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?

CRB

"CRB" wrote in message
om...
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.

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!

--
M Stewart
Milton Keynes, UK
http://www.megalith.freeserve.co.uk/oddimage.htm

CRB wrote:

[...]
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?

How about a technical justification for not replacing it? There's an
interesting article on cable life on the IEE web site - he
http://www.iee.org/Publish/WireRegs/...pdateApr04.pdf.

Basically 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.

--
Andy

In article ,
CRB wrote:
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.

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.

--
*Santa's helpers are subordinate clauses.*

Dave Plowman London SW
To e-mail, change noise into sound.

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).

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.

Christian.

(CRB) wrote in message . com...
Can anyone come up with a technical justification for replacing
such
cable purely on the basis that it is time-expired?

Many thanks for the responses - in particular to Andy for the data.
I'll implement all the suggestions in the replies.

CRB

(CRB) wrote in message . com...
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.

.......
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).

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 a

1. 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.

2. 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.

3. 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.

4. 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.


HTH

jim_in_sussex wrote:


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.

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.]

Stefek

(CRB) wrote in message . com...
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.
.......What were the Results of your Insulation test?, ie 500v applies
across all conductors. More than 2meg?.
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,
........The Wiring regulations recommend that your house should have
had 3 periodic inspections by now!.
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?
........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?.

Bob

Stefek Zaba wrote in message ...
jim_in_sussex wrote:




...... A cable running at a
constant 70deg will have a relatively short life - a few years, no
more.

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.

was speaking from experience :-(

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 :-;