RCDs in series in domestic house

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xafepiwe

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Electrical Contractor
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.
 
I'm getting a little lost with all the acronyms. Most I know, but some aren't familiar. Can you spell some of those out?
 
If I understand what you are asking, the short answer is that you are correct: redundant RCDs are unnecessary. A single RCD on a given circuit provides all necessary protection.
 
xafepiwe said:
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.

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.
Click to expand...

I hate to say this but I think you have come to the wrong place to get any good advice on working to British standards.

But even if you were doing a job here the best person to ask would be the qualified electrician that's going to have to hook up the install.
 
Its nice and a honor for you to ask us your questions, but for me its, Huh?. This seems to be a DIY post as they always start with a cheery greeting and a thank you.
And just ask your local qualified electrician.
 
RCD = Residual Current Detector, the IEC version of "Ground Fault", but per NEC rules, what we would call "GFPE" for equipment only. GFCIs that we require for personnel protection must trip at no more than 6mA, they allow 30mA. That's why Growler mentioned that this may be something that would not be best asked in a forum about the US.

SSO = Switched Socket Outlet, another thing that we don't really do much of here; a receptacle with an integral switch dedicated to just it.
TN-C = one of several types of grounding systems used in IEC world, almost all of which are variations of Wye systems. Link
RFC = Ring Final Circuit, again no direct meaning here, but it somewhat similar to an MWBC.
No idea on CCU
 
bwat said:
I'm getting a little lost with all the acronyms. Most I know, but some aren't familiar. Can you spell some of those out?
Click to expand...
I find doing a google search saying >electrical TLA< usually pops up a definition. But TLAs have a bad habit of multiplying!

(Just like drug companies getting away from calling an affliction by its real name, and just using the initials.)
 
Jraef said:
RCD = Residual Current Detector, the IEC version of "Ground Fault", but per NEC rules, what we would call "GFPE" for equipment only. GFCIs that we require for personnel protection must trip at no more than 6mA, they allow 30mA. That's why Growler mentioned that this may be something that would not be best asked in a forum about the US.

SSO = Switched Socket Outlet, another thing that we don't really do much of here; a receptacle with an integral switch dedicated to just it.
TN-C = one of several types of grounding systems used in IEC world, almost all of which are variations of Wye systems. Link
RFC = Ring Final Circuit, again no direct meaning here, but it somewhat similar to an MWBC.
No idea on CCU
Click to expand...
From some searching it appears that:

CCU = Commercial Consumer Unit, which is a distribution panel containing circuit breakers such as RCDs that is suitable for commercial use.
Our friends in the UK should be able to confirm whether this is correct.
 
I still think the original question is “is RCD necessary at a new outlet on an existing circuit that already has RCD protection at the panel?”
Replace RCD with GFCI to equate to a US residential installation and the answer is ‘no’.
 
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