Mineral wiring insulation resistance test failed, need certification!

[Krispy]

My house, built in 1960, has mineral insulated cabling and a voltage operated earth leakage circuit breaker. We moved in 7 years ago (don't suppose that is relevant though )

Due to some rebuilding work the meter needs to be moved, and in preparation for that my electrician has fitted a new distribution board; the supply connection, once changed, will become TNCS/PME.

In readiness for that he today tested the existing circuits and found that the insulation resistance showed as being very low - zero on some circuits, 100K-400K on others, and I believe that 1M is required?

He fitted a current operated RCB as an experiment to see what would happen - it tripped immediately.

Meter is due to be moved next Thursday. Whole house rewire wasn't in the plans ... in particular not doing it next week

My electrician (after a phone call to his affiliated membership organisation) says that we could fit RCBs on the lighting circuits in bathrooms and leave the rest of the lighting circuit unprotected, and could fit RCBs adjacent to, or within the socket, on all power sockets. I figure that would probably look a mess and it would be better to consider rewiring, but that's not going to happen overnight.

I'd appreciate any suggestions / advice you may have. Thanks.

 

[Marvo]

VOELCB's and RCD's aside, if a circuit is IR testing < 1Mohm it should be disconnected, the fault localised and repaired.

I'd also say the VOELCB should be replaced on sight as a matter of course. They were very unreliable at best and downright useless at worst. There's a school of thought that say it can remain as an isolator but I prefer if a device can't perform all it's designed functions it should be scrapped and replaced.

As for the mineral insulated cable I'd leave any that IR test okay and just replace the faulty sections. Obviously it would be wise and less stressful to do this whilst there's already building work happening.

Maybe in this particular scenario it would be worth looking at fitting a CU with individual RCBO's rather than a split RCD CU. This would prevent any future low insulation problems causing widespread power outages across multiple circuits.

 

[snowhead]

It's unlikely that all the MIC is low I.R, it's more likely to be a localised problem.
It will need the circuits splitting and testing then only the low sections will need replacing for now.

 

[ruston]

Has it been wired in MIC for insurance purposes?

I agree with snowhead and marvo, could be as simple as a bit of damp .

 

[Deleted member 9648]

As above,more thorough testing is needed than just tests at the DB. Only then will you know what you are up against.

 

[onions1066]

MICC is as tough as nails, as above narrow down the fault and rectify the faulty legs

 

[GLENNSPARK]

VOELCB's and RCD's aside, if a circuit is IR testing < 1Mohm it should be disconnected, the fault localised and repaired.

I'd also say the VOELCB should be replaced on sight as a matter of course. They were very unreliable at best and downright useless at worst. There's a school of thought that say it can remain as an isolator but I prefer if a device can't perform all it's designed functions it should be scrapped and replaced.

As for the mineral insulated cable I'd leave any that IR test okay and just replace the faulty sections. Obviously it would be wise and less stressful to do this whilst there's already building work happening.

Maybe in this particular scenario it would be worth looking at fitting a CU with individual RCBO's rather than a split RCD CU. This would prevent any future low insulation problems causing widespread power outages across multiple circuits.

rubbish....
its perfectly useable in its new found role as DP iso....complete with arc shutes!!
this means it can switch under load....

 

[Engineer54]

In readiness for that he today tested the existing circuits and found that the insulation resistance showed as being very low - zero on some circuits, 100K-400K on others, and I believe that 1M is required?

It won't be Zero, that would be a dead short, and things would have been going Bang!! Get an electrician in that knows about and understands the working of MICC cable.

I can't believe that ALL of these MICC circuit cables have failed IR testing, if they have, you are the most unluckiest householder in the country!! lol!! Far more likely, is that some buffoon that hasn't got a clue, has been playing around with this installation in the past!!

 

[Octopus]

I think that you should publish the test results - they sound odd to say the least.

 

[GLENNSPARK]

In readiness for that he today tested the existing circuits and found that the insulation resistance showed as being very low - zero on some circuits, 100K-400K on others, and I believe that 1M is required?

It won't be Zero, that would be a dead short, and things would have been going Bang!! Get an electrician in that knows about and understands the working of MICC cable.

I can't believe that ALL of these MICC circuit cables have failed IR testing, if they have, you are the most unluckiest householder in the country!! lol!! Far more likely, is that some buffoon that hasn't got a clue, has been playing around with this installation in the past!!

if they are flat....then it will no doubt be pots that have been interfered with....so just look for the pots first that have a `recently disturbed` look about em....then dismantle the joint....blowtorch time....drive out the damp....

 

[Rockingit]

The very fact the OP said the sparks had called his helpline suggests instantly that we have a less than experienced Domestic Installer here.

Get a second opinion!

 

[Rockingit]

It won't be Zero, that would be a dead short, and things would have been going Bang!!

Though I've always thought it very considerate of a fault to be low enough resistance to be able to trace an IR issue using a continuity stick!!

But yes, 0.00 Megohms does not equal 0 ohms!!

 

[ayjay]

the i/r will be in the pots unfortunatey sparkys these days are uesless unless their as old as me and grew up installing micc

 

[Knobhead]

I’m getting a distinct whiff of BS.

I’m amazed you lot have fallen for it.
(Or in it!)

