Minimum IR values

The honest answer is that 1 megohm is chosen as an easy number to remember.  Really any single figure is a bit of a fudge.

Any  PVC / polyethylene type insulation that is dry and in good condition should measure hundreds of megohms over the 50-100m cable length of a typical final circuit. So for that situation the 1Meg limit is set very low.

However for mineral insulated cables like pyrotenax, and loads like heating elements with ceramic substrates that are slightly porous, a happy reading may be a few hundred k ohms per metre, and for these you may end up having to warm things up and re-test to fiddle a pass, even though nothing is really wrong.

In terms of actual damage, it rather depends if all the leakage is in one place, and heating a small volume to incineration, or spread out over hundreds of metres and raising the temperature by fractions of a degree.

In terms of inspections, test the whole lot from the main double pole switch and if that passes, than all sub-circuits also pass.

If it fails sub-divide, if the sub-circuits pass, that is also OK. IF not then divide until you isolate a defective fitting or fixture or section of cable.

If you actually get a reading really close to 1Meg, on either side you could toss a coin to do something or nothing, but more professionally you could look at the circuit and decide if in the circumstances for that installation, cable type, dampness of location and what it is supplying, if it is a believable no fault reading, or it was worth investigating further or not.

Mike

The honest answer is that 1 megohm is chosen as an easy number to remember.  Really any single figure is a bit of a fudge.

Any  PVC / polyethylene type insulation that is dry and in good condition should measure hundreds of megohms over the 50-100m cable length of a typical final circuit. So for that situation the 1Meg limit is set very low.

However for mineral insulated cables like pyrotenax, and loads like heating elements with ceramic substrates that are slightly porous, a happy reading may be a few hundred k ohms per metre, and for these you may end up having to warm things up and re-test to fiddle a pass, even though nothing is really wrong.

In terms of actual damage, it rather depends if all the leakage is in one place, and heating a small volume to incineration, or spread out over hundreds of metres and raising the temperature by fractions of a degree.

In terms of inspections, test the whole lot from the main double pole switch and if that passes, than all sub-circuits also pass.

If it fails sub-divide, if the sub-circuits pass, that is also OK. IF not then divide until you isolate a defective fitting or fixture or section of cable.

If you actually get a reading really close to 1Meg, on either side you could toss a coin to do something or nothing, but more professionally you could look at the circuit and decide if in the circumstances for that installation, cable type, dampness of location and what it is supplying, if it is a believable no fault reading, or it was worth investigating further or not.

Mike

Typical at home or in a large factory?

I venture to suggest that 643.3.2 is not worded helpfully. If the whole board test is > 1MΩ, all is well. If not, check each circuit. If any one is < 1MΩ, that will drop the overall resistance below 1 MΩ because the resistance of a number of circuits in parallel (as Sparkingchip points out) is always less than the lowest one. Then it becomes a matter of either look more closely or FI depending upon what has been agreed at the outset.

So if the whole board test is > 1MΩ, all is well; but if it is not, it does not mean that there is a problem.

HTH.

mapj1 said:

Any  PVC / polyethylene type insulation that is dry and in good condition should measure hundreds of megohms over the 50-100m cable length of a typical final circuit. So for that the 1Meg limit is set very low.