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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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    However Im most interested about the dp protection on a TT install, is it a requirement that TT installs have DP protection ?

    BS 7671 has different requirements for "isolation" and disconnection by ADS. Isolation on TT systems requires the N to be broken in the same way as the line conductors, but for ADS only the line conductors need to be interrupted. So single pole switching RCBOs or MCBs are acceptable for ADS, if you can use a DP switch disconnector or upstream RCCB for isolation.


    Only if service conditions require that each circuit has individual isolation might you need DP switching MCBs or RCBOs.


    That said, RCDs won't discriminate on N-PE faults if the downstream devices don't open N as well - so DP RCBOs certainly have their merits if there's another RCD upstream - not to mention making testing and fault finding much much easier.


    Also +1 for mapj1's point of ensuring double/reinforced insulation before the first tier of RCDs - something that's sometimes been overlooked with the rush for metal CUs - but a fault from a pre-RCD L to the TT'd earthing system would be extremely dangerous.


      - Andy.
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    However Im most interested about the dp protection on a TT install, is it a requirement that TT installs have DP protection ?

    BS 7671 has different requirements for "isolation" and disconnection by ADS. Isolation on TT systems requires the N to be broken in the same way as the line conductors, but for ADS only the line conductors need to be interrupted. So single pole switching RCBOs or MCBs are acceptable for ADS, if you can use a DP switch disconnector or upstream RCCB for isolation.


    Only if service conditions require that each circuit has individual isolation might you need DP switching MCBs or RCBOs.


    That said, RCDs won't discriminate on N-PE faults if the downstream devices don't open N as well - so DP RCBOs certainly have their merits if there's another RCD upstream - not to mention making testing and fault finding much much easier.


    Also +1 for mapj1's point of ensuring double/reinforced insulation before the first tier of RCDs - something that's sometimes been overlooked with the rush for metal CUs - but a fault from a pre-RCD L to the TT'd earthing system would be extremely dangerous.


      - Andy.
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