Recommended checks before CU change

As the IR figures were really low (0.42mohms)

It's not that low - 20% higher and it would have actually have been acceptable under the 16th. That sort of value is quite normal where there's mineral insulation involved - e.g. traditional electric cooker heating elements or perhaps an immersion heater or two. Even if the elements aren't switched on at the time they'll still show on a N-PE test, and most likely on a L-PE test too if there's anything to bridge L to N - such as a timer or some other connected load.


The other common culprit for low IR is outside lighting or similar where water can get in.


The other gotcha for older installations is borrowed neutrals - especially on lighting. Lights were commonly wired in sheathed singles so N (and sometimes L) would be picked up from anywhere convenient rather than necessarily the correct circuit - landing light being fed from the downstairs lighting L (at the hall switch) but returning through the upstair's lighting's N was a common kludge - similarly wall lights (and occasionally outside lights) using a handy socket circuit for N.


  - Andy.

As the IR figures were really low (0.42mohms)

It's not that low - 20% higher and it would have actually have been acceptable under the 16th. That sort of value is quite normal where there's mineral insulation involved - e.g. traditional electric cooker heating elements or perhaps an immersion heater or two. Even if the elements aren't switched on at the time they'll still show on a N-PE test, and most likely on a L-PE test too if there's anything to bridge L to N - such as a timer or some other connected load.


The other common culprit for low IR is outside lighting or similar where water can get in.


The other gotcha for older installations is borrowed neutrals - especially on lighting. Lights were commonly wired in sheathed singles so N (and sometimes L) would be picked up from anywhere convenient rather than necessarily the correct circuit - landing light being fed from the downstairs lighting L (at the hall switch) but returning through the upstair's lighting's N was a common kludge - similarly wall lights (and occasionally outside lights) using a handy socket circuit for N.


  - Andy.