From time to time there is mention in uk.d-i-y on routinely replacing PVC T&E wiring on the basis of its age, and I need to decide whether to replace mine. In attempting to make this judgement on an informed basis, I have isolated, meggered and accurately measured the continuity of my individual ring final circuits (both statically and as a source impedance measurement), and all is satisfactory. I have also removed a 3 metre section , sliced it open and given it a close visual inspection. It looks virtually brand new, apart from surface dirt on the outer sheathing. Specifically, there is no visual sign of any degradation in the PVC insulation. I have owned the house since it was built, in 1965, and I know that the wiring has never been subject to overloads ( and in any case the correct fuse wire has always been used). The wiring has remained virtually undisturbed so I also know that the cable has not been subject to any undue mechanical stress. So although the cable is 40 years old, my conclusion is that it would be a waste of time replacing it, even though the cost of doing so would be irrelevant. Can anyone come up with a technical justification for replacing such cable purely on the basis that it is time-expired?
From memory, some forms of cavity insulation were reputed to damage the plasticisers in the PVC insulation, and I think there was a recommendation to keep the blown polystyrene bead type away from the wiring. It's 20 years now since I attempted to do that in my house, and apart from the electrics not giving any trouble, I've no idea whther there's trouble brewing in the walls or not!
How about a technical justification for not replacing it? There's an interesting article on cable life on the IEE web site - here:
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the life is highly temperature dependent (Arrhenius equation, etc.) If you take the equation in the IEE paper, with the constants given for PVC, and massage it slightly to give the life directly in years you get:
life in years = exp(15028/T - 40.678)
where T is the absolute (kelvin) conductor temperature of the cable.
Substituting some temperatures:
- at 70 deg. C (343 K) the life is 22.6 years (as per the paper);
- at 60 deg. C the life is 84 years;
- at 40 deg. C the life is 1,498 years!
70 deg. C corresponds, in principle, to operation at the full rated current. Except for cables feeding space heating installations, domestic wiring will spend the vast majority of its life at little more than ambient temperature.
I think you are worrying about nothing. In such an old installation though, you should consider, if you haven't already, adding or upgrading equipotential bonding to modern standards and perhaps adding RCD protection for socket-outlets likely to be used for supplying outdoor portable equipment.
Don't think so. What is often mentioned is that early PVC *lighting* wiring might not have an earth - or have had the earth cut off at all termination points.
PVC wiring - not exposed to the elements, as is the case with most domestic wiring, and not overloaded, has a pretty long life - more than the time it's been around so far.
I recently removed some dead 7/0.29 TW&E from my cellar which makes it about 40 years old, and I'd have described it as in perfect condition.
Don't bother. Replacement of PVC is "on condition". As you have proved, the condition is excellent and it passes the tests. The only advantage might be addition of earthing to lighting circuits if not already present.
However, if you feel you do want to do some work, then replace that old fuse wire board with a modern consumer unit, either split load, or RCBO'ed.
If it isn't obvious by the feel of the cable you're unlikely to detect failed cable by dissection. If it hasn't reached the end of the road you would need a chemical or engineering lab to detect how much life is left. Secondly wired fuses are a poor safeguard against current overload.
To explain, it may be useful to say why pvc cable fails. SFAIUI the main causes of deterioration are:
Heat - self generated ohms law and/or external ambient temperature. This causes the PVC to gradually leech its plasticer [SFAIUI PVC is actually a white powder until plasticer is added]. The result is a hardened inflexible cable which feels a bit like the texture of lead sheet: new cable has a slightly lively springy flexibility. You don't need to dissect it to sense this type of failure. Because it has lost its flexibility the cable is now at risk from movement, knocks etc, causing the internal Cu conductor to become a hazard by breaking through the insulation and/or sheath & shorting or exposing the Cu.
Heat is in fact the chief factor which limits the cable amp ratings given in the IEE tables. Depending on exposure to temperature, leeching can take from a year or so, to many decades, even theoretical centuries, before the cable is dud.
The maximum working temperature for normal twin & earth PVC cable is
70deg C - a rise of 40deg above an assumed max ambient of 30deg. If ambient is regularly over 30deg C either because you're in a warm country or the cable is running through a hot spot (eg near CH) then the max cable amps permitted is reduced. A cable running at a constant 70deg will have a relatively short life - a few years, no more.
