Commissioning a workshop sub-board: interpreting test results and deciding whether to energise
You are commissioning a new 400 V three-phase sub-board in a workshop. The board feeds six circuits, including a 20 A single-phase Type C MCB supplying outdoor floodlights installed last week. The floodlight fittings have been rained on and there is visible moisture in one enclosure. Test results, with all switches on and loads disconnected: insulation resistance at 500 V DC - 200 MΩ on five circuits, 0.4 MΩ on the floodlight circuit; main earth bar to sub-board earth bar continuity - 0.08 Ω; a socket outlet 40 m from the board reads 0.9 Ω on its earth pin using your long test lead; the 30 mA RCBO trips at 24 mA in 28 ms; the floodlight circuit's measured earth-fault loop impedance is 1.6 Ω.
1. Interpret each result. State clearly which pass, which fail, and which are likely to be test-method artefacts.
Read the worked answer
Insulation resistance: five circuits at 200 MΩ - pass (minimum 1 MΩ at 500 V DC). The floodlight circuit at 0.4 MΩ - fail. It is not a marginal reading; it indicates a low-resistance path to earth (moisture in the fitting, water in the enclosure, damaged cable sheath at a gland, or a degraded luminaire).
Earth continuity 0.08 Ω (main earth bar to sub-board earth bar): effectively a dead short through a healthy protective conductor. Acceptable, but compare it with the calculated value for the conductor length and record it.
0.9 Ω at the socket earth pin: almost certainly dominated by your 40 m test lead. Null or compensate the leads, or test with a low-resistance ohmmeter at the board end and calculate. Do not record 0.9 Ω as a fault until the method is corrected.
RCBO at 24 mA / 28 ms: pass - trip current is between 15 mA and 30 mA and the time is well under 300 ms. Also confirm no trip at 15 mA (50%).
Loop impedance 1.6 Ω on a 20 A Type C MCB: fail. For a Type C 20 A device the maximum Zs at 0.4 s is around 1.15 Ω. At 1.6 Ω the circuit will not disconnect within the required time on an earth fault.
2. What will you do about the 0.4 MΩ reading, and what are your options for the loop impedance failure?
Read the worked answer
0.4 MΩ circuit: do not energise it.
- Isolate and lock off, then disconnect the floodlight fittings at the junction boxes and re-test each leg separately to localise the fault.
- Inspect the wet enclosure, glands and cable entries; check the IP rating of the fittings suits the location.
- Dry the enclosure and fittings, remake any damaged terminations, and re-test at 500 V DC. A dry, sound circuit should read in the tens of MΩ or higher.
- If the reading stays low, replace the damaged section or the faulty luminaire - do not accept a low reading because 'it works'.
- Ensure any SPD or sensitive equipment is disconnected during testing so it is not damaged and does not skew the result.
Loop impedance 1.6 Ω - options:
- Increase the circuit protective conductor size and/or reduce the circuit length (largest effect).
- Change the device characteristic (e.g. Type B) only if the load's inrush allows it - likely not for floodlights with inrush, so verify.
- Provide a local sub-board closer to the load to shorten the run.
- If an RCD is relied on, confirm from AS/NZS 3000 that the specific clause permits it for this arrangement - an RCBO gives additional protection but is not a blanket substitute for the disconnection requirement, and the fixed wiring still needs a compliant earth-fault path.
Document the chosen solution and re-test.
3. What is the correct test sequence for this board, and what must be completed before you energise and hand over?
Read the worked answer
Sequence (de-energised first, then live):
- Visual inspection - enclosures, IP ratings, cable support, conductor identification, torque of terminals, barriers and shrouding.
- Continuity of protective earthing conductors and equipotential bonding (leads nulled).
- Insulation resistance at 500 V DC with loads and sensitive equipment disconnected.
- Polarity and correct circuit connections.
- Earth-fault loop impedance on each circuit against the device's maximum Zs.
- RCD/RCBO operation - 50% no trip, trip between 15 and 30 mA, time under 300 ms, plus the integral test button.
- Functional testing of each circuit and verification against the circuit schedule.
Before energising: all failures rectified and re-tested. The floodlight circuit stays isolated until it passes insulation resistance.
Before handover: results recorded on the Certificate of Test and Compliance, the Electrical Work Request lodged with the Electrical Safety Office if the work is notifiable, as-installed drawings and the circuit schedule provided, and the client shown the main switch and RCD test function. Any circuit left de-energised must be labelled and the client advised in writing.