Taking a spur off a spur: can it be done safely?

[SarahLund]

Hi there, I wanted to add an extra (2-gang) socket via a spur off the ring main, but on inspection it turns out that the last (also 2-gang) socket is actually a spur itself.

Given where the wires run, this is the only socket I could feasibly take a spur from - except that you are not allowed / supposed to take a spur off a spur... So my question is:

- Could I replace the existing (spur) socket with either a 2-gang RCD socket or something like this MK fused socket:

http://www.gil-lec.co.uk/mk-logic-k2737-3-gang-switched-socket-dual-earth

...and then take an additional spur from there?

- Alternatively, could I make the spur-on-spur arrangement safe by introducing a FCU at some point? If so, where would it go?

Thanks, and apologies if this is really obvious.

SL

 

[deadshort]

Hi, FCU >socket 1 > socket 2 total load on all sockets 13a.

This is safe and compliant -but remember 13a max on all sockets


Regards,

DS

 

[EFLImpudence]

- Alternatively, could I make the spur-on-spur arrangement safe by introducing a FCU at some point? If so, where would it go?

Yes that's the only thing you can do.

You must place the FCU before the first socket.

 

[SarahLund]

Brilliant, thanks so much for the quick replies. So 13A fuse in the FCU and 2 x 2-gang sockets. Sorted.

 

[333rocky333]

you sure it is a ring

 

[SarahLund]

Pretty sure. I haven't run a continuity test but the circuit was installed as part of an extension around a year ago. What would the implications be if it turned out to be a radial circuit? You can still run spurs off radials, right?

 

[JohnW2]

Pretty sure. I haven't run a continuity test but the circuit was installed as part of an extension around a year ago. What would the implications be if it turned out to be a radial circuit? You can still run spurs off radials, right?

It wouldn't be a problem. In fact, IF it were a radial, and IF the cable size were appropriate to the size of fuse/MCB protecting the circuit (usually 2.5mm² cable for a 20/25A MCB, 4mm² cable for a 32A MCB) then, IF you used the same size cable for your extra socket, you wouldn't need any FCU. [ BTW, branches from radials are usually just called "branches", rather than spurs (some people call them spurs if they use smaller cable that the rest of the radial circuit) ].

Kind Regards, John

 

[PrenticeBoyofDerry]

- Alternatively, could I make the spur-on-spur arrangement safe by introducing an FCU at some point?

That is the only way to comply to the requirements, when adding multi-spurred socket on a ring final.
The FCU should be at the point where the ring final socket is broken into, so all spurred sockets are protected 13A.

Such as the arrangement posted by deadshort below:

FCU >socket 1 > socket 2 total load on all sockets 13a.

This is safe and compliant -but remember 13a max on all sockets

Also to add new sockets and newly buried cable requires RCD protection if not already present and protecting that part of the circuit.

So configuration would be either:

RCD>Ring>FCU>newly buried cable/spurred sockets.
or
Ring>RCD protected FCU>newly buried cable/spurred sockets.
RCD/FCU

The method you posted of adding the three gang fused socket, then adding a spur to that, will not protect the cable against overload, so is not advised and was not RCD protected.

 

[JohnW2]

The method you posted of adding the three gang fused socket, then adding a spur to that, will not protect the cable against overload, so is not advised ...

I agree that that approach is not acceptable, but not for the reason you give. IF we are dealing with a ring, then IF it were possible to connect a further socket to the downstream side of the fuse in the 3-gang socket, then I would see no real problem with that - it would simply be a fused spur, with the fuse in a 3-gang socket, rather than an FCU. The cable upstream of the fuse could not suffer from overload, and has just as much fault protection as would a compliant unfused spur supplying one socket. However, I've never seen a 3-gang socket that would (certainly not without bodging) allow one to connect a further socket to the downstream side of the fuse - have you?

... and was not RCD protected.

Although it's obviously the case that any new sockets or buried wiring need to be RCD protected, no-one has told us or suggested that that the circuit isn't RCD protected. Given that the sockets circuits is said to have been installed about a year ago, it certainly should already be RCD protected.

Kind Regards, John