Discuss Garage Consumer Units in the UK Electrical Forum area at ElectriciansForums.net

You will need RCD protection somewhere is virtually every case. If it is fed via SWA cable so no need for protection on the distribution cable then it makes sense to have just a MCB at the house CU and then RCD and any other MCBs at the garage.

However, if you can't be sure of the sub-main installation then having RCD/RCBO at house and then just MCB at the garage is a better choice.

Two 30mA RCD in series without selectivity is poor design, not enough to worry about on an EICR beyond C3, but not something you should aim to do unless you have a very good reason.

Having said that, having MCB in series or RCBO+MCB will have negligible fault selectivity, even if they are OK on overload due to a 1.6:1 or greater difference in rated current. Again, not good design, but usually an acceptable trade-off if the final circuits in the garage are not critical as done in practically every case.
I knew there was an issue with RCDs in same circuit. I did not know MCBs would be an issue and quite surprised. I agree no other way of doing it. I am sourcing a garage unit with 2 MCBs and a 2 pole isolator. This will be run of the main CU which is a dual board with 32A MCB and RCCB type A, thankfully. Thanks for your help. You are very knowledgeable. I see alot of ppl using those plug RCDs when using power tools on circuits with RCBOs or RCCBs. Wonder what potential effect that would have?
 
I see alot of ppl using those plug RCDs when using power tools on circuits with RCBOs or RCCBs. Wonder what potential effect that would have?
It is perfectly safe - at least one of the RCD will trip - it is just inconvenient to have to reset two devices, and a possible hazard if the upstream device also feeds something like stair lights that you don't want failing.
 
Ya that's the main disadvantage of the upstream RCD

Loss of lighting on a socket fault in shed
If its a workshop shed then probably best option is emergency lights, that way any cause of power failure results in some lighting, not darkness and power tools still running!
 

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