between the nominaly live (bown wire) of the UPS output and earth will tend to tie the brown wire to earth and raise the bue wire to line voltage WRT earth.

    The load will still see normal line voltage and there is very little, if any, risk of dangerous electric shock. Touching the blue wire could be fatal, but it should be insulated and protected against touch to the same extent as the brown wire.



    That assumption is fine, if the UPS supplies only a single item of equipment that's not interconnected with other pieces of equipment. Unfortunately, fortuitous earthing often makes a mess of that in many ICT installations, for a number of reasons. This can leave exposed-conductive-parts at hazardous voltage in a fault.

    On a mains system (or a large generator) we go to a lot of trouble to promptly clear live to earth faults to protect human life. On a grid supplied system, during a live to earth fault, any person touching a class one appliance and in contact with true earth will probably be exposed to a shock at about half line voltage. Potentialy fatal hence the need  for prompt dissconection.


    On a modest size UPS system IN MAINS MODE then similar circumstances apply and any live to earth fault should promptly operate the OCPD.

    IN BATTERY MODE the UPS output is generally floating and a fault between either live conductor and earth is unlikely to be detected, or to have any serious consequences AT THE TIME. The OCPD would operate on mains restoration if the fault remained.


    Most small and medium size UPS systems do not have any bond between the neutral output connection and earth whilst in battery mode. Adding any such connection is unwise for several reasons.


    Apart from the fact that some UPS assume the N-E connection in the mains is permanent. If you want to use this kind of UPS safely, it really needs its own transformer immediately before the UPS, and/or monitoring for shutdown on neutral loss, otherwise maintenance on the neutral upstream of the UPS is impossible.