RCDs in series in domestic house

[Astolat]

Hi folks, just a query about having two RCDs in series. I want to have installed three new external 2 gang 13 amp SSO to IP66.

I currently have a 100 amp BG CCU installed by an electrician in 2016 with split load and 2 BG 63amp 30mA RCDs labelled ICE/EN 61008. The supply is what we used to call PME and I now think you call it TN-C-S.

One of the new SSO would be on a spur off an existing 32amp RFC [ spur length 300mm ] and two of the new SSO would be on a spur off a different 32amp RFC, [ spur length 10m max ]. All cables would be run in 20mm plastic conduit and all would be affixed to external walls of the house.

Before the new CCU was installed in 2016 in 1996 I personally [ before Part P ] installed one external 13 amp SSO to IP66 on external wall to kitchen. This was a spur off the kitchen RFC. This SSO has a built in RCD as at that time there was no RCD protection on the property, and I was going to use it for a lawn mower etc.

Both new spurs will be connected into an existing 13 amp SSO on an RFC which is just the other side of the brick wall. I do not see the need for a fused spur box at the start of the new spurs but would discuss that with my electrician when he turns up.

The load on the new SSOs would be lawnmower, lawn strimmer, power washer, Xmas Lights and electric drill etc. Not all at the same time though.

Should I ask for the 3 new 2g 13 amp SSO to have built in RCDs or should I just ask for plain 2g 13 amp SSO to IP66 without RCD?

I can not see the necessity for having RCD protection on the SSO as I can not imagine for one moment that there will be any discrimination between RCD on the main CCU and RCD in the SSO.

I will run the conduit and fix the boxes to the wall and then get my qualified electrician in to run the cables, test and connect. I can at least do the grunt work while self-isolated [ over 70 ] and save a bit of money.

Many thanks in advance for your help.

Astolat

Retired 73 year old DNO mains engineer.

 

[telectrix]

no need to add additional RCDs to a circuit that is already RCD protected. as for the outside socket with built-in RCD, this can be left (bearing in mind that 1 or both RCDs could trip) or replaced with a non-RCD socket.
and youse same age as me.

 

[DPG]

When you say both new sockets will be connected to an existing socket on the ring - do you mean connected to the same socket on the ring? If so you will need to either extend the ring or use an FCU.

 

[Astolat]

When you say both new sockets will be connected to an existing socket on the ring - do you mean connected to the same socket on the ring? If so you will need to either extend the ring or use an FCU.

Thanks. Looks like a FCU then as plan was cables from existing SSO to new SSO 1 and cable from SSO 1 to SSO 2 but i will leave that to the sparky.
thanks both of you for your help.