A circuit cable can be differently affected along its length by heat because current take off varies & because it may lose (radiate) heat at different rates at different points along the route. Look carefully at any circuits which have been used to routinely run electric fires for a over an hour at a time. Electric fires are best run off short dedicated circuits & kept off ring circuits.
UV light (ie from normal daylight, especially sunny days) - similar effect on the plasticer as heat. Cables which would be in daylight (even short lengths of a few inches going to an outside light) are best put in conduit or minitrunking.
Mechanical stress, PVC cable should be not be unduly restrained or stretched when laid, a light loose fit is correct. Avoid tight bends as when the conductor warms up it has different expansion characteristics to the surrounding PVC. Exposed and/or shorted conductors can result at the bend or twist. OSG has recommended minimum bend radii.
Contact with polystyrene (eg ceiling tiles): leeches the plasticer (affected cable looks rather like it has had PVA adhesive spilt over it & it's gone hard). Otherwise PVC cables seem to be fairly inert when in contact with most common chemicals. (SFAIUI, but haven't observed it, PVC cable can leech solvent from mastic used in roofing felt & adhesive, though that doesn't damage the cable itself, just the roof).
OSG (IEE On-Site Guide) page 96 lists some other factors.
To summarise, unless your cable has been exposed in these ways it is unlikely to need replacement because it is out of date.
In one respect you may be in a better position with the old 7/029 cable than the 2.5mmsq now standard as SFAIR it had a higher maximum current rating.
However you might be well advised to review how well installed other aspects of your installation are - eg terminal boxes, sockets, earth conductor continuity, proper (earth) bonding in the bathroom etc.
Since 1980 era there's been quite a few developments & rethinks which improve the safety of house installations. eg appropriate use of RCDs & MCBs in the CU.
Wired fuses are frowned on because their fusing current cannot be relied on, etc. Cartridge fuses are OK because they are factory manufactured to tight standards, even so MCBs are generally preferred nowadays. So upgrading to a modern Consumer Unit with MCBs & some, at least, RCD protection would be a good idea.
I'd concur with other posters that you consider those aspects more than the cable itself.
Actually it's taken to be 20 years - running at 70degs is "legitimate" long-term loading (though most of us would prefer most of our cables to run cooler than that). Paul Cook's "Commentary on IEE Wiring Regs" states in section 6.1.3, "Ageing of cables": "There is a general understanding that PVC cables with a continuous conductor operating temperatue of 70degsC have a life of 20 yers. There is also a rough guide that for each 8degsC increase in core conductor continuous operating temperature the life of the cable will be halved". (He goes on to give a "general formula" for cable ageing, which has the expected exponential form, but whose numbers don't in fact correspond to a halving-per-8-degrees... ho hum).
[Pauses to scream blue bloody murder at the idiot dysfunctioncal cat, which just came into the room and jumped onto the PeeCee. Which has the new replacement HD for the one which was fading fast resting, unfixed, atop. Knocks it off. Yes, it's powered up and spinning, and has my "main personal data" on it. AAAAARRRRRRRRRRGGGGGGGGGGGGGGGHHHHHHHHHHHHH!!!!!! Time for a fsck/CHKDSK, methinks! And later to fit it securely into the case, dammit.]
......What were the Results of your Insulation test?, ie 500v applies across all conductors. More than 2meg?. I have
.......The Wiring regulations recommend that your house should have had 3 periodic inspections by now!. my conclusion is that it would
.......Do you think that you have enough confidence in your Electrical Wiring to make a valid claim on your Household Insurance should the house be damaged in an electrical fire?.
circuit of mine running a convector fire (not overloaded, at least theoretically) lasted about 24 months, though I'd be the first to admit it probably didn't follow the best route to radiate heat & it was a 30 yer old 7/029 circuit.
It is another example of the unwisdom of running engineering things on the limit.
Result is that I tend to give priority to keeping cables cool by limiting prospective loading, eg by using 4mm sq sometimes even if
2.5mmsq would theoretically do.
Incidentally if you do the cost sums on the lower heat loss of 4mmsq cable v 2.5mmsq cable when running at 30 amps, you find that you save enough to pay for the difference in cable cost & have less volts drop at your machines. takes a long time to recover the cost though :-;
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