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Insulation resistance: what it proves, and at what voltage500 volts or 250?
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What an insulation resistance test confirms is simple: that the conductors are not touching anywhere inside that cable — that they run the whole way separate from each other. The test is applied across the load.
| Test voltage | Where |
|---|---|
| 500 V d.c. | All installations and circuits rated up to 500 V, except SELV and PELV |
| 250 V d.c. | SELV and PELV circuitry — and anywhere sensitive electronics cannot be disconnected |
A shaver socket is the example you will meet most in domestic work — and it is not SELV. Its isolating transformer gives electrical separation. The supply side, coming in at 230 V, is tested at 500 V, and so is the separation itself: Guidance Note 3 · 2.6.9 · Table 2.15 · p.75 gives 500 V d.c., minimum 1.0 MΩ. You do not drop to 250 V for it.
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- Building Regs
- PAT
- Fundamental Inspection & Testing
- Initial Verification
- Periodic Inspection & Testing