A lot of the fault scenarios mentioned would be aleviated by banning T+E cable.
It is only very recently that a manufacturer has introduced a stuffing gland with an T+E shaped insert. A nice innovation but totally incompatible with standard plastic accessories.
So for most of the past 60 years a cable has been in standard use with no method of securing the cable sheath. How was it ever approved in the first place? Why does it have reduced size earth? How many fires/accidents have been caused by these unsecured cables?
If the only acceptable cables were properly secured armoured or metallic sheathed we could also do away with totally nonsensicle safe zones. Also compulsory use of rcd's (with a laughable 8% failure rate) on lighting circuits could be abandoned
Not sure on the securing as I have used the metal strips for years with the slit in them you screwed or nailed them in and placed cable on them and secured similar to today's tie rap.
As to design the idea of putting two cables inside a metal sheave is as old as the hills it was when the outer lead covering was replaced with plastic that the rot set in.
The later adding of a centre core to be used as earth did improve the design slightly but the only way to return to the original is to replace the plastic outer with metal again.
This has been done and the Ali-tube range of cables designed to go inside stud walls Guardian, Earthshield, Flexishield, and Afumex are bring us back to the days when cable had lead covering although not sure if eating aluminium will kill rats in same way as lead maybe arsenic impregnation like the railways used is also required? Rats don't tend to eat lead cables.
The problem is supply and price. There are jobs where, until amendment 1 comes out which is it seems going to allow adding to a non RCD protected circuit, to be able to place the RCD at the socket end wiring in Ali-tube would make sense. However go to a DIY shed and ask for Ali-tube and one is met with a blank expression. So one has to buy it by the role. And it is expensive and most electricians don't want to buy roles of BS 8436 Ali-tube to use with the odd extension to a ring main.
The design with both outer earthed and a bare earth wire within does raise more questions as only the inner core is copper so we should not really include the outer tube with our earth loop calculations. However since the two are in contact with each other it is impossible to take separate readings for copper and aluminium so although the installation may pass when installed it is far more likely than twin and earth to deteriorate over time and then fail the ELI test.
We look in the 17th Edition and there is a page devoted to twin and earth and how it is to be rated when used in insulating walls (Table 4D5) but there is no table to give us the ratings for Ali-tube which was to have replaced twin and earth.
The temperature ratings are another matter and although Ali-tube is rated at 90 deg C where more twin and earth is rated at 70 deg C there is nothing to tell us if the wood beam we are attaching it to is rated at 90 deg C. The same applies to mineral insulated which I grant you is great for hiding in the mortar in a stone cottage so no cables are seen but can the oak beams take the temperature? It does of course give great ELI readings with the copper outer but as far as volt drop goes a ring main with 1.5mm cables it must be much reduced and can't see 100 sq meters being covered with 1.5mm live conductors in the cable!
I do see how round cables look far better on tray work and never look twisted and I have seen the Ali-tube being used in commercial buildings but in domestic I would think rare and where it is used in the main it's used as a cheap replacement for mineral cable in listed buildings.
But the question is really about if we should now be doing more calculations with the use of the RCD meaning the ELI can be very high around 200 ohms and circuit still pass it is now the line / neutral impedance which limits the length of a ring main due to volt drop. And the length of cable used in ring mains has increased slightly due to this to for a B32 MCB 106 meters. With this in mind I would have expected to see the prospective short circuit current being entered on the paperwork as this is a direct relationship to the volt drop.
However the IET forms do not require this to be recorded and even if they did many of the meters do not measure line / neutral but just measure line / earth for both readings. Yes I know top of range Robin does measure line / neutral but there are many which don't. If they did then flicking between the two readings would not give I x R = 230 as the "I" would be two x 2.5mm and "R" at 2.5mm and a 1.5mm. Yet so many meters multiply them together and one gets 230 with twin and earth. Or even worse 240 where conversion has not been recalibrated.
I play safe and use the wandering leads with an unknown meter and until tested I do not assume in meter switching where moulded leads are used.
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