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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?

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  • RCDs in series wont reliably discriminate unless the upstream RCD is of a time delayed type.

    Agreed - but I would add that the upstream RCD must also have a much higher residual current rating (e.g. min 3x the downstream rating) and be of a compatible type (e.g. a A type upstream of a AC type can't be expected to discriminate half-wave (rectified) residual currents).

       - Andy.

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  • RCDs in series wont reliably discriminate unless the upstream RCD is of a time delayed type.

    Agreed - but I would add that the upstream RCD must also have a much higher residual current rating (e.g. min 3x the downstream rating) and be of a compatible type (e.g. a A type upstream of a AC type can't be expected to discriminate half-wave (rectified) residual currents).

       - Andy.

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