Insulation resistance

The resistance between conductors that should be completely separated from each other. It is measured in megohms, line to neutral and each to earth, with a test instrument applying an elevated DC voltage. Healthy insulation reads very high; damaged cables, moisture or pinched conductors show up as a low reading long before they show up as a shock or a fire.

Where to look it up: BS 7671 Part 6.

Taught and practised in the Testing & Inspection section.