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Shared Protective Conductors

jbrameld

We've come across an installation where there are 6 x 10A MCB-protected circuits on a small distribution board.  Each circuit is wired to a 20A switch on a gridswitch plate and then wired out to a single socket outlet for each.

The CPC has been wired as a single run from the DB to the first socket, and is then daisy-chained to subsequent sockets. the CPC is the same size as the line conductor.  All circuits are wired in singles in containment.

Whilst this is clearly not the best way to do it, from my reading of the regs it is actually compliant. 543.1.2 refers to protective conductors common to two or more circuits, and requires that these are calculated or selected to the most onerous/largest of the circuits.  543.1.4 permits selection of the protective conductor size in accordance with table 54.7, which this appears to be compliant with.

So, whilst clearly a bit rough and not best practice, is it actually a non-compliance?

Jason.

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    jbrameld said:
    is it actually a non-compliance?

    Not at all. As Chris P mentioned, shared c.p.c.s are quite common with a range of wiring systems - not just conduit, but metallic trunking too, and often the case where multiple circuits are run in the same multicore cable (think theatre lighting for example - a 21 core cable might have 10 Ls 10 Ns and one c.p.c).

    The only situation I can think of where a shared c.p.c. might not be sensible is where there's a definite requirement to keep one circuit live while work is carried out on the other (perhaps a hospital situation) - you wouldn't want to have the live circuit without a c.p.c. just because some accessory on the other was removed. In most situations having to isolate both circuits together is acceptable.

       - Andy.

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  • 0
    jbrameld said:
    is it actually a non-compliance?

    Not at all. As Chris P mentioned, shared c.p.c.s are quite common with a range of wiring systems - not just conduit, but metallic trunking too, and often the case where multiple circuits are run in the same multicore cable (think theatre lighting for example - a 21 core cable might have 10 Ls 10 Ns and one c.p.c).

    The only situation I can think of where a shared c.p.c. might not be sensible is where there's a definite requirement to keep one circuit live while work is carried out on the other (perhaps a hospital situation) - you wouldn't want to have the live circuit without a c.p.c. just because some accessory on the other was removed. In most situations having to isolate both circuits together is acceptable.

       - Andy.

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