| Author |
VRI insulation failure
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| Beemer 2007-03-26, 9:25 am |
| 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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| Rheilly Phoull 2007-03-26, 9:25 am |
|
"Beemer" <Beemer@nowhere.com> wrote in message
news:vzMNh.2403$Fe4.619@text.news.blueyonder.co.uk...
>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
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| contrex 2007-03-26, 5:25 pm |
| On 26 Mar, 10:41, "Beemer" <Bee...@nowhere.com> wrote:
> 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?
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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| Andrew Gabriel 2007-03-26, 5:25 pm |
| In article <1174935314.310327.296430@y80g2000hsf.googlegroups.com>,
"contrex" <mike.j.harvey@gmail.com> writes:
> 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.
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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| Beemer 2007-03-27, 3:25 am |
|
"Rheilly Phoull" <rheilly@bigpong.com.au> wrote in message
news:4607985b$0$7396$5a62ac22@per-qv1-newsreader-01.iinet.net.au...
|
| "Beemer" <Beemer@nowhere.com> wrote in message
| news:vzMNh.2403$Fe4.619@text.news.blueyonder.co.uk...
| >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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| contrex 2007-03-27, 9:25 am |
| On 26 Mar, 20:22, and...@cucumber.demon.co.uk (Andrew Gabriel) wrote:
I believe oil and gasoline type substances can be damaging too.
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