Fourteen circuits. IR readings between 0.8 and 1.1 MOhm across all of them. Every single one above the mandatory minimum. Every single one would have passed a circuit-by-circuit review. But they were all in the same section of the same MSB — and that grouping is what the review flagged.

The IR values weren't random variation. They were clustered — all low, all in section 3, all above the pass threshold but anomalously so for a new installation. New cable assemblies on a fresh MSB typically read well above 100 MOhm before energisation. Values in the 0.8–1.1 MOhm range on 14 circuits simultaneously suggested something upstream of those circuits was the common factor.

Under the section 3 incomer access cover, the neutral termination block told the story. A 70mm² cable land had been torqued to 15 Nm. The specified torque for that termination was 35 Nm. The neutral conductor was making partial contact only — enough to give a reading above the binary pass threshold, not enough to give a reading that looked right against the installation standard.

Re-termination and re-torque to specification brought IR readings across all 14 circuits uniformly above 50 MOhm. Energisation was cleared the same day.

Pre- vs post-energisation intervention

A neutral termination fault identified pre-energisation is a mechanical correction — re-torque, re-test, documentation update. The same fault identified post-energisation under load is a live neutral fault on a section carrying current: voltage unbalance risk, elevated earth fault loop potential, and an MSB section that must be isolated under load to investigate.

45 min
pre-energisation correction
MSB outage + NCR cascade
post-energisation exposure

Why pattern-level IR analysis catches what circuit-by-circuit review misses

Circuit-by-circuit review applies a binary test: above threshold or below. The threshold exists because below it, there is a clear and present insulation fault. But threshold-passing does not mean the reading is normal — it means the reading cleared the minimum bar. On a new MSB, the normal condition is readings measured in the hundreds of MOhm, often immeasurable on standard equipment. A value of 1 MOhm is not a fault in isolation. Fourteen values of approximately 1 MOhm, all in one section, is a structural signature.

The mechanism here is straightforward. A partially contacted neutral provides an alternative path for leakage current across all circuits in that section. The shared neutral path means the apparent IR reading for every circuit in the section is influenced by the same degraded connection. Individual readings look different from each other because the downstream cable lengths and insulation characteristics vary slightly — but they all depress together, producing the clustering pattern. No single circuit reading reveals the cause. The section-level view does.

Threshold note: IR below 1 MOhm on a new installation is never acceptable regardless of the minimum. Values between 1 and 10 MOhm on a new installation warrant investigation before energisation — not a pass and move on. The question isn't whether the reading clears the minimum; it's whether the reading makes sense for the installation condition.

The intervention window

The neutral termination fault in this case was a 45-minute correction pre-energisation: access cover off, re-torque to specification, cover on, re-test, result documented. The same fault post-energisation under load is a live fault investigation. Section 3 carries current. The neutral is faulted. To correct it, the section has to be isolated — which means downstream equipment loses supply. The correction is the same mechanical task; everything around it is different. Independent pre-energisation review with section-level pattern analysis is what creates the 45-minute window. Standard circuit-by-circuit review is what closes it.