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TT system for garage, RCD question

Horace Horace
Hello, not used this forum in a while and have had to re-register. I hope someone can help. I have an upcoming job to replace an existing small consumer unit in a garage about 30m from the house. The supply at  the house is PME, and the current set up is a 4mm SWA fed from an MCB in the house CU,  I recommended a switch fuse but the customer does not want this as it means destroying the wooden cabinet the existing CU is in. So the feed to the garage will be from a 32A mcb..Ive specified an earth rod at the garage as its concrete floor.is damp and dont want to export the PME. I will earth the outer armoured wire via the gland at the house end only, not at the garage end, and provide a TT earth system to the new garage CU.


My question is regarding rcd's ? The 2 way garage CU will have x2 30mA  rcbo's for the outgoing circuits so then is there really a need for a 100ma main switch ? as it would serve no purpose. So to reiterate, the supply is SWA, protected by mcb at the house..If I use a 63A/100A mainswitch at the garage CU, will this be okay as it provides manual dp isolation., I want to reuse to existing garage CU and its metal.


The last time I installed an earth rod was over 20 years ago so want to
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    Wylex (and so schneider) and crabtree do double pole RCBOs in a  compact 'MCB format, and maybe others do by now as well.

    I will note that there are plenty of concrete floored garages with PME supplies, so unless there are complicating circumstances like an electric car, water or the floor is unusually conductive, TT may not be essential (though I tend to say do it when in doubt)

    You do need to take care that there is no credible fault that means  a pre-RCD Live might get to the case of the CU that is only TT earthed, so if anything an RCD as incomer in the garage CU might be preferred so the 'tails' are as short as possible. Or if it is only lights and sockets, then do you need a garage CU at all,  would a 32A radial on an RCBO from the house be OK, and use a fused spur as the light switch ?


    I'd not worry too much about discrimination between RCDs in the same box or in locations that are close, (not quite relevant here, but situations like the caravan pitch where there is one in the caravan and one on the post  beside it are OK) than ones that require a hundred yard trudge along a dark driveway to see which one has tripped.
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    Wylex (and so schneider) and crabtree do double pole RCBOs in a  compact 'MCB format, and maybe others do by now as well.

    I will note that there are plenty of concrete floored garages with PME supplies, so unless there are complicating circumstances like an electric car, water or the floor is unusually conductive, TT may not be essential (though I tend to say do it when in doubt)

    You do need to take care that there is no credible fault that means  a pre-RCD Live might get to the case of the CU that is only TT earthed, so if anything an RCD as incomer in the garage CU might be preferred so the 'tails' are as short as possible. Or if it is only lights and sockets, then do you need a garage CU at all,  would a 32A radial on an RCBO from the house be OK, and use a fused spur as the light switch ?


    I'd not worry too much about discrimination between RCDs in the same box or in locations that are close, (not quite relevant here, but situations like the caravan pitch where there is one in the caravan and one on the post  beside it are OK) than ones that require a hundred yard trudge along a dark driveway to see which one has tripped.
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