Ring final circuit testing explained
The ring final test is the one most often skipped, fudged or misunderstood. Done properly it proves the ring is genuinely continuous and finds broken legs and interconnections that no other test will reveal.
Updated 2026-09-30
What the test is actually proving
A ring final circuit is protected on the assumption that current divides between two legs. If one leg is broken, the remaining conductor carries the whole load and may overheat without the protective device ever operating.
The three-step test proves the ring is complete, that no legs are crossed, and that every socket sits on the ring rather than being fed by a hidden interconnection.
Step 1: end-to-end readings
Isolate the circuit, prove it dead, then disconnect both legs of line, neutral and cpc at the board. Measure the resistance of each pair of conductor ends joined together as a loop.
- · r1 — line to line, the two line legs joined at the board
- · rn — neutral to neutral
- · r2 — cpc to cpc
- · Null your leads first, or subtract the lead resistance from every reading
What the numbers should look like
On a standard 2.5 mm² twin and 1.5 mm² cpc ring, r1 and rn should be close to identical — within about 0.05 Ω of each other. r2 should be roughly 1.67 times r1, because the cpc has a smaller cross-sectional area.
A significant difference between r1 and rn usually means the conductors take different routes, which happens when a leg has been extended or when a supposed ring is really two radials joined somewhere in the middle.
Step 2: the cross-connection
Join the line of one leg to the neutral of the opposite leg, and the neutral of the first to the line of the second. This is the figure-of-eight connection. Measure line to neutral at every socket on the circuit.
Every reading should be substantially the same and equal to roughly (r1 + rn) ÷ 4. A reading that is noticeably higher at one socket means that socket is on a spur, not on the ring.
Step 3: line to cpc
Repeat the cross-connection, this time joining line to cpc. Measure line to earth at each socket. Each reading should again be consistent, and the highest of them is the circuit's R1 + R2.
Record that value. It feeds directly into the expected Zs for the circuit, which you then confirm with a live earth fault loop impedance test.
Faults this test finds
Each pattern of readings points to a specific physical problem.
- · One end-to-end reading open circuit: a broken leg, often a loose terminal at a socket
- · r1 and rn very different: crossed legs or an interconnection between points on the ring
- · One socket reading high in step 2: an unfused spur, acceptable if it is the only one from that point
- · Two sockets reading high from the same point: a spur off a spur, which needs remedial work
- · All readings identical to the end-to-end figure: the ring is broken and you are measuring a radial
Coding ring faults on an EICR
A broken ring is generally a C2 — the circuit is not as designed and a single conductor may be overloaded under fault or heavy load conditions. An excessive number of unfused spurs is usually a C3 unless loading makes it dangerous.
Record the actual readings on the schedule of test results rather than writing 'satisfactory'. The numbers are what the next inspector will compare against in five years.
Practical tips
Test at the accessory terminals rather than by plugging into the socket where you can, because a socket adaptor adds its own resistance. Label the legs with tape as you disconnect them — remembering which was which after four boards is how legs get crossed at reconnection.