Insulation Resistance

[JohnW2]

I’m hoping that I might be able to benefit from the experience of some of you here. Although my ‘experience’ spans very many years, since I’m not an electrician, that experience necessary relates to a very small number of electrical installations.

PVC is an incredibly good insulator, even when ‘old’. Over all the years, the (very) few times I have come across ‘measurable’ insulation resistance in domestic wiring has been when it has transpired that there has actually been a genuine problem of serious cable damage or an accessory full of water. On all other occasions, provided I’ve been very careful to eliminate everything other than cables and their terminations from the tests, I have invariably got ‘immeasurably high’ readings – commonly “>500MΩ” with 500V testing.

However, on the basis of what I’ve seen written and said, I get the impression that it’s not necessarily all that uncommon to see measurable, even if high, IR in installations that are deemed to be OK. Is this the case and, if so, is it genuinely due to deterioration of the wiring’s insulation, or something else? ... and, if one does find measurable IR, does the situation tend to deteriorate fairly rapidly thereafter. I have to say that, knowing how good PVC insulation is, if I came across a measurable IR I think I would take it to be an indication of ‘something probably going seriously wrong’ and would not want that situation to persist – but maybe I’m over-cautious because of my very limited exposure to real-world situations?

Any insights from the experienced would be much appreciated.

Kind Regards, John.

 

[westie101]

As regards installations I don't know. But as a comparison we test 11kV (6350 L-E) circuits with a 5kV megger, anything over 10MΩ in some circumstances is deemed acceptable, figures up to 5GΩ can be obtained

 

[JohnW2]

As regards installations I don't know. But as a comparison we test 11kV (6350 L-E) circuits with a 5kV megger, anything over 10MΩ in some circumstances is deemed acceptable, figures up to 5GΩ can be obtained

Interesting, although obviously a bit peripheral to my main interest!

Mind you, in terms of leakage current, 10MΩ on a 11kV circuit is equivalent to best part of 500MΩ on a 230V one. I also suspect that you may not be talking about PVC insulation. Even though I was brought up on 'wind-up'(hand generator) meggers, I don't think I've ever tested with more than 1000V.

Kind Regards, John.

 

[westie101]

A bit of a drift!

Even though I was brought up on 'wind-up'(hand generator) meggers, I don't think I've ever tested with more than 1000V.

I used a 2.5kV wind up once! Before XLPE insulation became common we used to test up to 13.5kV (the test equipment goes up to 50kV)

 

[flameport]

My experience - 'low' insulation resistance (i.e measured in the low 100s or sometimes 50-100 range) on PVC flat wiring is never the wiring itself, unless it has been physically damaged, or the circuits are excessively long.

Usually it is the terminations, or what is connected to them. Greasy old sockets and switches in kitchens being a common culprit, as are backboxes full of dirt/dust and other mess.
The worst examples are those where wiring has been run in the wall cavity as this leads to significant amounts of dirt entering through the holes in the backboxes.

Have had readings go from 5M to well over 100M just by opening socket outlets and removing the dirt and dust from the back of the sockets.

 

[JohnW2]

My experience - 'low' insulation resistance (i.e measured in the low 100s or sometimes 50-100 range) on PVC flat wiring is never the wiring itself, unless it has been physically damaged, or the circuits are excessively long. Usually it is the terminations, or what is connected to them. Greasy old sockets and switches in kitchens being a common culprit, as are backboxes full of dirt/dust and other mess.

Thanks. That's what I would expect. Do you therefore agree with what I said - namely that if one does find a 'measurable' IR in the wiring itself, that should not be allowed to go unresolved?

What rather startles me, to put it very mildly, is that (if I'm understanding correctly), the regs only require that IR of an installation be >1MΩ. Have I really got that right? I would be petrified if I knew the IR was even remotely close to that figure in my house!

Kind Regards, John.

 

[flameport]

the regs only require that IR of an installation be >1MΩ. Have I really got that right?

Yes, although the important word there is 'installation', rather than the individual circuits.
In a domestic situation, any single new circuit not in the 100M+ range would be cause for investigation.

However with installations containing larger numbers of final circuits, the overall insulation resistance can be much lower, simply due to the fact of parallel resistances. For example, a system with 20 final circuits each being 100M, the total would be 'only' 5M, however that would still be perfectly acceptable.

If an isolated section of cable tested under 100M, that would probably call for it to be replaced, however in the real world, such tests are rarely carried out, as that would require significant dismantling of the installation.

