Garage supply on old Wylex fuse - should I replace with RCD?

[juux]

I'm replacing a light in my garage and when I went to disconnect the supply from the house CU (an old Wylex fusebox - house is 15 years old) I found it was just a fuse, not an RCD.

After reading up a bit on such things I'm thinking it might be a good idea to replace the old fuse holder with a suitable RCD as they seem to be readily available.

The garage is about 10m away from the house and was built at a later date and consequently has a more modern CU with rail-mount MCBs. It is fed with SWA but does not have a separate rail-mounted RCD, despite supplying a pair of sockets in addition to the fluorescent tube (I'm thinking this is Bad and against current and possibly contemporary building code?).

A few questions arise from this:

A) Is an RCD in the main house CU a good idea, or a necessity?

B) Is an RCD in the garage CU a good idea, or a necessity?

C) I assuming that adding an RCU to either CU would not require any sort of notification or other remedial work?

D) From the admittedly limited information above, is it likely that the original installation complied with building regulations at the time of construction? I only ask as certain aspects of the house seem to have been bodged pretty badly and I'm wondering if this is yet another example!

 

[kkynaston]

Ideally you should have the SWA fed from the main CU from a non-rcd protected circuit. The swa should terminate in the garage cu. The garage should have a 30mA RCD as an incommer protecting all the circuits.
The reason you have the SWA fed from the a non-rcd circuit is to stop the house tripping when there is a fault in the garage

 

[juux]

Thanks, I can see this makes sense in a modern split-load CU but I have an old fusebox-style CU and I was thinking that £7 spent on an RCD for the house CU to replace the fuse would be worth it, even if it is just to avoid the hassle of changing the fuse should it ever go, but also in case of a complete breach of the SWA?

Since there appear to be no RCDs in the circuit at the moment (!), where should they ideally be fitted though? Just at the house CU, the garage CU, or both?

 

[BS3036]

Are we mixing up RCD and MCB?

 

[juux]

I hope not!

The house CU only has old-style fuses of varying ratings. The garage CU has a main switch and a 16A MCB for the sockets and a 6A MCB for the strip light - nothing else.

 

[juux]

Bump. Anyone want to advise on the best course of action here?

 

[davelx]

Ideally in the garage only. You do not need two RCDs in the same circuit - you cannot tell which will trip - or even both, and it may not be convenient to have to go back to the house in the event of a trip.

So - fuse in house protects cable to garage, RCD and MCBs in garage protect circuits in garage.

Also, you would not replace the fuse in the house with an RCD anyway, as this does not provide overload or short circuit protection - you would need both an RCD AND a fuse or MCB.