'Insulation Resistance'

[JohnW2]

'Insulation Resistance' seems to be a bit of a misnomer. As far as I can make out (and certainly within my very limited experience), unless one is dealing with 'pre-PVC' cables, when one sees a low 'IR' measurement, it is invariably due to mechanical/thermal cable damage (e.g. nails/screws, trapped within back box etc.), moisture/water, crud (dirt etc. - including dead animals!) or actual exposed conductor-to-conductor contact (e.g. conductors come adrift within accessories/enclosures etc.).

In the absence of any such factors, has anyone ever seen a low IR reading which (by elimination of everything else) they have believed/presumed was the result of deterioration of PVC insulation (i.e. true 'reduced insulation resistance')?

Kind Regards, John.

 

[ChrisRogers]

yes,

PVC cable does degrade with age, and I have removed cable with poor IR readings.

 

[JohnW2]

yes, PVC cable does degrade with age, and I have removed cable with poor IR readings.

Interesting. can you recall how low an IR have you seen (in the absence of any apparent mechanical damage)?

Kind Regards, John.

 

[333rocky333]

sometimes there is manufacturing faults also, i recall major problems with the cable installed at the British Library, and the contracters were sued i recall,

 

[JohnW2]

sometimes there is manufacturing faults also, i recall major problems with the cable installed at the British Library, and the contracters were sued i recall,

Yes, I forgot to mention that. Having a tendency to paranoia, I generally do an IR check on any new drum of cable I buy (and have never found a measurable IR) - but, as you imply, in the (I assume very rare) cases in which low IR is found in the absence of any obvious damage etc., it's possible that the cable in question has been like that 'since birth'. That's presumably become increasing less common in recent decades, because of the increasing proportion of installed cables which will have been IR tested in situ after installation, but there probably are still some defective cables that get installed without detection at the time.

Have any of you electricians had to remove an undamaged cable you've just installed because of failure of the IR dead test (assumed due to defective cable) at the time of installation?

Kindest Regards, John.

 

[holmslaw]

..

 

[ban-all-sheds]

They should not have sued.

They should have prosecuted for a breach of the CD&MR - a criminal offence.

 

[SimonH2]

Also, many materials (PVC is one I believe) are not waterproof. If you have a cable in a permanently wet environment, then unless it's got a suitable (undamaged) sheath it will absorb water over time - and that will eventually affect it's insulation properties.

 

[EFLImpudence]

it will absorb water over time

I think everything will - "over time".

Over time - your windows (the glass) will drip.

 

[JohnW2]

Also, many materials (PVC is one I believe) are not waterproof. If you have a cable in a permanently wet environment, then unless it's got a suitable (undamaged) sheath it will absorb water over time - and that will eventually affect it's insulation properties.

In a literal sense, you're probably right but, in the case of materials like PVC, one might be talking many decades, or even centuries, before such effects would become appreciably apparent. One certainly hears of PVC cables that have been in permanently wet, even underwater, environments for many years without any apparent ill effect. I also think that I once reported here my ongoing experiment with a length of submerged PVC T&E. At around 2 years, there was still no measurable IR at 500V, or even 1000V (that experiment continues - now about 3 years and counting).

Kind Regards, John.

 

[DetlefSchmitz]

Over time - your windows (the glass) will drip.

This turns out to be an urban legend. OK It may drip - but there is no practical evidence over several hundred years so far. I think there's a Wikipedia article on this.

 

[EFLImpudence]

It is a liquid.

I don't think it's been around long enough yet.

 

[seasickstevie]

Glass is a liquid?

Really?

 

[1john]

Glass is a liquid?

Really?

So I've been told too, aparantly if you look on really old stained slass windows in churches and the like it can be seen that is has "run".

 

[radweld]

So I've been told too, aparantly if you look on really old stained slass windows in churches and the like it can be seen that is has "run".

http://en.wikipedia.org/wiki/Glass#Behavior_of_antique_glass