Minimum values of insulation resistance

I would expect the IR test to be each conductor to each other conductor and each conductor to earth, since if you only measure conductor to conductor, one of the two conductors could have very low insulation to earth and you wouldn't know.


I don't think the figure of 1 MΩ is related to safe currents for personnel (though possibly someone will come up with proof that it is) as that is protected against by the RCD rather than by maintenance of the insulation. A figure of zero means you have a (potential) earth fault or short circuit while a very high figure means everything is ok. Somewhere in between it will be necessary to do something to prevent the earth fault/short circuit and a nice round figure of 1 MΩ is probably as good as any. As Chris says, it is the change that is of most interest. If the last reading was 4 MΩ and today's reading is 3 MΩ, whether I would be worried would depend on whether the last reading was five years ago or yesterday.

I would expect the IR test to be each conductor to each other conductor and each conductor to earth, since if you only measure conductor to conductor, one of the two conductors could have very low insulation to earth and you wouldn't know.


I don't think the figure of 1 MΩ is related to safe currents for personnel (though possibly someone will come up with proof that it is) as that is protected against by the RCD rather than by maintenance of the insulation. A figure of zero means you have a (potential) earth fault or short circuit while a very high figure means everything is ok. Somewhere in between it will be necessary to do something to prevent the earth fault/short circuit and a nice round figure of 1 MΩ is probably as good as any. As Chris says, it is the change that is of most interest. If the last reading was 4 MΩ and today's reading is 3 MΩ, whether I would be worried would depend on whether the last reading was five years ago or yesterday.