Which RCD for Garage Consumer Unit, mains coming from house

[Taylortwocities]

Discrimination between MCBs is quite hard to achieve. A ratio of 2:1 is often bandied about. There are proper tables provided by manufacturers of stuff.

An easier way is to have a FUSE at the source end feeding the downstream MCB.

But you are designing it, so you obviously know what you are doing.

 

[JohnW2]

NO! You must have discrimination between up and downstream devices. The downstream must be quite a lot smaller that the supply end, otherwise in the event of a fault the house MCB will trip.

I wouldn't say that there is really any "must" about it. Unnecessarily having two OPDs in series which won't 'discriminate' is silly, and a potential inconvenience, but that would seem to be about all.

If there were a "must", we would have somewhat of a problem, in that, as we all know, a 13A BS1362 fuse will often tonot discriminate from a B32 MCB (let alone a B20) and, perhaps 'worse', a 13A BS1362 fuse in a plug certainly will not, if plugged into a socket which is a 'fused spur', discriminate against the identical 13A BS1362 fuse in the FCU (not to mention 3A fuses for fans fed by B6 MCBs ) - so, if discrimination between OPDs were a "must", many of the common aspects of electrical installations would 'not be allowed'

Kind Regards, John
Edit: typo corrected

 

[Squiral1]

I'll look at various options and decide which is best to go down. I could fuse switch it, that is an option.
The cave CU is there if needed.

 

[Taylortwocities]