Hello I was jus wondering if anybody could help me...I would like to know how effective the insulation resistance test is. For example if you sliced the grey on your twin and earth and the insulation was damaged just slightly sheathed not to the copper would that be picked up in a Insulation resistance test ? Or does the Insulation have to be cut all the way to the copper for it to be picked up on the test. Also if the insulation was sheathed to copper at normal operating temperature 70 degrees what would happen to the cable?
insulation resistance
- Thread starter swift2
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You wont pick up damaged insulation such as you describe with an IR test - the insulation resistance test is more of a test that it is electrically OK as opposed to it being a shock hazard.
I don't follow what you are asking in the 2nd part - 70deg cable is designed to run at a maximum of 70deg.
I don't follow what you are asking in the 2nd part - 70deg cable is designed to run at a maximum of 70deg.
Hey thanx spark for the reply. For the 2nd part I meant if the cable was damaged, for example in or near a joist hole at the operating temperature would that be a fire hazard ?
Not even that.
Try a little experiment.
Get a bit of 2.5mm² T/E, strip back 10-20mm of sheath, cut the cpc away and then strip each core right back. Stick the ends into a 2-way piece of choc so that you can run conductors from there to attach your tester to without changing the spacing of the L & N cores. Do a IR test on them - see if you get a low reading.
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Takes more than 70°C to ignite wood. Or even char it.
thank u for the reply ban-all-sheds, could you just tell me what you mean by "changing the spacing on the L and N cores please.
Spark, by arcing would that mean both conductors would have to have no insulation on for it to travel through the air ?
If you just connect your clips to the ends of 2 long, free conductors a few mm apart they're going to move about.
Either apart which might make you question the validity of the test, or so that they touch, which would give you a low IR...
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I think the point is air is an insulator under normal conditions.
You can get ionisation of the atmosphere and then air conducts with some dramatic effects but unlikely to get this with an insulation tester.
What does conduct electricity is impurities in water. An example would be damp dust and where a cable has been damaged sometimes due to dust and moisture an insulation tester may find it.
As to power attracting or repelling cables with a 300 amp DC welding set used for air arc I have seen this effect. However not so easy to see with low amperage and non flexible cables.
Electricity can travel through air and I have watched electric storms many times. I have also used my radio to detect alternating current of certain frequencies. Capacitance and inductance can even allow electricity to transfer to other items when the insulation as A1.
The PAT tester has a function to test insulation with around 15000 volts and with that sort of insulation tester you may be able to detect thin insulation it is designed to test re-built items in case any screws are too long and are about to break through the insulation it is only done with re-built equipment and not a standard test. Mainly as it can completely destroy items if used in wrong way.
As to second part of question at 70 deg as already said not really a problem but as the types of cable which allow higher temperatures to be used then care is required. 90 deg is getting quite hot and would burn one if you touched it and it seems cables able to take well over this temperature are still rated at 90 deg one would assume because running hotter could cause problems to items touching them. Mineral insulated cables can run far hotter.
But time is also a factor and where my son works the cables have lasted around 3 years before the temperature has caused the insulation to creep until it caused a short. He works in a glass factory and of course it takes a lot of heat to melt glass. However it was the time that was surprising one would consider if it was OK for first 3 years then it was likely to run for ever but that was not the case and when the undamaged (It seemed) cables were inspected the insulation was no longer concentric and the copper was closer to inside on all bends.
You likely have a good reason for asking and if you relate as to why you want to know you are likely to get a far better answer.
For example where someone has caught a cable with a drill concealed in a wall using an insulation tester directly after the event is unlikely to find a fault. But repeat the test in 5 years time after dust and moisture has gathered then you very likely will detect a fault. This is why we are not only supposed to do periodic inspection reports but also we should compare results with last test and note any trends.
So why do you ask????
You can get ionisation of the atmosphere and then air conducts with some dramatic effects but unlikely to get this with an insulation tester.
What does conduct electricity is impurities in water. An example would be damp dust and where a cable has been damaged sometimes due to dust and moisture an insulation tester may find it.
As to power attracting or repelling cables with a 300 amp DC welding set used for air arc I have seen this effect. However not so easy to see with low amperage and non flexible cables.
