Readings at each socket should be substantially the same; higher values suggest interconnects or loose terminations.
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This calculator turns the three end-to-end measurements from a ring final continuity test — r1 (line), rn (neutral) and r2 (CPC) — into the readings you should see at each socket during the cross-connect stages. It saves UK electricians doing the (r1 + r2) / 4 arithmetic at the board and makes it obvious when a ring has a problem.
The test method is the standard three-step ring final procedure: measure each conductor end to end, then cross-connect line and neutral and check L-N at every socket, then cross-connect line and CPC and check R1+R2 at every socket. On a healthy ring, the socket readings are substantially the same everywhere and equal to a quarter of the sum of the relevant end-to-end values.
Expected at sockets: L-N = (r1 + rn) / 4, R1+R2 = (r1 + r2) / 4
r1, rn and r2 are the end-to-end resistances of the line, neutral and CPC loops measured with the ring opened at the board. When the ring is cross-connected (outgoing line to returning neutral, and so on), every socket sits on two parallel paths around the ring, and the maths works out so that the reading at each socket equals the sum of the two end-to-end values divided by four. The tool also flags when r1 and rn differ by more than 10% — same-size conductors should match — and notes that r2 is expected to be about 1.67 × r1 for 2.5/1.5mm² twin and earth.
With the ring disconnected at the board, measure line end-to-end (r1), neutral end-to-end (rn) and CPC end-to-end (r2) with a low-resistance ohmmeter, and enter them in ohms.
The tool shows the L-N reading you should get at each socket with line and neutral cross-connected, and the R1+R2 you should get with line and CPC cross-connected — each a quarter of the relevant sum.
It warns if r1 and rn differ by more than 10% (same-CSA conductors should read the same) and reminds you that r2 should be roughly 1.67 × r1 on 2.5/1.5mm² twin and earth.
Test every socket. Readings substantially equal to the expected values confirm a continuous ring; noticeably higher readings at some points suggest interconnects, spurs measured mid-test or loose terminations worth investigating.
This is the standard ring final continuity method described in the IET On-Site Guide and Guidance Note 3. Its value is that it proves the ring is complete and correctly connected — a broken ring still works as two radials and will pass a casual socket test, but it overloads conductors and shows up here as inconsistent readings.
The highest R1+R2 recorded at a socket (typically the point furthest from the board electrically) is the figure entered on the schedule of test results and used for the Zs calculation. Spurs read higher than ring sockets by the resistance of the spur leg, which is normal and worth noting rather than chasing.
If r1 and rn disagree by more than about 10%, stop and find out why before continuing — crossed conductors, a poor termination or different conductor sizes are the usual suspects. Guidance only; test methods and acceptance are per BS 7671 and GN3 for the actual installation.
Readings at each socket should be substantially the same; higher values suggest interconnects or loose terminations.
When the ring is cross-connected, each socket sees two parallel paths whose resistances sum to the full end-to-end loop. The parallel combination is at its maximum at the midpoint and works out to (r1 + r2) / 4, which is why every socket on a healthy cross-connected ring reads roughly the same quarter-of-the-sum value.
For standard 2.5/1.5mm² twin and earth, the CPC is smaller than the line conductor, so its resistance per metre is higher by the ratio of the conductor sizes — about 1.67 times. If r2 is far above 1.67 × r1 the CPC path deserves a closer look for poor connections.
On a correctly connected ring the cross-connected readings are substantially equal at every socket. Higher readings at some sockets point to spurs (expected), interconnections between legs, or loose terminations. Rising values towards one end suggest the cross-connection was made incorrectly and the test is effectively measuring around an open ring.
Record the end-to-end values r1, rn and r2, and the maximum R1+R2 measured at a socket during the cross-connect test — that maximum is the circuit's R1+R2 for the Zs calculation. Follow BS 7671 and GN3 for the full test and recording requirements.
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