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.

 

[Mr Hehound]

Yeah. It's there and would work if required.

 

[GLENNSPARK]

whats still there ?

supplementary (cross bonding)...?
and have you proved it..?

 

[GLENNSPARK]

very important love in the absence of RCDs for additional protection....

 

[Mr Hehound]

They show acceptable continuity.

 

[Knobhead]

Nothing tested then?

 

[Engineer54]

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.

I'm with Tony on this one. The major cause of a cables insulation failing/deteriorating is the temperature, environmental and stress levels that cable has been subject too over it's lifetime!! Correctly sized and installed cables in stable environments, will out last by a considerable margin those cables that have led a life of constant stress, and/or from being installed in environments that are not suitable for the type of cable used!!

 

[Marvo]

'Degradation' is a very unfocused term. Materials degrade in many ways therefore degradation can be measured in many ways such as durability(how brittle), discoloration, porosity etc but I guess in this context the main concern would be dielectric strength.

I doubt the relationship between the dielectric performance and time would be linear, most of these types of failure curves are somewhere between linear and logarithmic where degradation accelerates ever more rapidly in the late stages leading up to catastrophic failure.

 

[GLENNSPARK]

'Degradation' is a very unfocused term. Materials degrade in many ways therefore degradation can be measured in many ways such as durability(how brittle), discoloration, porosity etc but I guess in this context the main concern would be dielectric strength.

I doubt the relationship between the dielectric performance and time would be linear, most of these types of failure curves are somewhere between linear and logarithmic where degradation accelerates ever more rapidly in the late stages leading up to catastrophic failure.

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[Geordie Spark]

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View attachment 20336

Looks like one of Marvo's soldered copper pipe joints has just failed ...............

 

[Engineer54]

'Degradation' is a very unfocused term. Materials degrade in many ways therefore degradation can be measured in many ways such as durability(how brittle), discoloration, porosity etc but I guess in this context the main concern would be dielectric strength.

I doubt the relationship between the dielectric performance and time would be linear, most of these types of failure curves are somewhere between linear and logarithmic where degradation accelerates ever more rapidly in the late stages leading up to catastrophic failure.

I would think that all these forms of cable degradation would be affecting it's dielectric performance to one extent or the other...

I think most of us here have removed cable that was installed 50 /60 years ago, and found it to be in almost perfect condition, along with other just as old installations where the cables degradation is found, to say the least ''extensive''!!

I have removed old TRS 7/029 (circa 1955) from my parents house during a rewire only to find cable degradation at a few locations at points of termination to socket outlets. The rest of the cable being completely as new, with both Visual and IR values to prove!! It was in such good condition that i wired my parents garage out with it, installed in steel trunking and conduit!! Saved a few bob on PVC singles, and the TRS will be far more suitable for the low winter temperatures.

As far as i know, those T&E TRS cables are still in place, a good 20/25 years after i installed them!! lol!!

 

[7029 dave]

Yes pulled out a lot of 7029 pvc just as good as when it was installed, cant say that with vir in my experience.

 

[Engineer54]

Yes pulled out a lot of 7029 pvc just as good as when it was installed, cant say that with vir in my experience.

Unless it's been really stressed (constant heavy loadings) and or subjected to high ambient temperatures, then VIR /TRS/CRS will generally only show signs of insulation degradation at termination points, especially on lighting cables that are terminated into incandescent lamp fittings. High heat, 60 C and above for longish periods is a real killer for those older rubber cables, but unaffected by sub zero temperatures....