 

[Darkwood]

Anyway with only about 6 people left in the country proficient in MICC terminations and most of them posting on this thread i suggest you beg for our help on site!

 

[Engineer54]

But yes, 0.00 Megohms does not equal 0 ohms!!

So what sort of ohm value would you equate a Zero reading on a modern digital IR or MFT tester??

 

[Krispy]

Thanks for all your input, I will discuss with my Sparks and report back.

It won't be Zero, that would be a dead short, and things would have been going Bang!!

As I understand it the test was from Neutral to Earth - I'm presuming that is why its not going bang?!

My Sparks is certainly not young, and has come across it before but I expect that domestic installers don't bump into mineral wiring much. The only reason I can think of why the house was wired with mineral is that the whole thing is very solidly built - concrete floors and roof for example - so probably built for an "engineer"

 

[Lucien Nunes]

Got caught out with a similar set of results a few years ago. Very low IR readings at the first EICR carried out on an MI installation in a church were found to be caused by widespread seal failure, rather than the odd one or two bad ends. Early 60s job, reasonable standard of workmanship with all 660V cable neatly dressed and terminated, about 20 circuits with lots of 7 core 0.002 in the lighting. We had planned to test it and if generally OK as we expected, replace the Bill Vertex DB and make localised repairs as needed.

Initial IR tests were all disappointing so we started breaking some circuits down into single runs. IIRC about 1 in 3 runs was <1MΩ, the leaky seals were spread all over the building not just the obvious damp and inaccessible areas. Because it was all imperial cable and would be a very slow job to remedy, plus the likelihood of further problems, we agreed with the customer that rewiring was the most sensible option and we started pulling in FP as soon as they could get an emergency faculty granted.

It seems one or more of the installers was not fitting and filling the pots properly, they had many decades of letting in moisture before this first test was done. I was disappointed because I have great faith in MI and had not experienced this level of widespread failure before. I would gladly have redone it all in new MI but time was against us!

 

[Deleted member 9648]

Thanks for all your input, I will discuss with my Sparks and report back.



As I understand it the test was from Neutral to Earth - I'm presuming that is why its not going bang?!

My Sparks is certainly not young, and has come across it before but I expect that domestic installers don't bump into mineral wiring much. The only reason I can think of why the house was wired with mineral is that the whole thing is very solidly built - concrete floors and roof for example - so probably built for an "engineer"

It's unlikely (but not impossible) that you would get only a N-E dead short reading on an existing MICC with the live being clear ......it is more likely that the reading is from connected equipment.

 

[Rockingit]

So what sort of ohm value would you equate a Zero reading on a modern digital IR or MFT tester??

My MFT on 500V IR test resolves to 0.000 Meg, so in effect that's 1K. Quite often I'll then use the continuity setting (<2k) and see readings of a few ohms (at which point my continuity buzzer will work quite happily).

Potentially, this could catch the unwary as a resistance of say 1k ohms would fail an IR test but not always pull sufficient current for ADS, even with an RCD, so you could actually see 0.000 on your IR test result and still have a 'working' circuit.

 

[Engineer54]

My MFT on 500V IR test resolves to 0.000 Meg, so in effect that's 1K. Quite often I'll then use the continuity setting (<2k) and see readings of a few ohms (at which point my continuity buzzer will work quite happily).

Potentially, this could catch the unwary as a resistance of say 1k ohms would fail an IR test but not always pull sufficient current for ADS, even with an RCD, so you could actually see 0.000 on your IR test result and still have a 'working' circuit.

Not for long i'd warrant!! lol!!

 

[GLENNSPARK]

Got caught out with a similar set of results a few years ago. Very low IR readings at the first EICR carried out on an MI installation in a church were found to be caused by widespread seal failure, rather than the odd one or two bad ends. Early 60s job, reasonable standard of workmanship with all 660V cable neatly dressed and terminated, about 20 circuits with lots of 7 core 0.002 in the lighting. We had planned to test it and if generally OK as we expected, replace the Bill Vertex DB and make localised repairs as needed.

Initial IR tests were all disappointing so we started breaking some circuits down into single runs. IIRC about 1 in 3 runs was <1MΩ, the leaky seals were spread all over the building not just the obvious damp and inaccessible areas. Because it was all imperial cable and would be a very slow job to remedy, plus the likelihood of further problems, we agreed with the customer that rewiring was the most sensible option and we started pulling in FP as soon as they could get an emergency faculty granted.

It seems one or more of the installers was not fitting and filling the pots properly, they had many decades of letting in moisture before this first test was done. I was disappointed because I have great faith in MI and had not experienced this level of widespread failure before. I would gladly have redone it all in new MI but time was against us!

and please be my guest Lucien in continuing to do so....
poor potting is not poor MI is it....

 

[telectrix]

and please be my guest Lucien in continuing to do so....
poor potting is not poor MI is it....

and, as it was done in the 60's we can't blame the Electrical Trainee's, can't blame maggie thatcher, nor part pee, i know, qe'll blame winston churchill. he was around then.

 

[Lucien Nunes]

No that's true enough, I had just never had such a bad experience of low IR even on a system of that age before, perhaps I've been lucky.

 

[telectrix]

if the N's are still connected in the DB, you may only have 1 or 2 N-E faults. ( parallel paths, and all that.)