Garage/Outbuilding electrical installation

I'm not clear why you think you need to change anything at all - is there a specific problem?


You do not say how your house is earthed - I assume some DNO supplied earth terminal, so the MCBs covering the alarms and so on would operate OK with an fault to earth. IF they were installed today those would probably be RCBO (*) not MCB

However, if the garage supply requires RCD cover or not rather depends on the cable arrangements. Cables like SWA with an earthed armour, or an external equivalent, like earthed metal trunking or conduit over a soft skinned cable, do not need RCD protection. The assumption is that any misplaced drill or pick axe or whatever, will be earthed by the armour, or the conduit, and when it reaches the live core there will be a flash and a pop, but the  fuse or breaker will be enough  to disconnect, as the prospective fault current will be many hundreds of amps.

In your case it sounds like  it is an armoured cable, on a supply with a hard wired earth, so no RCD is needed at the origin.

Note that if any part of the run was in twin and earth (or any similar cable without an earthed armour) in a route that is not either obvious (from the surface), or more than 50mm from the wall or floor finish, so serious tools wouldlbe needed to damage it by mistake, then it would need RCD protection after all..

In the garage itself you may prefer there to be the RCD there, rather than run back to the house if it goes dark.

If you want to add RCD cover to the wiring in the garage itself, (perhaps that is soft skinned cable ?)  then changing the local CUs main switch for an RCD may be easiest.

You do not need to make the garages into a TT island, unless you have things in the garage that require it - a car charging point or socket for  a  caravan would be classics, or indeed if you had a ham radio station you might like it, if antennas are the kind that are outdoors and need a large earth electrode (VHF ones that look like TV antennas do not normally).

If your garage had running water coming in metal pipes then a 10mm bond is needed back to the house if you are using the house earth, but you have that anyway.


regards mike.


* An RCBO, combines RCD and MCB functions, and senses overloads as well as earth faults- an RCD does not sense overload, just L-N imbalance to detect earth faults, and needs further MCBs or fuses to handle the overload case.


PS a standard 40A RCD is still looking for a 30mA difference current, so unless it is s time delay type, it will race with any 30mA RCD further downstream so either or both may trip - to discriminate reliably you need special time delay RCDs, - usually set up in cascades in factors of 3

A possible example of a robust system where faults trip out only the minimum of circuits,  may be a 300mA RCD with 1/3 second delay, feeding a few branches each with 100mA RCD with 0.1sec delay ('S type') feeding final circuits each with instant 30mA types for personal protection. Normally in domestic, it is not worth that level of complexity.

The only thing I think I can add to Mike's is to note that the latest regulations do ask for 30mA RCD protection for domestic lighting circuits (in addition to the usual requirement for soft-sheathed cables concealed in walls, serving bathrooms etc) - so that does sound to be one point where the current arrangement falls slightly short. It is quite a minor deviation however and wouldn't normally require fixing unless you were making a change to the circuit - it wouldn't result in anything more than a C3 on an EICR for example.


  - Andy.