 

[JohnW2]

the regs only require that IR of an installation be >1MΩ. Have I really got that right?

Yes, although the important word there is 'installation', rather than the individual circuits. .... However with installations containing larger numbers of final circuits, the overall insulation resistance can be much lower, simply due to the fact of parallel resistances. For example, a system with 20 final circuits each being 100M, the total would be 'only' 5M, however that would still be perfectly acceptable.

Sure, but therein would be one of my main concerns. You say that an overall IR of a 20-circuit installation of 5MΩ would be 'perfectly acceptable' - but that's because you are thinking of it as 20 circuits each having an IR of 100MΩ. However, if all you have is your 5MΩ overall figure, how can you know that there are not 19 final circuits with very high IRs and one with a (very worrying) IR not much over 5MΩ??

Doing an installation-wide IR test is all very well if the result is very high, but with a very low acceptance limit (as in the regs) it seems to be a recipe for missing very serious problems. Am I missing something?

Furthermore, the regs appear to regard an overall IR acceptable if it is >1MΩ, regardless of how many (or few) circuits may be involved. Small domestic installations with, say, 8 or less final circuits cannot be that uncommon, but would you really 'pass' one of them (without investigation) if the overall installation IR was, say, only 2.5MΩ?

Kind Regards, John.

 

[PrenticeBoyofDerry]

Bearing in mind most electrical installations are wired in parallel, this will have an effect on the overall insulation resistance of the system being tested. As the more resistances there is in parallel the lower the reading will be.
Although the readings can be quite low, this does not mean there is a fault.
If the reading was one of concern you then break down the tests.
This can be done by testing smaller sections or individual circuits.
Also remember that capacitors and neons should be disconnected as this will give you false readings.
Then if a circuit does test out at say about 2 Mohms although an except-able reading, would be worth further investigation.
My findings have often been that circuits run in damp areas, cause resistance to lower, ie under floorboards at ground level, just done one were a spring runs under the house lots of moisture in the air and ground, was measuring out at 12 Mohms. It didn't help that junction boxes were also down there connecting up spurs and extending the circuit. Newly plastered walls can also bring the readings down, as the moisture still in the plaster can effect the cable.

 

[JohnW2]

Bearing in mind most electrical installations are wired in parallel, this will have an effect on the overall insulation resistance of the system being tested. As the more resistances there is in parallel the lower the reading will be....

Sure, that's what I've was discussing but, as I pointed out to flameport, there's a problem. If one only knows the over IR of all circuits in parallel, then one can only guess what that individual circuit's IRs may be.

Flameport said that an IR less than 100M&#937; in a single final circuit would be a cause for investigation, and that sounds sensible to me; admittedly he said 'new', but (perhaps over-cautiously) I personally would want to look into the cause for an IR <100M&#937; in a single circuit, no matter what the age. He then went on to say that an overall IR of 5M&#937; would be 'perfectly acceptable' for an installation consisting of 20 final circuits. As I wrote to him, that's because he was thinking of the 'best case scenario' in which each of the 20 circuits had an IR of 100M&#937;, hence 5M&#937; when in parallel. However, as I pointed out to him, there are far more worrying situations which could result in an installation IR of 5M&#937;. Choosing figures which make the sums easy, if, for example, 19 circuits each had an IR of 570M&#937; and the 20th one had an IR of 6M&#937;, the overall IR would be exactly 5M&#937; - and I would have hoped that no-one would regard that at being acceptable without some investigation. However, in terms of single circuits, you go on to say:

Then if a circuit does test out at say about 2 Mohms although an except-able reading, would be worth further investigation.

...which is essentially what the regs imply, but would you really be comfortable not to investigate a single circuit whose IR was only just over 2M&#937;?

My findings have often been that circuits run in damp areas, cause resistance to lower, ie under floorboards at ground level....

Yes, one would expect that, if moisture had got into JBs and/or accessories (dampness should not affect the cable itself).

Newly plastered walls can also bring the readings down, as the moisture still in the plaster can effect the cable.

Yes, I can believe that, too -but, again, that's only going to happen if dampness from the plaster gets into accessories - PVC is very impervious to water. In the past, I did experiments in which I left PVC cables submerged in water for months, and that had no effect on IR.

