UK Low Insulation Resistance Test

[DazzaL]

Hi guys,

Hoping someone can clarify some things here. I had a socket replaced in my kitchen recently and as part of this, the electricians did an Insulation Resistance test and told me the reading was very low at just over 1 mega ohms. They advised me to basically rewire the ring (at significant cost) as it needed to be 300MOhms plus from the consumer unit (actually replace the unit too as they wanted to split the ring into 2 or 3 rings as it turned out the ring covered the kitchen and lounge plus a bedroom. All in all it would cost in the region of £4-5k to rectify this once re-tiling/plastering was done.

I spoke to another electrician friend of mine about this and the first thing he asked was: did they disconnect all appliances from every socket before testing? Well, no they didn't do that - there were loads of things plugged in still (tv, smart speakers, wifi routers and lots of kitchen appliances all plugged in and switched in the on position). At this he just said forget it, the test is invalid unless you disconnect everything first.

So is it fair to say that this may be a non-issue (the low reading of 1Mohms) as they didn't disconnect everything first? or should I get it retested with everything out (I was quoted £400 by the same company to do more diagnostics though they discouraged that as an option and wanted to go straight into a full on rewiring job).

 

[pc1966]

Don't forget 1Mohm Is 1,000,000 ohms! Works out at something like 0.05Watts at 230V, so not going to be starting any fires from excessive heating, even if the 1Mohm was localised to one small position.

It is not the 1M that concerns me as such, it is why it is there and what might happen with a bit of extra damp, movement, or another rodent having a bite.

 

[cliffed]

Anything less than 2M ohms need investigation,& thus should sort out that 1 M ohm reading.
I doubt that a rewire is needed,just a decent caring Sparkie

 

[pc1966]

Anything less than 2M ohms need investigation,& thus should sort out that 1 M ohm reading.
I doubt that a rewire is needed,just a decent caring Sparkie

Definitely.

The problem with simple pass/fail thresholds like, say, 1M is it fails to take in to account the extent of what is being tested. So 1M on a circuit of 100-200m cable and dozens of sockets or light fittings in dusty and damp conditions is not so worrying, where as a simple 10m radial to an appliance in a dray flat means something serious has gone wrong.

 

[Risteard]

BS7671 Table 64 actually quotes Minimum values of insulation resistance to be 1Mohm! upto 500v

So if 'just over 1Mohm' has been recorded recorded it is 'compliant' but worth getting a second opinion and checked out if still low after disconnecting all appliances.
Probably only half a days work at most to determine cause, not £400 worth!

It should be remembered that this is for the entire installation though, and not just for a tiny section of cable. So you would either need to conduct the test on the installation in its entirety or calculate the resistances in parallel.

 

[UNG]

If you are worried about it call in a qualified electrician, preferably one that is a member of NAPIT or the NICEIC for peace of mind. You can always try 'Which' they have a good reliable database of professionals.

Yes get a properly qualified electrician in that knows what they are doing. Using scheme "electricians" doesn't necessarily give any guarantee of their ability other than being able to pay the annual fees

 

[DefyG]

It should be remembered that this is for the entire installation though,

Or an individual circuit! You wouldn't necessarily test the 'whole' installation if installing a new radial circuit serving a double socket in an office block ...... would you?

 

[Risteard]

Or an individual circuit! You wouldn't necessarily test the 'whole' installation if installing a new radial circuit serving a double socket in an office block ...... would you?

The minimum values in BS7671 are for the installation (or distribution circuit with all final circuits connected in larger installations) - not for an individual final circuit.