Test results checker
Type in a reading from your tester and see whether it meets BS 7671. Pick the test first.
- V – Voltage
- I – Current
- R – Resistance
If your tester shows an over-range reading such as >299, type it as it appears.
Insulation resistance
Limits are from BS 7671:2018+A4:2026 and IET guidance. Check against your copy of BS 7671 or the On-Site Guide before relying on a result.
Checking an earth fault loop reading? Use the Max Zs calculator: pick the breaker and enter your measured Zs.
Insulation resistance
Test with the DC voltage for the circuit's band and compare the reading with the minimum (BS 7671 Table 64).
| Circuit | Test voltage | Minimum |
|---|---|---|
| SELV and PELV | 250 V DC | 0.5 MΩ |
| Up to 500 V, including 230 V and 400 V | 500 V DC | 1 MΩ |
| Above 500 V | 1000 V DC | 1 MΩ |
Since Amendment 2 (2022), where equipment that could be damaged by the test is left connected, a 250 V DC test between the live conductors and the protective conductor must read at least 1 MΩ. Some manufacturers still want their equipment disconnected first.
RCD trip time
Test at the RCD's rated residual current, 1 × IΔn (30 mA for a 30 mA RCD).
| RCD type | Trip time at 1 × IΔn |
|---|---|
| General (non-delay) | 300 ms or less |
| Type S (time-delayed) | Between 130 ms and 500 ms |
BS 7671 has not required a 5 × IΔn test since Amendment 2, though it can still help with fault finding. Check the RCD type is right for the load too: Amendment 4 makes the Type AC, A and F requirements clearer.
Ring final continuity
- End to end. Measure line (r1), neutral (rn) and CPC (r2) from one end of the ring to the other. r1 and rn should be about equal: within 0.05 Ω is a common rule of thumb. For 2.5/1.5 mm² twin and earth, r2 should be about 1.67 × r1, because the CPC is smaller.
- Line to neutral. Cross-connect line and neutral of the two ends. Every socket should then read about (r1 + rn) ÷ 4.
- Line to CPC. Cross-connect line and CPC. Every socket should read about (r1 + r2) ÷ 4. This is R1 + R2 for the ring, which you record.
A socket that reads noticeably higher than the rest is likely to be on a spur, or to have a poor connection. The 1.67 ratio comes from the conductor sizes; the real resistance ratio is slightly lower, so the checker only flags a ratio more than about 10 % away.
Worked example
A 2.5/1.5 mm² ring reads r1 = 0.52 Ω, rn = 0.52 Ω and r2 = 0.86 Ω end to end. Line to neutral at each socket: (0.52 + 0.52) ÷ 4 = 0.26 Ω. R1 + R2 at each socket: (0.52 + 0.86) ÷ 4 = 0.345 Ω. The ratio 0.86 ÷ 0.52 = 1.65 is close to 1.67, so the CPC is continuous and the right size.