Before working on any electrical circuit isolate the supply by switching off the main switch and remove the fuse to the circuit that you are working on, If it is a circuit breaker switch off the main fuse and circuit breaker and remove the wires from the top of the circuit breaker. Place a sign saying that you are working on the circuits on the consumer unit, then nobody will restore the power whilst you are working on it.

If you are not 100% certain what you are doing call a qualified electrician. Building regulations are changing all the time and modifying your home electrics could be against new rules and could invalidate your home insurance, if in doubt check first!

The ring circuit or ring main as it is often called starts at the consumer unit and returns to the consumer unit. It can serve an area up to 120 square yards. It is normally protected by a 30 amp fuse or 32amp circuit breaker, it can have any number of sockets or fused connection units on it but the maximum load is 7200 watts. For this reason I normally install three ring mains per house, 1 on the ground floor, 1 on the upstairs floor and 1 serving the kitchen alone, as this is where most of the high consumption appliances are.

A typical ring main diagram would look like this-

The 2.5mm² wire starts at the consumer unit then runs  from one socket to another and returns to the consumer unit.

The connections at the consumer unit would look like this-

Here you can see that Both cables from the ring circuit are both connected to the same terminals in the consumer unit, The two Earth wires connect to the earth block and the two Neutrals to the Neutral Bar and the two Live wires go into the same circuit breaker or fuse.

The connections at the sockets are pretty simple-

HOW DO I KNOW IF I HAVE A RING MAIN?

If you are unsure as to whether you have a ring main installed in your home you can soon find out pretty easily. Unplug any appliances which are plugged into the sockets. Isolate the main switch at the consumer unit and remove the fuse or switch off the MCB for the socket circuit.  Plug in an appliance that you know is working to make sure it is isolated, now remove the socket from the wall by removing the two retaining screws and gently move the socket clear of the back box. If there are three cables in the back box refasten it as this will make it difficult to test, you could assume that it is a ring main feeding a spur but you still need to test another socket. Find the next socket and test it is not live by plugging in an appliance you know is working, now remove the socket from the back box, hopefully this will have two cables in the back box. Undo the Live (RED) terminal and remove the two red wires, now using a multi meter switch it to OHMS and place one of the leads on each of the red wires or use a continuity tester, the meter should read Zero or very low resistance, some meters will emit a bleep also, this means that there is a circuit there and that it is a ring main!

If there is  resistance present it means that you have a radial Circuit -

FITTING A SPUR OR FCU

You might realise that your existing ring main does not have sockets in the correct places or perhaps you might want to connect a FCU (fused connection unit) to the ring main to power a extractor fan or similar appliance. This is possible as long as the total load on the ring circuit does not exceed 7200 Watts. In theory each socket on the ring circuit could supply a spur to a single or double socket or FCU as long as the area is less than 120 square yards and the total load is less than 7200 Watts.

Find a socket which you would like to supply your new socket or FCU. Isolate the main switch and remove the fuse or MCB for that circuit. Remove the cover from the socket and look to see how many cables are in the back box. If there's one cable you cannot use it as this is already a spur, if there is three wires you cannot use it as it is already feeding a spur, if there are two wires test to see if it is a ring main.

When you have found a suitable socket you can start installing the spur. Route the cable as described in routing a cable. Connect the three wires at the spur- Earth to The Earth connector using yellow and green sheathing, Red to L or Live and Black to N or Neutral, if you are using a metal back box this will need earthing also, you can do this using a piece of wire enclosed in Green and yellow sheathing, simply connect it to the Earth terminal in the back box and wire the other end into the Earth terminal on the socket. Now connect the wires at the Feed side of the circuit, This can be quite tricky as you are trying to get three wires into one terminal. Ensure that all three Reds go into the Live terminal, all three Blacks go into the neutral and all three earths go into the earth terminal. Gently push the socket to the back box ensuring that none of the wires are twisted or trapped, be aware that if any wires are trapped you could crack the socket when you tighten it! Replace the two retaining screws and tighten slowly but do not over tighten or you will crack the socket unless it is a metal socket!

Replace the fuse or circuit breaker at the consumer unit and switch on the main switch, now test the socket that is feeding the spur and the new socket with an appliance that you know is working.