Interpret low-resistance measurements without mistaking an alternative path for a sound conductor.
Continuity testing must confirm the intended conductor path. A low reading through connected metalwork can hide a broken circuit protective conductor (CPC). Plan the dead test so you understand the paths being measured; do not disconnect protective conductors from an energised installation to chase a reading.
A worked ring example
Opposite-end line-to-neutral links divide a sound, uniform ring into two equal-resistance test paths. If the line and neutral loops each measure 0.48 Ω, their equal size and common route give an expected socket reading of:
(r1 + rn) ÷ 4 = (0.48 + 0.48) ÷ 4 = 0.24 Ω.
Each test path has half the total end-to-end resistance, and putting those equal paths in parallel halves it again. That gives the divisor of four. This example excludes spurs and additional parallel paths; include both end-to-end readings in the sum.
Line-to-CPC readings need another judgement
Where the CPC has a smaller cross-sectional area, its resistance is higher. Cross-connected line-to-CPC readings can vary with position even on a sound ring. Interpret the expected pattern for that cable arrangement before declaring a break.
Decision check: similar socket readings are useful evidence, but do not replace the separate end-to-end checks of each conductor.
Methods, and what the readings tell you
- Safely isolate and prove dead before any continuity test.
- In method 1 (R1 + R2), link the line to the protective conductor at the board and measure between them at each point. In method 2, a wander lead runs from the earthing terminal to the remote protective conductor. Null the instrument for the leads, or subtract their resistance, and null again whenever the leads change.
- R1 + R2 helps verify continuity of the intended line and CPC path. Added to a valid Ze it supports the usual Zs calculation. Record the highest appropriate radial-circuit value: branches, conductor sizes and connections mean the physically furthest point is not invariably the worst case.
- Confirm protective conductor continuity before any live loop test. An open protective conductor found at initial verification is corrected before the circuit is energised.
- Conduit, bonded pipework and structural steel give parallel paths that lower readings and can hide a broken protective conductor. A reading that changes as a clamp is moved suggests a connection problem; first confirm the leads and probe contacts. Check the intended main bonding path, including the pipe contact, allowing for parallel routes. A reading to the clamp body alone can miss poor contact with the pipe. Assess diverted-current hazards before any protective connection is separated.
- On a ring, test each conductor end to end first. Line and neutral of the same material, size and route should have closely comparable end-to-end readings. A commonly used good-practice comparison is for these matching line and neutral readings to differ by no more than 0.05 Ω. It is a rule of thumb, not a pass mark: allow for the expected resistance and for instrument and contact uncertainty, then complete the remaining ring tests. Investigate an unexpected difference for a poor joint, different conductor or measurement error. In 2.5/1.5 mm² cable the smaller protective conductor reads about two-thirds as much again end to end (r2 ≈ 1.67 × r1).
- Investigate a socket reading above the expected pattern. A legitimate spur, poor test contact, connection defect or circuit problem may explain it; one high reading does not tell you which. A broken ring may still supply appliances, while its current distribution and conductor protection are compromised. The load may not all move to one leg.
- Cross-connect each line end to the opposite CPC end. The reading at a socket is the parallel combination of two line/CPC routes. For a sound uniform ring without spurs or other parallel paths, (r1 + r2) ÷ 4 gives the mid-point maximum when the conductor resistances differ. Use the appropriate result when deriving Zs.
Worked example. For that uniform ring with r1 = 0.60 Ω and r2 = 1.00 Ω, the mid-point maximum is (0.60 + 1.00) ÷ 4 = 0.40 Ω. Near the cross-connected ends, the parallel value is (0.60 × 1.00) ÷ (0.60 + 1.00) = 0.375 Ω. At the mid-point, adding Ze = 0.35 Ω gives an expected Zs of 0.75 Ω, before any required temperature adjustment.
In a medical location, supplementary bonding helps limit potential differences between relevant connected parts. Verify and record the required resistance; it is one part of the protective arrangement.