Kind Regards, John

 

[PrenticeBoyofDerry]

If an installation were new I would only expect very high readings.
If the installations was not new, it would be handy to have previous test results to compare to, but we know this is not always the case.
As far as low readings a concerned, say you were doing a PIR and you got good readings on all circuits bar one, and that being a reading of 5 Mohms.
I would be happy to say it complied, I would talk to the person ordering the work, if they wished for the circuit to investigated further I would. If not, I have done my job, providing installation checks out and disconnection times are met and it is fit for continued use.
It may be a case that the next inspection date would be brought forward to an early date than normal, to see how the system is fairing, with that particular reading in mind.

 

[ericmark]

When doing "Transmittion Lines" in uni they were represented as a series of induction and capacitive links together will some resistance. You will note how the resistance reading changes as the lines charge up. As the installation gets bigger and bigger the time increases and if you study the charge curves for a capacitor it never is fully charged.

So we don't want to wait for ever and a day therefore limits are set.

Although 1 meg is often considered the pass point in real terms we look at how long the cable is and for a normal house one would expect 1mohm pass for whole, but much higher for part of the installation.

If when one tries to lay down rules for a single circuit one must remember in an industrial installation we could have a mile of cable. So hard to set limits.

I would agree at 100mohm I would not investigate but that does not mean that at 5mohm I would. It would depend on what I was testing. The main problem is dust and damp and of course this is not found inside the cable but on it's termination. Normally we don't completely disconnect a cable and clean ends to test but simply switch off.

Where monitoring devices are used these can introduce a path to earth so one has to allow for this.

The biggest problem is where people think they should not show you previous results seems they want one to re-invent the wheel.

To me armed with previous results one is looking for readings within 5% of previous readings rather than looking for a pass limit.

It is often a case of using our experience and brains rather than try to follow a rule book. When working on a TBM had I condemned all items under 1mohm we would not have cut much rock.

 

[JohnW2]

If an installation were new I would only expect very high readings. .... As far as low readings a concerned, say you were doing a PIR and you got good readings on all circuits bar one, and that being a reading of 5 Mohms. I would be happy to say it complied, I would talk to the person ordering the work, if they wished for the circuit to investigated further I would.

Thanks. As I've been discussing, the problem is that, unless you test each circuit separately, you would not actually know that you were dealing with the situation you describe (just one low IR, all other circuits high). If all you had was the overall IR for the whole installation, the only thing of which you could be certain was that no circuit had a lower IR than that overall figure.

I've let this thread drift a bit in the direction of discussion, but what I really was hoping was to get a feel for the sort of IR measurements people are commonly getting on domestic installations, and what 'trigger levels' they are applying for further investigation ... so can you (or anyone else) give me some idea as to the sort of IR figures you usually see (for entire, not new, domestic installations) and what value of (whole installation) IR would make you want to measure individual circuits, and maybe investigate further?

If It may be a case that the next inspection date would be brought forward to an early date than normal, to see how the system is fairing, with that particular reading in mind.

Yes, that certainly makes sense. As in so many testing situations, a deteriorating result is much more significant than an unchanging (altghough not perfect) one.

Kind Regards, John.

 

[JohnW2]

Thanks, Eric. I am, of course, specifically interested in and talking about domestic installations, so very long cable runs are not an issue. Certainly within a domestic environment, I don't think capacitance of the cable is really an issue and I presume that there is as small delay after applying the DC before the testers 'make the measurement', so as to allow the 'capacitors' to charge to fairly near the full voltage (i.e. so that very little 'charging current' is still flowing) - that's probably only a few milliseconds for the sort of capacitances we're likley to be dealing with. It's certainly the case that if I test a full 100m reel of any cable, I invariably get an 'unmeasurable' result.

Although 1 meg is often considered the pass point in real terms we look at how long the cable is and for a normal house one would expect 1mohm pass for whole, but much higher for part of the installation. I would agree at 100mohm I would not investigate but that does not mean that at 5mohm I would. It would depend on what I was testing.

Thanks. What, say, if you were testing a bog-standard domestic installation, in which you'd done all you could to eliminate any possible factors which might affect IR measurements? Would you then feel the need to investigate if the total installation IR was around 5M&#937;?

Kind Regards, John.

 

[PrenticeBoyofDerry]

I find that when I am testing the whole installation, it would normally be a new installation measuring form the tails.
When doing work on older installation this is not always possible or practical.
So I generally test each circuit individual. (I mostly work in the domestic sector)
But If measuring from the tails again the guideline would be 2Mohms.
You could have say 6 circuits on a board with the overall resistance reading being 1.85Mohms.
But once all circuits are tested individually there would be no fault found and all the readings complied.