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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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    On a single phase TT system as neutral is considered to be a 'live' conductor (I know, it should not be.. ), yes you would normally break L and N double pole, at least as the first RCD. N and your TT earth may not be at the same voltage, especially if there are other loads on the substation that pull the substation N-E connection a bit (remember the electrode at the substaion may be as high as 20 ohms on a hot site, though on a TNC-s street I suggest a substation electride equivalent of a couple or ohms max, more likely  less than an ohm between N and terra-frma is more like it.)

    The recommendations for TT include the case where everyone on a small pole-pig transformer is TT, and I know that is not the case here, but the advice to break the neutral remains.
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    On a single phase TT system as neutral is considered to be a 'live' conductor (I know, it should not be.. ), yes you would normally break L and N double pole, at least as the first RCD. N and your TT earth may not be at the same voltage, especially if there are other loads on the substation that pull the substation N-E connection a bit (remember the electrode at the substaion may be as high as 20 ohms on a hot site, though on a TNC-s street I suggest a substation electride equivalent of a couple or ohms max, more likely  less than an ohm between N and terra-frma is more like it.)

    The recommendations for TT include the case where everyone on a small pole-pig transformer is TT, and I know that is not the case here, but the advice to break the neutral remains.
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