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Selectivity/ Discrimination Comercial site

Philip Conlan

I'm wondering if could get a clear explanation or guidance on selectivity on an commercial/ industrial installation.

For example I have drawn out part of an installation below to help illustrate.

Is there a set process for determining if all MCB / RCD protection is adequate when moving from submain to submain?

Parents

  • RCDs in series wont reliably discriminate unless the upstream RCD is of a time delayed type. Try to avoid more than two RCDs in series and make certain that the upstream device is a time delayed type.

    MCBs in series wont reliably discriminate on short circuits, only on overloads. Avoid MCBs in series and use fuses for protecting upstream sub mains with MCBs reserved for final subcircuits..

    BS88 fuses in series will usually discriminate if of different ratings, and will almost always discriminate if TWO standard sizes apart.

    32 amp and 40 amp BS88 fuses in series will probably discriminate, 32 amp and 50 amp will almost certainly discriminate.

Reply

  • RCDs in series wont reliably discriminate unless the upstream RCD is of a time delayed type. Try to avoid more than two RCDs in series and make certain that the upstream device is a time delayed type.

    MCBs in series wont reliably discriminate on short circuits, only on overloads. Avoid MCBs in series and use fuses for protecting upstream sub mains with MCBs reserved for final subcircuits..

    BS88 fuses in series will usually discriminate if of different ratings, and will almost always discriminate if TWO standard sizes apart.

    32 amp and 40 amp BS88 fuses in series will probably discriminate, 32 amp and 50 amp will almost certainly discriminate.

Children
  • in reply to broadgage

    A very clear response thank you!

  • in reply to broadgage
    broadgage said:
    MCBs in series wont reliably discriminate on short circuits, only on overloads.

    Yes, if it's the mythical zero resistance fault. However, if it's at the business end of a hand tool, you might stand a chance.

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