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Insulation resistance on appliances, and when to drop to 250 VOne megohm or two?

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Insulation resistance

The opposite question to continuity. Continuity wants a path; insulation resistance wants no path at all.

?Insulation resistance checks that the live conductors are not touching the earth or the body. Line and neutral are connected together and tested to the protective conductor or the body — the earth pin, or accessible metal. So here you want the resistance as high as possible.
Before the test: power switches on, all covers in place. Where 500 V could damage sensitive circuitry, test at 250 V DC or measure protective conductor or touch current instead — and check the manufacturer’s instructions.
EquipmentMinimumTest voltage
Class I1 MΩ500 V DC
Class II (and class II with functional earth)2 MΩ500 V DC
What the meters actually show. A dedicated PAT tester may only display "> 20 MΩ" because that's the top of its range. A multifunction tester will resolve much further — "> 2000 MΩ" on a healthy appliance. Both are passes.
✋Write the "greater than" symbol on your paperwork. If the meter shows > 20 MΩ you record > 20 MΩ, not 20. Marks are lost on this.

Unlike continuity, insulation resistance is done on class I and class II alike. Of the earth and insulation tests, a class II appliance gets insulation resistance only — plus the functional check every item gets.

Why old cable fails it. Insulation goes brittle and cracks with age, so readings drop over time. A high reading on an old cable in good condition is a good sign, not a suspicious one.

Section 10 · 10.6 · insulation resistance · 500 V DC (250 V alternative) Table 10.2 · 1 MΩ and 2 MΩ · p.72

When to drop to 250 V

500 V DC is the standard test voltage. There is one reason to reduce it.

⚠Where SPDs or other sensitive electronic equipment are built into the item, test at 250 V DC instead. 500 V through electronics expecting 230 V can destroy them.
ItemWhy
Surge-protected extension leadContains an SPD
Sockets with USB outletsElectronics behind the faceplate
Switches with neon indicatorsSensitive components
IT equipmentTreat with extra care

Section 10 · reduced test voltage for sensitive equipment

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