Free guide · Inspection and testing
Ring final continuity test, step by step
A ring final circuit starts and finishes at the same protective device, so every socket is fed from two directions. The continuity test proves the ring is complete, with no breaks and no cross-connections, and gives you the R1 + R2 value you need for the schedule of test results.
The test has three stages. Try each one in the interactive visualisation below, then read the steps.
Before you startTesting the continuity of a ring final circuitRead first
What this shows
This interactive visualisation shows a ring final circuit with six sockets. The brown line, blue neutral and green cpc each run round the ring and back to the consumer unit.
The tabs follow the three steps of the test. Step 1 measures each conductor end to end. Steps 2 and 3 cross-connect the ends and measure at every socket. The fault buttons break the line or the cpc.
Key words
- r1
- The end-to-end resistance of the ring's line conductor.
- rn
- The end-to-end resistance of the ring's neutral conductor.
- r2
- The end-to-end resistance of the ring's cpc.
- Cross-connection
- Joining the line of one end to the neutral (or cpc) of the other end at the board.
- R1 + R2
- The line plus cpc resistance to a point: the highest step 3 reading on a ring.
What to look for
- On step 1, read r1, rn and r2. r1 and rn match, and r2 is about 1.67 times r1.
- Change to step 2 and tap every socket. Every socket reads about (r1 + rn) ÷ 4.
- Press Broken line and tap the sockets again: the readings jump up, because each socket is only fed from one side.
- Change to step 3 to see the line–cpc readings and the R1 + R2 for the circuit.
On the job
- Disconnect both ends of the ring at the board before step 1, and identify which end is which.
- Record r1, rn and r2 on the schedule of test results, and the highest step 3 reading as R1 + R2.
- A ring that is broken still works, so the fault is easy to miss without this test, and one leg of the ring can then be overloaded.
- Test at every socket on the ring, including any spurs, so that no socket is left untested.
Step by step
- 1Isolate and identifyCarry out safe isolation, then find the two ends of the ring at the board and identify which line, neutral and cpc belong to each end.
- 2Stage 1: end to endMeasure each conductor from one end to the other: line (r1), neutral (rn) and cpc (r2). The line and neutral readings should be within 0.05 Ω of each other. With 2.5/1.5 mm² cable, r2 should be about 1.67 times r1.
- 3Stage 2: line to neutral, cross-connectedJoin the line of one end to the neutral of the other, and the other line to the other neutral. Measure between line and neutral at every socket. The readings should be about the same all the way round, roughly (r1 + rn) ÷ 4.
- 4Stage 3: line to cpc, cross-connectedRepeat with line and cpc cross-connected. Measure line to cpc at every socket, operating the switches as you go. The highest reading is the circuit's R1 + R2, roughly (r1 + r2) ÷ 4, and a spur will read higher than the ring sockets.
- 5Reconnect and recordRemove the cross-connections, reconnect the ring correctly and write r1, rn, r2 and R1 + R2 on the schedule of test results.
Always follow the current edition of BS 7671 and its guidance notes.
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