Final ring circuit or radial or both Ask questions and find answers to many subjects relating to electrics and electrical work

[Lawrence]

I need to install two RCD fused spurs to run 300W appliances.

I've opened up all the plug sockets on the first floor of the property I am working in (a fairly old cottage) and all the sockets all have just one set of wires to them.

The house has been partly rewired at some time and there is a modern consumer unit. There is a 32A MCB controlling the sockets to the first floor which is definitely a final ring circuit i.e. two sets of cables terminating at the MCB

Does this mean that the first floor sockets are a radial spur off the final ring circuit?

Am I still okay to spur off the first floor circuits to connect the RCDs, especially given the low rating of the appliances and the fact there appears to be little else on the circuit?

[jackhawk]

Hi Lawrence,
Can you confirm your first floor and ground floor sockets?
Does the property have ring up and ring down?
Is there an RCD fitted to the fuse board on the socket side?
Is the property TT TNS TNCS.
The more you put in the better you get back.
Regards
Jackhawk

[sparx]

Hi Lawrence,
first is it a proper ring? just because there's 2 cables at MCB they may be not joined any where, you need to test for circuit, ie remove one set from MCB make off into term. blocks and see if when mcb on that you get a live/neutral back to term blocks using mains tester,[ thats the quick way without proper test gear,]
Even if it is a ring you must assume that each socket with single cable is already a spur & you are only permitted one outlet point from each spur, unless... if you can trace and prove a socket is the only one on its spur, then a way out is to reroute the cable via a Fused Connection Unit [fused spur] BEFORE the socket & then you can loop onto more sockets [ or RCD/fused spurs] as total load on each spur can only be 13A max.
hope this helps, regards SPARX

[Lawrence]

thanks Sparx. how can I prove each socket is the only one on the spur without ripping out the entire floor?

[Lawrence]

thanks Jackhawk

the earthing is TNCS and there is no RCD
there are separate rings up and down

[sparx]

Hi agn Lawrence,
only way is to go to nearby skts and look for ones with 3 cables in, doing one at a time, disconnect the lives safely and put in temp. term blocks apart from each other, and find out any skts. now dead which must be a spur from disconnected one, note and put wiring back, move on and repeat until all spurs identified.
good luck,SPARX

[electricman]

It used to be 'par for the course' in old installation for a 'large' cable to be run to a mid point and a huge joint made somewhere under the floor, then a series of smaller cables coming from this joint out to each socket, This method which was very common has long since been outlawed, check that this os NOT what you have. A clue would be that the fuse protecting this circuit would be bigger than 30 amps, so look for a big fuse (or even a 30 amp fuse if you don't have a bigger one), switch it off and see if the sockets in question go off.