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Insulation resistance practical: big numbers this timeOne megohm on a brand new circuit. Pass?
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The last of the main dead tests. Polarity is confirmed after it — mostly proved already by your continuity tests, with Edison screw lampholders checked on their own (except E14 and E27 to BS EN 60238). You are proving the insulation is intact: line not touching neutral, line not touching earth, neutral not touching earth.
So you want big numbers: megohms, millions of ohms. Ideally the tester runs out of range and shows its over-range reading — >999 MΩ on one meter, >200 or >2000 on another. Off the scale is exactly what you want.
| Circuit | Test voltage | Minimum |
|---|---|---|
| SELV and PELV | 250 V DC | 0.5 MΩ |
| Up to and including 500 V (ordinary single and three phase) | 500 V DC | 1 MΩ |
| Above 500 V | 1000 V DC | 1 MΩ |
Guidance Note 3 · 2.6.7 · insulation resistance · from p.68 Guidance Note 3 · 2.6.7 · minimum values; below 20 MΩ on new work · p.70
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- Fundamental Inspection & Testing
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