Legacy Installation EICR

Question

Greetings all. 
 Hopefully this will be an easy one for the hive mind, and I've tried the 'Search' but to no avail: 
 I manage an old industrial complex built in 1960. During the EICR testing most of the fixed wiring circuits have failed the insulation tests, with readings of less than 1Mohm. These cables are all MICC and have been untouched since installation. To replace all these cables would be an epic task and these works would need to be done in the next three months to satisfy the handover of the premises to the landlord. 
 Is there a method to test these 'legacy' cables other than to test to current standards (500v)? Istr mention of testing such cables at 50v but I can't find where I saw this. Is there a method to implement mitigation measures instead of replacing all the cables, or spending many hours isolating sections to fault find? 
 As you may be able to tell, I'm no electrician and every contractor I've approached has insisted that the entire building needs to be recabled, which isn't currently viable. 
 Thanks.

alanblaby · Accepted Answer

I find it hard to see the insulation being below 1M without causing problems somewhere. I suspect you may need a more experienced person to do the inspection. 
 I had exactly this fault with an industrial unit around 10 years ago, I was there to do the remedials. 'All lighting circuits low IR' was the wording. It was an easy fix, in fact no fix required, there were rows of fluorescent lighting down the unit, quite old, and all had a capacitor between L+N internally. The Inspector had clearly never encountered this before, had not disconnected the lighting before the IR test, and had concluded it was a wiring fault to cause the low IR readings.

AJJewsbury · Answer

What sort of values are you getting? And for what sort of circuit lengths? 
 I'd start by pointing out that 1M is the minimum standard expected for the initial verification of new installations - BS 7671 doesn't explicitly say that the same should be expected for periodic inspections. Do the readings suggest that safety of the installation is impaired? 
 - Andy.

mapj1 · Answer

Mineral cable is famous for absorbing moisture if not kept warm and dry and this lowers the IR. However it does not usually matter unless very severe. A few hundred K ohms over tens of metres is fine. The same resistance over a few feet less so. 
 Be wary of anyone too young to have installed it - modern plastic cables are very different with gigohm resistances. 
 It can often be dried by gas torch, starting a metre or so back and moving the hot spot to a free end. It is important the end can vent, as pressure from trapped steam can split the tube or fire the fittings off... The past is a different place in terms of what is considered reasonable.

Legacy Installation EICR

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