Low Insulation Resistance Figure

[rightsaidfred]

My mother has just had a light fitting replaced out of necessity. The electrician who did the work gave her a "Minor Works Certificate" to cover the work. On it he drew attention to the fact that the wiring was over 40 years old and that the insulation resistance test on the lighting circuit concerned was only 1.2 Mega Ohms (I've read on here a figure of 2 Mega Ohms quoted as the minimum). He recommended that a rewire was necessary and should be carried out in the "near future". Now, I'm pretty happy that a rewire is a good idea (Victorian house and we know it is at least a 1960's electrical installation) but can anyone tell me, based on this resistance figure, is this urgent (ie before Christmas) or can this wait until the New Year?
Thank you in advance

 

[breezer]

the reason there is such a "low " figure is the insulation is old (as you said) and is starting to break down.

The question is when will it finally break down, that no one can answer

for peace of mind i would get it done soon, but when is soon, and to leave it till later raises the question, when is later and will you remeber?

 

[rightsaidfred]

I want to plan it in for the end of Jan beginning of Feb next year when my Mother will be away for a couple of weeks visiting my Sister. If there's no technical problem with waiting until then, I'll be booking the Electrician to come back then later this week. Does that sound sensible?

 

[breezer]

sounds good to me, but phone in jan to confirm

 

[rightsaidfred]

It's a plan then. Thanks for the help.

 

[dingbat]

I've tested plenty of 40+year-old installations and had IR values off the scale. I've also tested lots of relatively recent installations and found very low IR values.

When measuring IR you are effectively measuring the electrical separation of the individual conductors of each circuit. The cause matters not if the values are too low to comply.

The low values may be because of the breakdown of the insulation but more often it is due to other causes - damp, nipped conductors, poor terminations, cramped enclosures, dust, etc, etc...

I'm not saying it doesn't need a rewire - I would certainly recommend one - just that the insulation doesn't 'break down' in quite the way many imagine it might. In fact, with PVC cable, as the plasticiser leaches away with age and the insulation becomes more brittle, its electrical insulation actually improves!