Rewire After 25 Years?

[JohnW2]

Late to the party again, but this is one of the three major changes to cables designed for use in domestic properties since 1970, the others being a change from imperial to metric conductor sizes and the harmonisation of conductor insulation colours.

True, although, even though the CPC size of 2.5mm² cable very occasionally might, the latter two don't really have any relevance to the ongoing suitability of a cable.

The other potentially relevant change is not directly to do with the cable, per se, but is the much more recent introduction of the concept of "Cmin" (decades after it should have been introduced ). That change meant that a circuit which had previously had a Zs 'just low enough' for adequate fault protection (by ADS) might have come (after the introduction of 'Cmin') to have 'inadequate fault protection' with the existing cable size. Returning to the first 'change' ...

The CPC in 2.5mm² twin and earth cable was upgraded to 1.5mm² in the 80s (sorry, can't remember when exactly) as it was found to be insufficient when the circuit was protected by BS3036 fuses. A quick fix at the time was to replace the 3036 with a 3871 breaker.

Are you talking specifically about a 30A BS3036? If so, does anyone know what the I²t for such an animal is?

Kind Regards, John

 

[RF Lighting]

Yes it was an issue uniquely related to a 1.0mm² CPC protected by a 30A BS3036 fuse. I’m sure I did the calculations on here years ago to prove the theory.

 

[JohnW2]

Yes it was an issue uniquely related to a 1.0mm² CPC protected by a 30A BS3036 fuse. I’m sure I did the calculations on here years ago to prove the theory.

Fair enough. As you will presumably realise from my question about the I²t of such a fuse, I wanted to repeat that calculation, since I was surprised at the suggestion that a 1mm² CPC would be adiabatically inadequate - given that such calculations usually show that CPCs of T+E are considerably larger than they theoretically need be (adiabatically-speaking). Admittedly, however, I think I've only ever done the calcs for MCBs.

Kind Regards, John

 

[securespark]

the latter two don't really have any relevance to the ongoing suitability of a cable.

I'm not sure whether the imperial equivalent of 2.5 may have suffered the same issue as the earlier incarnation of metric 2.5.

I worked under the 14th for less than a couple of years and at that time it was the metric edition.

I never did any calcs on imperial cable.

 

[SUNRAY]

I'm not sure whether the imperial equivalent of 2.5 may have suffered the same issue as the earlier incarnation of metric 2.5.

I worked under the 14th for less than a couple of years and at that time it was the metric edition.

I never did any calcs on imperial cable.

7/0.029" is ~3mm² amd the earth [being 3/0.029"] is ~1.25mm² which were I believe rated at 20A and 8A. I have never understood the silly discrepancy between imperial and metric cable sizes [learnt at a young age from my father at school age] and will be more than happy to be told they are incorrect.

 

[securespark]

Can't find my copy of the 14th....the perils of moving office from one room to another!

 

[SUNRAY]

7/0.029" is ~3mm² amd the earth [being 3/0.029"] is ~1.25mm² which were I believe rated at 20A and 8A. I have never understood the silly discrepancy between imperial and metric cable sizes [learnt at a young age from my father at school age] and will be more than happy to be told they are incorrect.

To clarify, the sizes are close enough but I feel the current rating of imperial cables ought to be higher. Viz talking to an ex SEEBoard electrician last night, they used to install 7/0.036" for 3.6KW Night Storage Heaters.

 

[teaboyjim]

I’m currently working in a building constructed in 1973. All the original fit cable is PVC/PVC and is still in great condition and certainly does not need rewiring. The only real ‘problems’ are that all the 2.5mm² cables have a 1.0mm² CPC, a general lack of outlets and undersized submains for current useage. One of the meeting rooms has just been rewired from 3 single sockets to 8 twin sockets to give an idea of how modern requirements are much different.

In general terms is the "type" of cable insulation a key factor in determining if a complete rewire is done and is that in laymans terms "grey wire" is pvc and "rubber wire" would need redoing as it can degrade?

 

[EFLImpudence]

If the 1mm² CPC in 2.5mm² T&E had to be increased, why was it there in the first place?

 

[sparkwright]

Not so many houses out there with old rubber now, many have been re-done over the years.

However, many have been re-wired badly, and so often these end up getting re-wired or partially re-wired.

Even more so if a new owner wants extra sockets, sockets removed from skirtings and raised up, light switches and lights moved to different positions.

With those things in mind, and possibility of faults such as rings not as rings, no earth on lighting circuits, nasty joints under the floors and rodent damage - it really is sometimes easier and more satisfactory to start again.

 

[teaboyjim]

Not so many houses out there with old rubber now, many have been re-done over the years. However, many have been re-wired badly, and so often these end up getting re-wired or partially re-wired. Even more so if a new owner wants extra sockets, sockets removed from skirtings and raised up, light switches and lights moved to different positions.

I think my house falls squarely in the category that you describe.Was there a reason why skirting boards had holes cut in them for plugs? Mine has got a few rooms like this

With those things in mind, and possibility of faults such as rings not as rings, no earth on lighting circuits, nasty joints under the floors and rodent damage - it really is sometimes easier and more satisfactory to start again.

Precisely - There is a value in peace of mind and being able to tell new owners that it's been done properly

 

[JohnD]

Was there a reason why skirting boards had holes cut in them for plugs?

it seemed like a good idea at the time, because it was easy.

Only when they kept getting smashed by hoovers and chair legs did disadvantages become obvious.

Sockets near the floor also seemed like a good idea, if you live in a country where there are no pregnant people, no bad backs, no stiff knees, no arthritis and no fat people or people in wheelchairs or on crutches.

 

[sparkwright]

In those days, people didn't really have the tools or incentive to start cutting sockets into the wall, particularly if the house was nicely decorated.

Everything seemed an effort then.

 

[teaboyjim]

In those days, people didn't really have the tools or incentive to start cutting sockets into the wall, particularly if the house was nicely decorated.
Everything seemed an effort then.

Do you mean both proud standing boxes and the habit of installing them onto the skirting which I presume they chiselled out?

 

[JohnW2]

I'm not sure whether the imperial equivalent of 2.5 may have suffered the same issue as the earlier incarnation of metric 2.5. ....

I suppose that might have been possible, but I have assumed that, since iot had been around for decades, someone would have noticed (and corrected) the problem, just as they subsequently did with 2.5mm²/1.0mm² within a few years of its appearance.

I never did any calcs on imperial cable.

Nor have I, until today!

As I said last night, the problem I had was in trying to ascertain the appropriate value of I²t for a 30A BS3036 fuse to use for an adiabatic calculation. The best I can currently do is to look at the 'bottom end' of the characteristic curve for a 30A BS3036 in the Appendix of BS7671. That indicates a disconnection time of 0.1 sec at a current of about 430A.

That equates to an I²t of 18,490 A²sec. If one takes "k"=115, an adiabatic calculation then indicates the minimum CSA of the CPC of about 1.18 mm².

As being discussed, 1.18mm² is more than 1.0mm², confirming that the CPC in a 2.5/1.0mm² cable would theoretically not be adequate with a 30A BS3036 fuse.

However, I believe that 7/0.029" imperial cable had a 3/0.029" CPC. If that is the case, then the CPC would have had a CSA of about 1.278mm² - hence adequate for use with a a 30A BS3036 fuse (if above calculation is correct).

However, as I've said, those calculations assume that the I²t figure for a 30A BS3036 I've used is roughly correct.

Kind Regards, John