Discuss Insulation resistance of Pink Fibre in the Electrical Forum area at ElectriciansForums.net

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Hi there,
Does any one know what kind of insulation resistance to expect for pink fibre wall/ceiling insulation? I'm trying to figure out if the cheap insulation meter I have is actually giving me a reasonable answer.

here's a bit of background:
I have a GFCI mounted in an electrical panel for a bathroom floor heating system that keeps tripping, and it seems to be erratic - sometimes it can stay on for weeks or months, other times it can trip within minutes it's not usually instantaneous. The heating control thermostat is also GFCI protected - I know I have 2 GFCIs on the same circuit, but it's one at each end of the same cable with nothing else connected. I'm reluctant to change the panel GFCI for a breaker until I know what's going on.

So, I completely disconnected the cable from the power panel and everything at the other end - which should have left a totally isolated wire. I then measured the insulation resistance with this cheap insulation meter that I was sent by accident (I had ordered something else) and I was getting a reading of about 10-20Mohm at a purported 1KV. In principle, not particularly concerning, except I was expecting an out of range reading.

I pulled the cable from the wall I know had been involved in a renovation, thinking that maybe the guy who did the bathroom had nicked the sheathing, but it was clean. I was still getting a 20-30Mohm from the freed cable end. However, I noticed that the readings changed dramatically. depending on where I probed (one probe on the cable ground wire, the other just hunting).

I found that putting the probes into the pink glass fibre insulation actually gave me a 20Mohm reading, no cable or copper involved, just probing the insulation. When the probes were close together, I got a reading of around 5-6Mohms - separating the probes in resulted in about 20Mohm, which more or less stayed constant, even when the probes were stretched as far as I could make them. Is this what you'd expect?

I suspect that the fact that I'm actually getting a reading means that the cable has been exposed somewhere, maybe squirrels, mice or a raccoon (we've had all three at some stages in the the last 20 years), and possibly the conductors are now touching the pink insulation, but not each other. But, before I cut the cable to try and isolate the problem area, it would be useful to know if I'm fighting a phantom insulation reading from a cheap Chinese meter. Just seems odd to me that the insulation resistance doesn't seem to change dramatically with probe separation. I actually have a ROD-L M100BVS5 Hipot Tester, but I would have to repair it before I could use it - I want to avoid that route if I can.

Thanks in advance for any suggestions, or help.
Peter
 
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Petej999

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Hi there,
Does any one know what kind of insulation resistance to expect for pink fibre wall/ceiling insulation? I'm trying to figure out if the cheap insulation meter I have is actually giving me a reasonable answer.

here's a bit of background:
I have a GFCI mounted in an electrical panel for a bathroom floor heating system that keeps tripping, and it seems to be erratic - sometimes it can stay on for weeks or months, other times it can trip within minutes it's not usually instantaneous. The heating control thermostat is also GFCI protected - I know I have 2 GFCIs on the same circuit, but it's one at each end of the same cable with nothing else connected. I'm reluctant to change the panel GFCI for a breaker until I know what's going on.

So, I completely disconnected the cable from the power panel and everything at the other end - which should have left a totally isolated wire. I then measured the insulation resistance with this cheap insulation meter that I was sent by accident (I had ordered something else) and I was getting a reading of about 10-20Mohm at a purported 1KV. In principle, not particularly concerning, except I was expecting an out of range reading.

I pulled the cable from the wall I know had been involved in a renovation, thinking that maybe the guy who did the bathroom had nicked the sheathing, but it was clean. I was still getting a 20-30Mohm from the freed cable end. However, I noticed that the readings changed dramatically. depending on where I probed (one probe on the cable ground wire, the other just hunting).

I found that putting the probes into the pink glass fibre insulation actually gave me a 20Mohm reading, no cable or copper involved, just probing the insulation. When the probes were close together, I got a reading of around 5-6Mohms - separating the probes in resulted in about 20Mohm, which more or less stayed constant, even when the probes were stretched as far as I could make them. Is this what you'd expect?

I suspect that the fact that I'm actually getting a reading means that the cable has been exposed somewhere, maybe squirrels, mice or a raccoon (we've had all three at some stages in the the last 20 years), and possibly the conductors are now touching the pink insulation, but not each other. But, before I cut the cable to try and isolate the problem area, it would be useful to know if I'm fighting a phantom insulation reading from a cheap Chinese meter. Just seems odd to me that the insulation resistance doesn't seem to change dramatically with probe separation. I actually have a ROD-L M100BVS5 Hipot Tester, but I would have to repair it before I could use it - I want to avoid that route if I can.

Thanks in advance for any suggestions, or help.
Peter
What were you measuring the IR of the pink fiber against?
 
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What were you measuring the IR of the pink fiber against?
I measured the IR in serval places directly in tha attic, and I also happened to have a bat of the stuff in an area of the house that hasn’t been renovated yet. Without being particularly scientific about it, it seemed that I was getting the same magnitude of reading no matter where I placed the probes, but both probes had to be in the fibre.

Measuring across/along a wooden joist showed 60-100Mohm. Probes in free air showed over range.
 
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Petej999

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I measured the IR in serval places directly in tha attic, and I also happened to have a bat of the stuff in an area of the house that hasn’t been renovated yet. Without being particularly scientific about it, it seemed that I was getting the same magnitude of reading no matter where I placed the probes, but both probes had to be in the fibre.

Measuring across/along a wooden joist showed 60-100Mohm. Probes in free air showed over range.
Don't understand what you are trying to achieve.
 
