A ring circuit with a leg taken away

The find

Same property as Find 005, a licensed HMO in York. Circuit 10 fed the first-floor sockets and the central heating: 2.5mm² cable, 32A device in the consumer unit.

On test there was no continuity round the line and neutral legs. The circuit protective conductor was continuous, so the earth path was intact, but the ring itself was not a ring. That is the classic signature of an open ring, and the usual cause is a single loose connection.

The giveaway was in bedroom 4. The socket there had one cable at it. In a ring, every socket sits in the loop and has two cables, one arriving and one carrying on. A socket with a single cable is either a deliberate spur, or it is sitting at the cut end of something that used to be continuous.

It was the cut end. One leg of the ring had been disconnected from the socket loop, run up into the loft, and used to supply the immersion heater.

What you’re looking at

A ring final circuit is two legs leaving the same breaker, running round the property through the sockets, and arriving back at the same breaker. Every socket sits in the middle of the loop, so current can reach it by either direction.

That sharing is the whole point, and it is the reason a 2.5mm² cable is allowed to sit behind a 32A device at all. Neither leg carries the full load on its own, because the load divides between the two paths.

Cut the loop and the sharing stops. What is left is a long 2.5mm² radial with a 32A device in front of it, and now a single run of cable can be asked to carry more current than it is rated for. The breaker will not necessarily see anything wrong, because 32A is exactly what it has been told to allow.

The immersion heater

The leg that was taken did not go somewhere harmless. It went up into the loft as a single 2.5mm² conductor and fed the immersion heater, connected back to that same 32A device with nothing fusing it down to something the cable could live with.

An immersion is a heavy load and it runs for long stretches at a time, which is exactly the sort of load that wants its own circuit and its own correctly sized protection. The guidance for a water heater on a storage cylinder of any real size is a dedicated circuit, not a branch taken off the sockets.

Why it matters

That is what makes this one worth writing up. There was no scorching, no smell, nothing a homeowner could reasonably be expected to spot. It only showed up because the circuit was tested properly, and then because a socket with one cable at it was worth stopping to think about rather than writing off as a spur.

The fault code

C2. Potentially Dangerous. Twice over, and they are recorded separately on the report. The open ring leaves 2.5mm² conductors protected by a device rated beyond what a single run can carry. The immersion supply is an unfused leg off that same device, feeding a sustained load. Neither is a switch-it-off-now emergency, which is why it is a C2 rather than a C1, but both need putting right rather than noting for later.

The fix

Two jobs again, and they are related.

The sockets come first. Either the ring goes back to being a continuous loop and gets re-tested for continuity, or the first-floor sockets are reconfigured as a radial with a protective device that actually matches the cable. Both are legitimate. What is not legitimate is leaving it half way between the two, which is where it was.

Then the immersion comes off the socket circuit altogether and gets its own final circuit, sized for the load, with its own correctly rated protection and a double pole switch so it can be isolated properly.

Someone did this deliberately, and I would guess it felt reasonable at the time. There was a cable near the airing cupboard, the immersion needed a supply, and the sockets carried on working afterwards. Everything appeared fine. That is the thing about protection: when it is wrong, the installation usually carries on behaving perfectly normally right up until the day it does not.