Ring final circuit continuity test
A ring final circuit can look perfect and still be broken or interconnected. This three-step test proves the ring is complete and gives you R1+R2 at the same time.
Why it matters
A break in a ring turns it into two long radials. A 32 A device then protects 2.5 mm² cable that can overheat. An interconnection (a “figure of eight”) hides the same problem. Neither can be seen by eye.
Step 1 — end-to-end readings
With the circuit isolated, identify the two ends of each conductor at the board. Measure end to end:
- r1 — line to line
- rn — neutral to neutral
- r2 — cpc to cpc
r1 and rn should be within 0.05 Ω of each other. With 2.5/1.5 mm² cable, r2 should be about 1.67 × r1 because the cpc is smaller.
Step 2 — cross-connect line and neutral
Join the line of one end to the neutral of the other end, and vice versa. Now measure line to neutral at every socket. Each reading should be about the same — roughly (r1 + rn) ÷ 4. A socket that reads noticeably higher is on a spur or there’s an interconnection.
Step 3 — cross-connect line and cpc
Repeat with line and cpc cross-connected and measure line to earth at every socket. Readings should be about the same, roughly (r1 + r2) ÷ 4. The highest reading is R1+R2 for the ring. Testing line to earth at each socket also confirms polarity.
What faults look like
| Result | Likely cause |
|---|---|
| One end-to-end reading open circuit | Broken ring in that conductor |
| Sockets nearer the middle read higher | Ends cross-connected wrongly (same-end pair) |
| One socket much higher than the rest | Spur or poor termination at that point |
| Readings drop at some sockets | Interconnection (figure of eight) |
iZapp’s 3D Testing Bench lets you do this test on a virtual board with real-looking kit: place the probes, read the tester and record the result on a practice schedule. Some attempts have a fault planted for you to find.
Frequently asked questions
What should r1 and rn be?
They should be within 0.05 Ω of each other, since line and neutral are the same size.
Why is r2 higher than r1?
In 2.5/1.5 mm² twin and earth the cpc is smaller, so it has more resistance — about 1.67 times r1.
How do I get R1+R2 for a ring?
From step 3 (line and cpc cross-connected): the highest line–earth reading at any socket is R1+R2.
Last updated 2026-10-14. Training summary of BS 7671 and IET Guidance Note 3 practice, written by NAPIT-registered electricians. Always check the current edition and amendment of BS 7671 and your scheme’s requirements.