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Automatic disconnection cannot be achieved in the time required

Julien Leblanc
I was reading art. 411.3.2.6 of BS 7671...

I'm wondering: except for the case where Zs is too high (caused by the length of the circuit, together with the CSA), are there other situations where automatic disconnection times cannot be achieved ?

Maybe some special types of loads or circuits ?
  • 0
    Yes ... for example where only inverters are supplying loads.
  • 0
    One example might be IT equipment (as in data processing equipment) fed from an uninterruptable power supply (UPS) - UPSs can operate in several different modes - such that the loads might be supplied by direct mains at some times, from battery at some others and perhaps mains but via a transformer at others again. As a result it's impossible to guarantee satisfactory Zs at all times (when running from batteries there'll almost certainly never be enough current available even during faults to open overcurrent devices promptly), Even RCDs aren't necessarily helpful as you would need a N-PE link on the output side of the UPS for them to provide ADS when the UPS is running on battery, but in other modes such a link isn't permissible.


    As a result we look for other methods of providing shock protection - sometimes that'll be adding supplementary bonding, other times relying on the supply voltage to collapse to a safe level if there is a L-PE fault, other times ensuring that the system runs as a separated or unearthed (IT earthing) system when ADS isn't operational. Which (or which combination) depends in the fine detail of the circumstances (e,g. the UPS's design).


       - Andy.
  • 0
    Another example where bonding is more important, is in TT installations that can suffer more serious ground rise during faults, not necessarily just faults on the circuit  in question.

    And generator derived supplies which are no able to provide a significant over current.

    There are other cases where there may not be ADS of the conventional kind (protection omitted) for certain safety critical things (sprinkler pumps come to mind as not being the sort of thing you may want on an RCD) and limiting the possible touch voltage is an alternative. I should say that the modern RCD has knocked this into a cocked hat, as in all but the most controlled situations, bonding everything  is not that practical.
  • 0
    Thank you all for your answers.

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