Does cable insulation degrade at a linear rate?

[Mr Hehound]

Was doing some IR tests at a property today. Built in 1976 according to the homeowner. Nothing to worry about regarding regs, lowest reading was 99Mohms, which I thought was ok for a installation going on for 40 years old.

Got me thinking though (and not something I ever asked in college) - does insulation degrade at a linear rate or get to a point and then go downhill rapidly?

Just for the sake of the question, take a cable that had a set voltage and amperage through it without any change or external factor to interfere.

Would it degrade at a set rate over time or get to a certain point (in time) where it started to degrade more quickly?

Apols if this is a stupid question.

 

[Knobhead]

Simple answer:
No.
Temperature, climatic conditions and air bourn contaminates all add together.

I had samples of 50 year old VIR SWA tested as part of a modernisation project. The MCC panel was replaced, the cables reused. The cables were in perfect condition due to the cool and stable environment.

 

[Mr Hehound]

Thanks Tony

 

[davethsparky]

Cable manufacturers these days often rate cables with a lifetime of 20 years if used at their max current rating 24/7 and say that the lifetime will be longer if run at lesser currents or not in use full - time. This is of course assuming there are no external factors affecting the cable such as those mentioned by Tony.

Older cables can be more susceptible to age and degradation if they weren't put in very well to start with, but if installed properly they can still be just as good today as they were when they were put in.

 

[GLENNSPARK]

Was doing some IR tests at a property today. Built in 1976 according to the homeowner. Nothing to worry about regarding regs, lowest reading was 99Mohms, which I thought was ok for a installation going on for 40 years old.

Got me thinking though (and not something I ever asked in college) - does insulation degrade at a linear rate or get to a point and then go downhill rapidly?

Just for the sake of the question, take a cable that had a set voltage and amperage through it without any change or external factor to interfere.

Would it degrade at a set rate over time or get to a certain point (in time) where it started to degrade more quickly?

Apols if this is a stupid question.

cable...(certainly a flat twin)..will tend to settle over time....
i have been into umpteen installs whilst carrying out an ECR and its around about 11.5 Meg on an L-N....

 

[Mr Hehound]

Cheers Dave.

There's nothing wrong with this install imho other than it being an old 3036 wylex board so no e/l protection (though I've always thought that other than this those old boards are pretty bulletproof?)

Not looking to alter it. I did work with a guy who used to push for rewire based on his 'own rule of thumb' that if the wiring was more than 30 years old it needed doing, regardless of test results.

 

[GLENNSPARK]

Simple answer:
No.
Temperature, climatic conditions and air bourn contaminates all add together.

I had samples of 50 year old VIR SWA tested as part of a modernisation project. The MCC panel was replaced, the cables reused. The cables were in perfect condition due to the cool and stable environment.

yes but we can take it further than that Tony...
lets not forget how may faults the OPD at origin of circuit has cleared and of course the magnetude (Ipf)

 

[Mr Hehound]

Thanks Glen (on the mobile app so don't know how to add thanks other than post it up)

 

[Mr Hehound]

Glen - (again sorry if this is a stupid question) how far downstream are you talking about when you say origin of circuit?

 

[Geordie Spark]

Simple answer:
No.
Temperature, climatic conditions and air bourn contaminates all add together.

I had samples of 50 year old VIR SWA tested as part of a modernisation project. The MCC panel was replaced, the cables reused. The cables were in perfect condition due to the cool and stable environment.

A "cool and stable environment" with you around???!! :lol:

 

[Knobhead]

yes but we can take it further than that Tony...
lets not forget how may faults the OPD at origin of circuit has cleared and of course the magnetude (Ipf)

I’m not going in to the protection of the cables as it’s a complex subject. The cables I was referring to had no fuses or MCB’s just “group overload protection” which opened the 1000A OCB.

If you look in the drawings I sent you there is an unfinished drawing of group protection.

 

[GLENNSPARK]

Glen - (again sorry if this is a stupid question) how far downstream are you talking about when you say origin of circuit?

well...
if you are talking finals then surely the OPD (.4 or 5s) offering protection for the cable would be the origin...of that circuit (think sub boards)

for subs (5s) then it would be of course the cable supplying the sub board/cascade....
think about it sunshine...
and theres the parrallel path to consider here as well....so think on....

 

[Knobhead]

A "cool and stable environment" with you around???!! :lol:

The building has 18” concrete walls and roof with half of it buried in solid rock. Not even me going off on one would make a blind bit of difference.

 

[GLENNSPARK]

The building has 18” concrete walls and roof with half of it buried in solid rock. Not even me going off on one would make a blind bit of difference.

thats yer bloody feet man...

 

[GLENNSPARK]

Cheers Dave.

There's nothing wrong with this install imho other than it being an old 3036 wylex board so no e/l protection (though I've always thought that other than this those old boards are pretty bulletproof?)

Not looking to alter it. I did work with a guy who used to push for rewire based on his 'own rule of thumb' that if the wiring was more than 30 years old it needed doing, regardless of test results.

nowt wrong with the board (as long as the enclosure is still good for continued service...and disconnection times are still being met)..
but no mention of suplementary bonding by the O/P...
and if it aint in...its a fail...