Type AC RCD upstream of Type A

[OldKettle]

Hi,
I am planning to install an outdoor socket as a spur from an existing socket on a 16A radial circuit.

When initially looking at the work, I realised that the circuit is not RCD protected.

Because I wanted to keep the internal socket, I decided that I would buy a FCU with RCD, put that upstream of the internal socket and then have the outdoor RCD socket after that (please see the picture at the bottom). I thought that would be good because it would RCD protect the internal socket, and also RCD protect the cable run from the internal socket to the outdoor socket. This is the one I purchased.

SafetySure RCD Spur White

• Passive (does not need resetting after a power loss)• Conforms to BS7288:1990, BS1363

www.toolstation.com

I have now realised that you are not supposed to put a Type A RCD downstream of a Type AC and, as far as I can tell, the outdoor socket is Type A.

I've struggled to confirm that the outdoor socket is Type A, but I did find this listing for it

2 Gang 13A Double Pole Switched Weatherproof Residual Current Device Sentry Socket Outlet (Passive) IP66 Grey

Part of Masterseal Plus range

www.cef.co.uk

which states "A.C and pulsating - D.C fault current sensitive" and I think that suggests that it is Type A.

Because of this, I think that I can do one of two things.

Alternative 1 - Branch the outdoor socket off upstream of the FCU, so that its Type AC is not upstream of the Type A socket.
Alternative 2 - Get rid of the internal socket, forget the FCU and just go CU -> Outdoor Socket
Alternative 3 - The same as Alternative 1, but losing the FCU and just using an RCD protected internal socket

Both of these alternatives would have to forego RCD protection on the cable to the outdoor socket.

I would prefer to keep the internal socket, and so would favour Alternative 1, or 3.

Any advice on this situation and specifically whether Alternative 1 is OK, would be much appreciated.

Also, could anybody please advise if an RCD socket also provides RCD protection for downstream sockets?

In the circuit CU -> RCD Socket -> Non-RCD Socket, is the Non-RCD socket protected by the RCD on the upstream socket?

If so, would the following fall foul of the same mixing RCD type problem

CU -> Type AC RCD Socket -> Type A RCD Socket (by putting Type AC protection upstream of Type A)?

 

[Murdochcat]

There is absolutely no advantage of putting 2 RCDs in series

 

[ericmark]

There is no point having a 30 mA feeding a 30 mA, as to type AC, A, F, B and S, the problem with type AC is DC can freeze their operation, so the big question is there any items which could cause DC?

I find it a problem as we have no idea what may be plugged in, and even type A is only good for 6 mA which is next to nothing. So unless there is some device which will trip if it detects over 6 mA DC it seems a bit pointless.

With a TT supply we rely on the RCD, but with TN it is secondary protection, so with a TN I would not really worry unless it supplies an EV charging point or solar panels.

But at cost of a RCBO v RCD FCU, I would fit in the CU, and one has little option since type B not avaiable as single width RCBO's, so as long as RCD protected I would not worry unless feeding an EV charging point, and if that is the case there is a lot more involved like loss of PEN.

 

[OldKettle]

Thanks for all of the advice.

The outdoor socket will be used for power tools (including lawnmower, strimmer, mitre saw, plunge saw and various others). It will also potentially be used to power portable lights. The only other thing that it is likely to be used for is a bench top power supply which I sometimes use to work on projects in that location.

The internal socket may be used for networking equipment and PC equipment, any potentially for other tools.

There is absolutely no advantage of putting 2 RCDs in series

Is this because the upstream RCD socket protects the downstream RCD socket?

The problem I have is that I want to protect the upstream socket, but have already bought an RCD version of the downstream (outdoor) socket.

If the upstream RCD socket protects the downstream socket, could I go CU->Outdoor RCD socket->Indoor Non-RCD Socket, and have the Indoor socket protected by the outdoor socket's RCD?

That sounds a strange solution, but I am planning to put the indoor and outdoor sockets back-to-back on the wall, so could go outside and back in with not much more trouble.

Failing that, the simplest solution (aside from an RCBO on the CU) would appear to be to add a branch, with one leg going to the external socket, and a parallel branch going to an RCD protected internal socket.

 

[Murdochcat]

In think you are going about this incorrectly

You are modifying a circuit, so you have to make the whole circuit compliant to the latest version of the regs.

This means rcd protection at the board for the whole circuit

 

[Risteard]

Type AC RCDs should be illegal.

 

[flameport]

The problem I have is that I want to protect the upstream socket, but have already bought an RCD version of the downstream (outdoor) socket.

You have purchased the wrong items.
An FCU with an RCD only protects items connected to the load side of the FCU. That Toolstation effort is obsolete and entirely unsuitable.
An RCD socket only protects items plugged in to the socket.

The RCD should be in the consumer unit which in anything semi-recent is installing an RCBO.
If RCBOs for the consumer unit do not exist, then the solution is an RCD in an enclosure next to it.

 

[OldKettle]

You have purchased the wrong items.
An FCU with an RCD only protects items connected to the load side of the FCU. That Toolstation effort is obsolete and entirely unsuitable.
An RCD socket only protects items plugged in to the socket.

The RCD should be in the consumer unit which in anything semi-recent is installing an RCBO.
If RCBOs for the consumer unit do not exist, then the solution is an RCD in an enclosure next to it.

Yes, but I bought the FCU with the original intention of CU->FCU->Internal Socket->External Socket

Which should have been fine except the FCU is Type AC and the External Socket is A.

I never expect that the RCD would protect the supply side.

I've decided that, for now, I am going to simply remove the internal socket and just install the Type A external socket.

I would prefer RCBOs on the CU, but getting electricians in my area interested in small jobs is very difficult. I couldn't even get one interested in a new kitchen circuit. The only one that did turn up to quote was not fully qualified (could not yet sign off their own work), took a look at the CU and declared that the cooker circuit was under RCD (which it isn't).

If I do reinstate the internal socket, I will just add an appropriate RCD socket.

 

[ebee]

With ref only to the question about two RCDs operating in series then for discrimination could be an issue but sometimes it is not actually a great problem. If one RCD fails (questionably up to 7% failure risk - I`m not suggesting that using 2 of them might result in a 0.49% risk but hey it might reduce it from 7%) .
So , is discrimination a problem in your case?
Yes they really should be type A not type AC (however there are many type AC in existence and might often work well in actual practice) .
I`d rather have a circuit with a (or part of the circuit with a) type AC than none at all, however at my age I have in the past wired several installations with rewireable fuses and no RCD at all. I don`t lose much sleep over it. No piles of dead bodies littering the streets.
In short, put the whole circuit(s) on type A RCD and if it makes you happy then add a type A or type AC or none at all to that extra socket.

 

[ericmark]

Is the RCD primary or secondary protection? Or asking another way is the installation TT or TN? If TN no real worries, if TT then all circuits should be RCD protected.

 

[ebee]

Well if TT it would a least be near impossible to expect a disconnect without an RCD so you might well be relying on its 7% failure rate whereas in a TN system it`s additional protection. in a TT system I prefer two RCDs even if one is 100mA time delayed

PS (I would actually prefer two 30mA RCDs in tandem and say on adjacent shared walls between two rooms = different environments providing discrimination issues are not really much of a problem, some might prefer them to be different makes too.)