Continuity and resistance testing of protective and bonding conductors
The instrument and the method
Low-resistance ohmmeter, reading to 0.01 Ω, leads nulled or zeroed before use - including any wander lead. Measure protective conductor continuity as R1+R2 with the circuit isolated and line and neutral linked at the board, or measure R2 alone with a long lead. Ring final circuits need the three-step continuity test plus the cross-connected figure-of-eight test to prove the ring and check for interconnections.
What to verify
Circuit protective conductors, the earthing conductor from the MET to the means of earthing, main protective bonding from the MET to gas, water, oil, structural steel and other extraneous-conductive-parts, and supplementary bonding in special locations. Disconnect parallel paths when testing a bonding conductor on its own - otherwise the reading is artificially low and proves nothing.
Expected values
R = ρL/A; for copper at 20 °C this gives roughly 0.018 Ω/m for 1.0 mm², 0.0073 Ω/m for 2.5 mm², 0.0046 Ω/m for 4 mm² and 0.0018 Ω/m for 10 mm². Worked example: a 20 m, 2.5/1.0 mm² radial gives R1 = 0.0073 × 20 = 0.146 Ω and R2 = 0.018 × 20 = 0.36 Ω, so R1+R2 = 0.51 Ω. Main bonding conductor resistance is commonly expected to be no more than 0.05 Ω.
Trap: recording R2 on the schedule where R1+R2 is required.
Trap: leaving bonding connected in parallel and accepting a suspiciously low reading.
Rule of thumb: null the leads, note the length, calculate what you expect, then compare.