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Understanding 560.6.12

Mitchell Jamieson Smith

Hi Everyone,

I am writing to ask if anybody can signpost me to a document that explains the reasoning behind the requirements of 560.6.12, particularly point (i).

The reason I ask is, that based on my current understanding, this requirement - "[a UPS shall be able to] operate distribution circuit protective devices" would mean that a UPS used on a safety service supply would have to sized to blow all of the fuses that it feeds when on battery backup.

However, almost all of the UPSes I have encountered only directly feed one board. This means the UPS has to be sized to be capable of blowing that one single fuse.

But why would that matter? I can see why there could be a problem if the UPS tries to lower the inverter output voltage during a fault, but on battery backup, what would be the difference between trying to blow that single fuse that cuts off the power to the distribution board, and just letting the inverter turn off and try to switch back to maintenance bypass?

Hope that all makes sense! Thanks in advance.

  • 0

    There are some BIG UPSs out there looking after essential services in major infrastructure, that feed a number of distribution boards.

    Notwithstanding that, the UPSs discussed in 560.6.12 don't power a rack of business-critical servers, but the safety services discussed in 560.1.

    The basic idea, though, is if the UPS is sized to operate the distribution circuit protective devices, because of selectivity, a fault of a final circuit supplied by that distribution circuit ought not to be able to deprive other circuits of power.

  • 0 in reply to gkenyon

    Thanks for the answer - appreciated.

    The UPSes I'm looking at do feed safety services, including some large mechanical vent systems.

    I get that the UPS should be able to clear final circuit fuses, and fuses within some parts of the distribution system so that one fault doesn't turn the whole safety system off.

    What I am unsure about, is in a scenario where an industrial UPS feeds a single board, is it still right that the UPS must be sized to clear the fuse between it and that board? The reason I am unsure, is that in this situation, I am not clear on what the functional difference between clearing the fault by blowing the fuse, and clearing the fault by turning off the inverter is.

  • 0 in reply to Mitchell Jamieson Smith

    In practice for that case it may not matter if the fuse blows or the supply collapses but exactly what happens will need to be very well documented with the UPS makers if you are to rely on it. There is a general problem that the fault current from any kind of UPS is not a large multiple of the full load current, as it would be with the mains supply, and the system designer needs to be sure they have considered all the cases. What you do not want is it smoking away at a small multiple (1.5 - 2  times ) the full load current and nothing ever blowing. Earth fault relays are sometimes needed as well, to speed things up a bit.
    Mike.

  • 0 in reply to Mitchell Jamieson Smith
    Mitchell Jamieson Smith said:
    The reason I am unsure, is that in this situation, I am not clear on what the functional difference between clearing the fault by blowing the fuse, and clearing the fault by turning off the inverter is.

    'protective devices' covers a whole host of ills - but usually protection against overcurrent, and protection against electric shock.

    For protection against overcurrent, you have determined that this is a 'fuse' (although let's substitute 'circuit-breaker' in there too) ... but is that really the correct interpretation?

    Now, have a look at Regulation 533.1.1. Unlike the requirements for RCDs, which are prescriptive regarding standards, there is a list of standards, but then the following statement:

    The use of another device is not precluded provided that its time/current characteristics provide a level of protection not less than that given by the devices listed above.

    So, for protection against overcurrent, is a power converter's current-limit precluded from consideration in this regard by BS 7671 ?

    And Regulation 419.2 tells you what to do if you are also using a power converter's current limit for protection against electric shock.


    Going back to Regulation 560.6.12, the requirement you picked out is:

    Where an uninterruptible power supply is used, it shall:
    (i) be able to operate distribution circuit protective devices ...

    So, does this preclude the current-limit from consideration? It certainly doesn't say 'fuses' ?

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