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Fault protection for distribution circuits

Baldyhugh

Hi all

I’d like your opinion on the following setup please. Last year wired a storage outbuilding for a customer comprising lights, sockets, roller door etc. The supply to the whole premises is single phase TT. I ran a supply from the meter position via a 100mA time delayed rcd and 63a mcb to a distribution board in the new building. The customer has since built two extra buildings adjacent to the first building, a mechanics workshop and a storage shed. I have installed two distribution boards in the mechanics workshop to supply it and the storage shed. My question concerns the earth fault protection for the distribution circuits. The building wired last year has a distribution board containing a main switch and rcbo’s. I ran a supply from this board to the first board in the mechanics workshop via a 50a mcb. The board in the workshop then contains rcbo’s for all the final circuits. I then ran a supply from the first board in the mechanics workshop to a second board via a 40a mcb in the first board. The second board contains rcbo’s for all the final circuits. As it stands the earth fault protection for all my distribution circuits is provided by the 100ma time delayed rcd at the meter position. Is this acceptable or should I install an rcbo in the first board to supply the other two boards? This then means that everything in the new buildings is on one 30ma rcbo which isn’t very practical, nuisance tripping etc. is it acceptable for distribution circuits to have fault protection via the main rcd at the meter position?

thanks

Peter

  • 0

    I don't think the regs decree what level of selectivity would be acceptable - so it's more a matter of agreement between yourself as designer and the customer. In one sense what you have is equivalent to having one distribution circuit and one large DB - in the sense that the subsequent DBs are acting as extensions of the first - that might not be unacceptable.

    Getting a 3rd level of discrimination with RCDs gets tricky I suspect - 30mA and then S-type is easy enough, but the next tier would need a longer delay (and higher rating) so you'd be looking at programmable earth leakage relays, which are getting a bit more complicated and more to the point from the customer's point of view, expensive. The other obvious option is to run separate distribution circuits back to the origin for each of the buildings ... again likely expensive. So it's really a customer decision to my mind - how fussed would they be if a single fault took out all three buildings, and how much would they be willing to pay to avoid that.

       - Andy.

  • 0 in reply to AJJewsbury

    Hi Andy

    Thanks for your reply. The second and third buildings were never envisiged when the first building was built so I didn’t have anything to consider regarding these. The supply from the meter position to the first building is buried swa about 130 metres so running two additional supplies back to the meter position for the additional two boards would be impractical and as you mentioned, expensive, however would probably be a better way of doing it as each building would have its own time delayed rcd so a fault on a distribution circuits would only take out one building rather than all three. Couldn’t see my customer agreeing to this however! I was just curious if it was acceptable for the time delay rcd at the meter position to be used as earth  fault protection for further distribution circuits to the additional two boards rather than just the first distribution circuit to the first distribution board as originally designed.

    Thanks again

    Peter

  • 0 in reply to Baldyhugh

    Personally, as a new install, I'd go for 300mA time delay at the source to the first DB. Then feed each seperate DB from a 100 mA time delayed. Of course, all final circuits on 30mA no delay.

    There will then be some discrimination between the DBs, so a fault on one DB shouldn't, (unless a massive fault), cause the 300mA at the source to trip. 

  • 0 in reply to alanblaby
    alanblaby said:
    There will then be some discrimination between the DBs

     This is not guaranteed, but in practice, it might happen for faults less than 1.5 A, especially if the manufacturers of the devices are the same. For any fault that exceeds 1.5 A on a distribution circuit, all bets are off.

  • 0

    The other thing to bear in mind is that downstream RCBOs should ideally be double pole to avoid a final circuit N/E fault pulling the whole show out.

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