4mm2 twin and earth cable

Correct Mike, the accuracy of the numbers is not justified by the measurment accuracy, although this is quite good. Was that your typing or mine?


Regards

David

Well, if you want to be super-accurate with your micrometer, be careful how you pick it up. Thus we were taught at school.


I'm with Mike, 3 sf may be the best that you can hope for.


After retirement, I am beginning to forget the statistical confidence limits for (repeated) measurement, let alone significant change. I reckon that if I am really lucky, on my lathe, I can get within 1/2 thou and under those circumstances I would measure with more than one instrument.


Now then, does nobody claim that sort of accuracy with an MFT?

There has never been a 4mmsq single core cable in UK, When metric first appeared in 1970 wiring regulations 2.5mmsq was single core 1/1.78 (mm), Next up was 4mmsq 7 strand. 7/0.85 (mm)


Aluminium started at 16mmsq.


Copperclad Aluminium, as our fellow member Michael mentioned, turned up in 1974, same sizes/cores/strands as the above.


For wiring regulation buffs its worth noting that in 1974 the late great Electricity Boards used a "special" 2.5mmsq 7 stranded cable for use in their current/voltage transformer work. (A single core 2.5mmsq was not considered practical).

This cable though did make it into the regs, 7/0.67(mm)


Moving on to the 15thed regs, a previous poster has perhaps misread the radial requirements, a 30/32amp cartridge/mcb is backing up a 4mmsq cable, a 20 amp cartridge/mcb backs up 20 amp cables.


But the circuit in question has happily taken care of kitchen loads for many years, so the idea of downgrading the mcb is perhaps not needed in this case. In any event downgrading protection is something we dont take lightly.


Hope this helps


Regards, UKPN

a 20 amp cartridge/mcb backs up 20 amp cables.

Unless of course you consider the famous ring circuit...


 

Aluminium started at 16mmsq.

On the DNO side perhaps, but plain aluminium was certainly available in common T&E sizes before the wiring regs prohibited the smaller sizes - e.g. (pinched from another forum):



  - Andy.

"A 20 amp cartridge/mcb backs up 20 amp cables"


Well spotted, should be 2.5mmsq  cables!!!


As regards Ally cable, Im going to bed, check tomorrow. Thanks again.


Regards UKPN

"OK ebee, fair enough to suggest that a vernier is not really accurate enough, but is accuracy the problem, it more likely to be resolution. Here is a picture of a fairly modern digital micrometer, resolution 1 microm (1/1000 mm). Its accuracy is usually +- 2 or 3 microns."


Hi Dave, microns - often referred to as "mouses tails" .


Yes just to clarify a little yes most of those engineers knew about accuracy and resolution and it was during  the pre calculator era.

They made bits for jet engines and the engineers had instruments, inspection had better ones, quality control had many sets of different usage and calibration in a temperature controlled environment. Monthly all the shop floor instruments were (in rotation) collected one day, left to stabilise temperature next day then tested and returned on day 3. If they passed they got a brand new "METROLOGY" sticker on them. Precision slip gauges and interference bands and all sorts of lovellys. Non were allowed on the shop floor without the current or last months sticker on them - including their own private tools. All traceble back to the NPL (National Physics Lab) I took mine in a few times, it was an old 0 to 1 inch micrometer, my Grandad gave it to me, tested OK but for measuring face up to 2/10 thou over the outer one third of the faces, so OK for flat work but not on round work, Obviously OK for my home use.

There was a laugh one day when they sent a brand new vernier caliper gauge , say 4` or 5` I think it was. They complained it was almost 2 though out on some of its range. Moore and Wright wrote back that this caliper was OK, the accurate part was the last foot of its range and that was only guaranteed to within 5 thou when new. They`d been using them to nearest thou i.e. with half a though presumed accuracy for years - probably shows just how good Moore & Wright and others were back then and how disciplined staff with robust regimes could be in practice much of the time I suspect.


I suppose in our world we might liken some of it to the practice of using a club knife or "stanley knife" etc to slide down the earthwire of a T & E cable without nicking it or the insulation, takes a lot of practice before it becomes second nature, our old R E B used to get this taught at college, I think sucking of teeth might happen nowadays. (Actually it might be one reason that I have an aversion to using a cpc as a cheeswire!)


Of course I might be a little deluded !