Testing and verification
Radial line and CPC resistance
- where
- line conductor resistance (Ω)
- circuit protective conductor resistance (Ω)
- line resistance per metre (mΩ/m)
- CPC resistance per metre (mΩ/m)
- length (m)
Explain this equation
Gives the expected R1 + R2 value for a radial circuit from cable resistance per metre and length. Use it to know what reading to expect at the far end during continuity testing.
r1 is the line conductor resistance and r2 the protective conductor resistance, both in milliohms per metre. L is the one-way circuit length in metres. R1 + R2 is the combined end-to-end resistance of both conductors in ohms.
The test loop runs out along the line conductor and back along the CPC, so each per-metre value applies once over the circuit length. Adding them gives the loop resistance per metre, and the 1000 converts milliohms to ohms.
2.5/1.5 mm² copper is about 7.41 + 12.10 = 19.51 mΩ/m at 20 °C, so 20 m gives about 0.39 Ω. Double the length and the reading doubles.
Watch out: don't double the length as well, because both conductors are already counted. Per-metre values are usually quoted at 20 °C, so correct for temperature when comparing with design limits. A calculated value does not prove continuity.
Add the per-metre resistances of the line and circuit protective conductor, then multiply by the one-way circuit length. Both conductors are already included in the sum.
The calculated value helps interpret a continuity test. The test itself requires safe isolation and competent procedures; an expected value does not prove continuity.
Enter your values to see the result.
Common mistake
Doubling the length after already adding line and CPC resistance.
Limits
Use conductor resistances at the same stated temperature. Joints, test leads, spurs and cable variation affect readings. This calculation does not prove continuity.
Sources
The tools calculate the displayed equation. They do not automatically choose cable sizes, approve installations, supply current regulatory tables, certify equipment or replace inspection and testing.