James

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Here is a useful guide

http://www.electriciansforums.co.uk/Inspection-and-Testing-Insulation-Resistance-Test.php
 
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I'm hoping to get some confidence that the results I'm seeing are realistic.

It's odd to me to see a measurable resistance in something that I would have expected to see an over range for. Glass and air are supposed to be good insulators (electrical as well as thermal)
 
James

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The resistance of glass fibre itchy stuff is irrelevant.
at no point should it be in contact with anything live.

also, dust and moisture are conductive and also can be flammable.
 
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The resistance of glass fibre itchy stuff is irrelevant.
at no point should it be in contact with anything live.

also, dust and moisture are conductive and also can be flammable.
That makes sense.

I’m getting a measurable reading between the conductors of an isolated cable and the pink insulation. That indicates to me the cable insulation has been breached - probably by rodents, because the cables have been installed neatly and with a lot of care. The task I have is finding out where. Knowing the characteristics of IR for the insulation I’m hoping .will help - I don’t relish digging through the itchy stuff if I can help it.

Theoretically the IR should reduce the closer I probe to the breach. I’ve used a similar technique with success in vehicle wiring

(Edit) what I didn’t mention is the cable run is 30 to 40’ long before the cable gets to a point where the pink stuff ends (it’s not a small house)
 
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I'm sure someone will correct me if I'm wrong. After a bit of looking around on the web, it would appear that this is a real world instance of the infinite lattice resistance problem. The maths and analysis is more suited to someone with a more agile mind than mine.

If you are interested, George Hnitiuk gives a run down of the problem in a series of Youtube videos starting here

But an answer on physics.stackexchange.com here, shows:
"the increase in the number of paths between points can grow fast enough to balance the increase in path length"

and

"the resistance between two points asymptotes to a constant as the points diverge"

Hopefully I've not taken this out of context, but from empirical observation (and roughly showed) it appears that no matter how far the probes in the fibre bat are separated, the IR will be roughly constant. That is, unless the probes are close together.

For my particular issue this means, I think, that the cable sheath breach could be 1ft from where I'm looking to 30 ft away, and the only way to find out where it is, it to physically look.

Ah well! I'll just have to resort to cutting the cable, putting in an electrical box and splicing a replacement cable. What a pain.
 
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I'm sure someone will correct me if I'm wrong. After a bit of looking around on the web, it would appear that this is a real world instance of the infinite lattice resistance problem. The maths and analysis is more suited to someone with a more agile mind than mine.

If you are interested, George Hnitiuk gives a run down of the problem in a series of Youtube videos starting here

But an answer on physics.stackexchange.com here, shows:
"the increase in the number of paths between points can grow fast enough to balance the increase in path length"

and

"the resistance between two points asymptotes to a constant as the points diverge"

Hopefully I've not taken this out of context, but from empirical observation (and roughly showed) it appears that no matter how far the probes in the fibre bat are separated, the IR will be roughly constant. That is, unless the probes are close together.

For my particular issue this means, I think, that the cable sheath breach could be 1ft from where I'm looking to 30 ft away, and the only way to find out where it is, it to physically look.
A simpler way of looking at the above (I think), is the concept of resistance per square.
If you take a thin square section of a resistive material, and measure the resistance across it, the resistance will always be the same, whatever the size of the square. 1cm square, 1 metre square, or 1 km square, the resistance will be the same.
 
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A simpler way of looking at the above (I think), is the concept of resistance per square.
If you take a thin square section of a resistive material, and measure the resistance across it, the resistance will always be the same, whatever the size of the square. 1cm square, 1 metre square, or 1 km square, the resistance will be the same.
I personally wasn’t aware that such a unit existed, but it now makes perfect sense and ties really nicely into the phenomenon I am seeing. I guess this unit came into favour after I stopped being active in Electronic design Engineering.

I think that for the general electrical practitioner all they need to know is that if there’s a measurable resistance between an isolated cable conductor and fibre insulation, then there’s probably a cut or exposed cable touching the insulation somewhere along the cable. There’s probably a few caveats that go along with this too. For the most part I think this is a very useful bit of knowledge.

I’ve not had a chance to confirm this on my cabling yet, but I think it’s looking pretty certain that’s the answer.
 
Avo Mk8

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This has been an interesting foray into the resistivity of thermal insulation. It even prompted me to look into how glass fibre/wool was made, and what with the lubricants they use spinning the glass, and the polymers they add, it seems the stuff may not be as electrically insulating as one might suppose.
But returning to your initial post, none of the resistance values you report would cause a GFCI to trip, unless I'm missing something. You might not yet have found the cause 🤔?
 
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This has been an interesting foray into the resistivity of thermal insulation. It even prompted me to look into how glass fibre/wool was made, and what with the lubricants they use spinning the glass, and the polymers they add, it seems the stuff may not be as electrically insulating as one might suppose.
But returning to your initial post, none of the resistance values you report would cause a GFCI to trip, unless I'm missing something. You might not yet have found the cause 🤔?
I totally agree. GFCIs typically need about 5mA to trip or 24Kohm. My readings are about 1000 times greater than that.

I’m pretty sure what I’m seeing is a secondary effect. The primary effect is possibly a localized bridging by contaminants of live to ground, which are separated by millimeters. As the tripping is erratic I suspect weather and seasonality to change the humidity in the insulation, or whatever’s touching the cable. Anyway that’s speculation until I find the real cause - I see the IR as a clue at this stage.
 

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