What is a ring final circuit and how is it different from a radial?

The ring final circuit is a British peculiarity, born of post-war copper shortages, and it is why a UK house has one 32A circuit serving a whole floor rather than several small ones. It works well and it is safe when intact, but it has one failure mode that no other arrangement has: it can be broken and still appear to work perfectly.

Step by step

1. Picture the ring.Two cables leave one MCB. They run to the first socket, on to the next, and so on around the house, and the last one returns to the same MCB terminal. Every socket therefore has two cables in it and two paths back to the board.
2. Understand why that allows 2.5mm cable on a 32A breaker.Current splits between the two legs according to where the load is, so no single length of cable carries the whole circuit current. That is the whole trick, and it is also why breaking the ring is so serious — one leg then carries everything.
3. Picture the radial.One cable leaves the MCB, visits sockets in turn, and stops at the last one. Each socket has two cables except the last, which has one. Because one cable carries everything, either the cable is bigger or the MCB is smaller.
4. Learn the standard sizes.Ring: 2.5mm twin and earth, 32A MCB, conventionally serving a floor area up to about 100 square metres. Radial: 2.5mm on a 20A MCB for up to about 50 square metres, or 4mm on a 32A MCB for up to about 75. Kitchens are often given their own circuit regardless of area.
5. Understand spurs.A spur is a single cable branching off a socket on the ring to feed one more point. One unfused spur per socket, feeding one single or one double socket, and never a spur from a spur. A fused connection unit spur can feed more because the fuse limits it to 13A.
6. Understand why a broken ring is invisible.If one leg is cut, disconnected at a socket, or was never connected properly, every socket still works — it just becomes a long radial on 2.5mm cable protected at 32A. Nothing trips, nothing looks wrong, and the cable can be overloaded by half as much again under sustained load.
7. Know that only testing finds it.An electrician tests ring continuity by measuring end-to-end resistance of each conductor and then cross-connecting them. This is a standard part of an EICR and it is why an EICR is worth having on a house you have just bought or where DIY work has clearly been done.
8. Do not attempt to convert one to the other.Turning a radial into a ring, or splitting a ring, changes the design current of the circuit and the protective device. That is design work with certification requirements, not an alteration.

Tips

• Modern practice increasingly favours radials on RCBOs for new installations because they are simpler to test and a fault affects less. Both are permitted; neither is obsolete.
• A house extended over the years often has a ring with several spurs added ad hoc. That is the commonest source of EICR observations on socket circuits.
• If you ever have floorboards up, photograph any cable runs and junctions you can see. That record is worth a lot later.

Common mistakes

• Counting cables at a socket to identify the circuit type — Two cables could mean a ring, a mid-point of a radial, or a socket feeding a spur onward. One cable could mean the end of a radial or a spur. The front of a socket does not tell you the topology.
• Assuming a working socket means a healthy ring — A ring with a break works perfectly at every socket. Function proves nothing about the integrity of the circuit.

If it doesn't work

Questions people ask