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Demand and Diversity Assessment

Philip Conlan

I have come across an old commercial site with several boards/sub-boards non of which have been considered for selectivity with MCBs or RCDs.

When looking at the demand and diversity from 1 supply nothing seems to add up, or either I'm looking at this all wrong.

DB2 I would assume that the demand after diversity is too great for 1-the 6mm cable CCC and 2-the 32A MCB supply from DB1 is underrated.

if anyone could provide some clarity on this installation that would be helpful.

Currently, I'm working my way through the IET design guide and trying to get a better understanding.

Thanks

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  • 0

    Discrimination is more a nice-to-have rather than a blanket requirement - there are very many cases where it's not achieved - "split" CUs where an earth fault on once circuit will trip the RCCB and so cut the power to half the house - between 13A fuses in FCUs and upstream 32A MCBs for faults - between pitch and caravan RCDs - the list goes on an on. Usually it's a balance between convenience and cost  - ask a customer if they want full discrimination and they'll say yes of course - explain that it'll add an extra £20/way for their 48-way DB and they often change their mind! Not so long ago entire installations, large as well as small, had a single up-front ELCB.

      - Andy.

  • 0 in reply to AJJewsbury
    AJJewsbury said:
    Not so long ago entire installations, large as well as small, had a single up-front ELCB

    Indeed, and not only that, one of the guidance notes a few years back said it in itself did not break that discrimination part either.

    I doubt that opinion would be stated nowadays but it was quite explicit back then and I admit it surprised me a little bit (but not entirely) at the time when the front enders were all the rage.

  • 0 in reply to AJJewsbury

    I think discrimination becomes much more important if the upstream protection isn't easily accessible - e.g. in an industrial estate or flats where upstream is in a locked cabin where it will take two working days for the landlord's managing agent to attend with a key.

  • 0 in reply to wallywombat

    I will certainly ditto that one, wally

    In fact these days I often opt for RCBOs for that reason

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  • 0 in reply to wallywombat

    I will certainly ditto that one, wally

    In fact these days I often opt for RCBOs for that reason

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