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TT system for garage, RCD question

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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  • 20A power and 6  A lights does sound light enough that  you could have just the RCD, no MCBs  and a fused spur for the lights. I bet you do not need the full 6A for lights either. Note that against a dead short a 13A fuse in a plug, the  20A MCB and the 32A MCB will probably  draw lots for the order they fire in, and may all go  together if the both MCBs have delatched and started to move before the contacts of the fastest one have actually opened wide  enough for the arc to flame out and cut the power - funny things happen  in the time it takes to go zup-click !.
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  • 20A power and 6  A lights does sound light enough that  you could have just the RCD, no MCBs  and a fused spur for the lights. I bet you do not need the full 6A for lights either. Note that against a dead short a 13A fuse in a plug, the  20A MCB and the 32A MCB will probably  draw lots for the order they fire in, and may all go  together if the both MCBs have delatched and started to move before the contacts of the fastest one have actually opened wide  enough for the arc to flame out and cut the power - funny things happen  in the time it takes to go zup-click !.
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