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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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    Well if the feed to this dist board is via 32A MCB then your Garage mainswitch is not needed to be more than that is it? You didn`t state what rating of the Garage RCBOs are but unlikely they`ll be more than 32A in total anyway. in fact with discrimination in mind any might be difficult to acheive. Personally I`d prefer the RCBOs to be double pole switching to acheive isolation of both live conductors as it`s a TT anyway (you can get such RCBOs in single module format).Ref the 100mA mainswitch, I`d prefer it to acheive cascading therefore a little resilience against RCD failure rates (note 100mA is not classed as personal protection though and also it should be a Time Delayed one). I`d prefer this 100mA unit to be at the feed end - not just to give the SWA RCD protection but to mitigate against RCD failure caused by "stiction" because it would offer different enviroments to cascaded RCDs. Whilst your concrete base would bring earthy potential into the Garage in reallity it would probably be as effective as an earth rod or plate etc in itself in reality, although yes it should be bonded to the Garage earth, Is there room at the house consumer unit to put a 100mA time delayed RCD?
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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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    There is no room at the house to fit an upstream time delayed RCD,or anything else for that matter. DP rcbo's,are a good option thanks for recommendation, the garage needs 20A for power, and a 6A for lighting.

    Recommendations for dp rcbo's and a 100ma mainswitch in the garage board to protect against rcbo failure sounds like the ideal option.


    However Im most interested about the dp protection on a TT install, is it a requirement that TT installs have DP protection ? Would a mainswitch provide this if single pole rcbo's were ever used? , as a mainswitch is not  a "protective device", its a manually operated isolator..





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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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    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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    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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    If you have an installation with TN-C-S PME earthing and you create a TT installation within it to supply a garage as a island, does the neutral of the supply cable from the house into the garage stop being part of the main house installation?


    Andy Betteridge
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    If you have an installation with TN-C-S PME earthing and you create a TT installation within it to supply a garage as a island, does the neutral of the supply cable from the house into the garage stop being part of the main house installation?

    I'm not sure, I'm correctly anticipating the question behind that, but ...


    The N would need to be treated as part of the TT system once you're in an environment where anyone working on it might be in contact with the TT system's PE conductors rather than the main installation's PE conductors. It's not so much the absolute voltage on the N that's the concern but the voltage difference between N and PE.


    An uncleared L-true Earth fault can result in the N floating a long way from true Earth - on a TN system PE will follow N, on TT it won't.


       - Andy.

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