Electricity can travel through air and I have watched electric storms many times. I have also used my radio to detect alternating current of certain frequencies. Capacitance and inductance can even allow electricity to transfer to other items when the insulation as A1.
The PAT tester has a function to test insulation with around 15000 volts and with that sort of insulation tester you may be able to detect thin insulation it is designed to test re-built items in case any screws are too long and are about to break through the insulation it is only done with re-built equipment and not a standard test. Mainly as it can completely destroy items if used in wrong way.
As to second part of question at 70 deg as already said not really a problem but as the types of cable which allow higher temperatures to be used then care is required. 90 deg is getting quite hot and would burn one if you touched it and it seems cables able to take well over this temperature are still rated at 90 deg one would assume because running hotter could cause problems to items touching them. Mineral insulated cables can run far hotter.
But time is also a factor and where my son works the cables have lasted around 3 years before the temperature has caused the insulation to creep until it caused a short. He works in a glass factory and of course it takes a lot of heat to melt glass. However it was the time that was surprising one would consider if it was OK for first 3 years then it was likely to run for ever but that was not the case and when the undamaged (It seemed) cables were inspected the insulation was no longer concentric and the copper was closer to inside on all bends.
You likely have a good reason for asking and if you relate as to why you want to know you are likely to get a far better answer.
For example where someone has caught a cable with a drill concealed in a wall using an insulation tester directly after the event is unlikely to find a fault. But repeat the test in 5 years time after dust and moisture has gathered then you very likely will detect a fault. This is why we are not only supposed to do periodic inspection reports but also we should compare results with last test and note any trends.
So why do you ask????
Hello Ericmark, thank you for your reply. The reason for me asking the question was a scenario where cables run through joist holes and sometimes where there is a nail going through the hole, or the hole has not been cut out properly which results in the cable being sheathed and sometimes even the insulation. I was wondering if that was the case inside the joist would that be a fire hazard or not, also would an insulation resistance test pick up on that. Would the P.A.T testers test be allowed to be done on twin and earth cables or is it only for appliances ??
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At my parents-in-law's place, I installed a fused spur in the downstairs loo. There was plenty of space, so I gave it its own breaker on the RCD side of a 16th split board.
There was part of the run I could not access to clip, so the cable hung down.
Several months later the rains came & the underfloor space got very soggy.
The RCD started popping off.
I IR'd the cable and it was woefully down.
Having ripped it out & replaced it, I examined it to see the fault.
After 30 minutes close inspection, I found a pinhole in the sheath. Whatever had caused this had penetrated (just) the neutral conductor's insulation.
There was part of the run I could not access to clip, so the cable hung down.
Several months later the rains came & the underfloor space got very soggy.
The RCD started popping off.
I IR'd the cable and it was woefully down.
Having ripped it out & replaced it, I examined it to see the fault.
After 30 minutes close inspection, I found a pinhole in the sheath. Whatever had caused this had penetrated (just) the neutral conductor's insulation.
Hello Secure thank you for your post, it was very helpful...for example if you were clipping direct and you nicked the cable later on after the cable has worked at its operating temperature the IR will go down ? however if you were to have done a IR test just after you had installed it would the result have been >2000? is this correct ?
Im not saying at all you nicked that cable just giving that as an example just in case you were thinking that.
Im not saying at all you nicked that cable just giving that as an example just in case you were thinking that.
swift2, it's nothing to do with whether the cable has been at operating temperature or not.
Insulation resistance is measured between two conductors. Nicking the insulation of one, or even both, will make no difference. In fact as BAS has said you could remove the insulation completely and the conductors will still be insulated by the air gap between them. However nicking the insulation and then letting the cable get damp and dirty will allow current to flow through the dirt, which will reduce the insulation resistance.
Insulation resistance is measured between two conductors. Nicking the insulation of one, or even both, will make no difference. In fact as BAS has said you could remove the insulation completely and the conductors will still be insulated by the air gap between them. However nicking the insulation and then letting the cable get damp and dirty will allow current to flow through the dirt, which will reduce the insulation resistance.
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holmslaw
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