Inspection, Testing and Commissioning
This domain covers the initial verification sequence, the meaning of each result, and the handover of a compliant installation. Expect questions on test order, instrument settings, and which certificate covers which scope of work.
Order of initial verification
- Visual inspection first, with the installation dead.
- Continuity of protective conductors, including main and supplementary bonding, then ring final circuit continuity.
- Insulation resistance at 500 V DC with a minimum of 1 M ohm, loads and electronics disconnected and SPDs isolated.
- Polarity, then earth electrode resistance where one applies, then Ze with the main earthing conductor disconnected.
- Zs at the far end of each circuit, prospective fault current, and RCD operation: no trip at half the rated residual current, tripping within 300 ms at the rating and within 40 ms at five times it for a 30 mA device.
- Functional testing and phase sequence on three-phase supplies.
Commissioning
Prove protective settings against the discrimination study, confirm rotation, check labelling, energise in stages, load test and record, prove interlocks and alarms, then hand over the operating manuals and as-built drawings.
Worked example
A 20 m 2.5/1.5 mm2 radial runs from a TN-C-S origin with Ze = 0.35 ohm. At 20 C, conductor resistance is 7.41 milliohm per metre for 2.5 mm2 and 12.1 for 1.5 mm2. So r1 = 0.148 ohm and r2 = 0.242 ohm, giving R1+R2 = 0.390 ohm and an expected Zs of 0.35 + 0.39 = 0.74 ohm. A 20 A Type B device has a tabulated maximum of 2.30 ohm, corrected to 1.84 ohm, so the circuit complies with margin. If insulation resistance instead read 0.4 M ohm against the 1 M ohm minimum, the installation would fail and the circuits would need sectionalising to find the low reading.
Traps that cost marks
- Insulation testing with dimmers, electronics or SPDs connected, producing a false low reading or damage.
- Failing to null or compensate the test leads before continuity testing.
- Using a high-current loop tester on an RCD-protected circuit and tripping it; use the low-current setting.
- Confusing Ze, measured at the origin with the earthing conductor disconnected, with Zs at the end of a circuit.