VRI insulation failure

I have just seen a domestic earth fault involving a cotton-braided Vulcanised Rubber Insulation (VRI) cable. A 60A branch fuse had blown and a
friend asked me to look at the problem.
The live cable and its neutral had passed through a brass gland but to me the insulation appeared undamaged. I did not have a 500V insulation tester with me but I could find no low resistance to earth with an ohm-meter.
I replaced the fuse and when I closed the isolator the fuse again blew but this time the fault revealed itself as the live cable had shorted to earth on the inside of the brass gland.
The cable and fuse were was renewed and the problem was cleared.
My question is with 40 year old VRI cable what changes take place in the insulation? I know it gets brittle but assuming the cable is undisturbed and not under any vibration why would it fail like this? Could there have been a chemical change going on which gradually carbonised the insulation?
Beemer
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Well the VRI does get sorta fragile with age, any chance of condensation? Usually the carbonisation is the result of the arcing and heat and dampness in the porous old VRI sure helps with arcing :-).
--
Cheers .......... Rheilly P



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| >I have just seen a domestic earth fault involving a cotton-braided | > Vulcanised Rubber Insulation (VRI) cable. A 60A branch fuse had blown and | > a | > friend asked me to look at the problem. | > | > The live cable and its neutral had passed through a brass gland but to me | > the insulation appeared undamaged. I did not have a 500V insulation | > tester | > with me but I could find no low resistance to earth with an ohm-meter. | > | > I replaced the fuse and when I closed the isolator the fuse again blew but | > this time the fault revealed itself as the live cable had shorted to earth | > on the inside of the brass gland. | > | > The cable and fuse were was renewed and the problem was cleared. | > | > My question is with 40 year old VRI cable what changes take place in the | > insulation? I know it gets brittle but assuming the cable is undisturbed | > and not under any vibration why would it fail like this? Could there have | > been a chemical change going on which gradually carbonised the insulation? | > | > Beemer | > | > | Well the VRI does get sorta fragile with age, any chance of condensation? | Usually the carbonisation is the result of the arcing and heat and dampness | in the porous old VRI sure helps with arcing :-). | -- | Cheers .......... Rheilly P | | No dampness or sunlight and normal domestic inside temperature band. I guess it must just have been long term pressure of the cable against the brass gland unless of course the guy had been moving the wires about without telling me.
thanks,
Beemer
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You got it. It doesn't just get brittle and crumbly, it becomes poorer at insulating. Disturbance is a BAD idea unless disconnection and replacement is the next step.
The extent to which the insulation and sheath deteriorate in service depends very much on whether the cable has been subjected to overloading and/or excessive temperature, or the rubber has been exposed to direct sunlight. Deterioration results in a loss of insulating properties, with the rubber becoming dry and inflexible - perhaps with a tendency to crumble.
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Ozone is another detrimental factor. Over a period of time, it affects rubber near switch contacts, motor brushes, and anywhere else near a source of sparks. Any accidental use of VRI in old electromechanical telephone exchanges was quickly a disaster.
--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]
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On 26 Mar, 20:22, snipped-for-privacy@cucumber.demon.co.uk (Andrew Gabriel) wrote:
I believe oil and gasoline type substances can be damaging too.
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