Part of the testing for an electrical report involves testing for something called “Insulation Resistance”. In a nutshell, this test measures the condition of the insulation between whichever cables are under test.
It is tested with everything removed from the circuit (all sockets removed or light bulbs removed) as these can provide false readings on the sensitive test equipment.
The level of the “insulation resistance” is measured in Mega-Ohms, literally Millions of Ohms resistance. The cable must be over 1M Ohms minimum to pass the test (although anything lower than 2-3M Ohms would be regarded as faulty and investigated).
Low insulation resistance readings can be caused by a number of issues:
- Extremely low readings indicate a dead (or nearly so) short between 2 of the conductors. Causes of this include physical damage to cables (rodent damage, drilled cable, melted cable), or potentially a loose wire that has caused a short. Loose wires would fall into a different category of fault as it is not related to the insulation.
- The same issue affecting the old rubber insulated cables (where the insulation breaks away, as above) is the cause of this. It gives low readings where the insulation has failed
- There are some other issues that can cause this: Inexperienced testers can often find a fault similar to this by not removing loads from the circuit. Everything must be unplugged or disconnected (including switches with neon lights on) as these items give false readings