Any idea why the UK started off with TN-S but moved away from it?
I have in moments of irritation / dark humour suggested that it is because it makes it easier for the DNOs as their workers no longer need to be able to count any higher than four .. though there are some very capable folk in DNO land, so that is probably unfair to judge all by the actions of a few.
In reality the CPC came almost for free in the era of underground lead sheathed cables and sweated (wiped solder) joints, and once the hessian serving had rotted off or been eaten by mice, the lead sheath also served as as a kilometer length distributed electrode. Overhead 2 wire service was always TT from day one, but the lead water pipe serving as the electrode was miles long was quite capable of blowing the low current service fuses of the day even though they were the old hot wire type.
The lead sheath was never seen as a credible current carrying conductor, so was never used as the neutral, and the TNS earth Zs is allowed to be slightly higher because of that.
OF course under tension or in heaving ground, it is possible the lead cracks all round and the cores stay connected, and so an 'off earth' fault is possible, and not always spotted straight away.
The modern cable dispensed with the lead sheath some time ago, and those cables are now failing and being replaced, in many cases by aluminium clad cable where the outer is as good a conductor as the cores, and so is used as the neutral. The problem is that in turn that aluminium jacket rots off quite fast once water gets in, hence the open PEN problems.
It would be perfectly possible to make a modern cable with 4 segmented cores inside a metal jacket instead of 3, but there is some economy in not having 2 conductors at (well, nearly) the same voltage. It is essentially about cost, and the fact that the open PEN fault is rare enough, like car crashes, to be acceptable.
Any idea why the UK started off with TN-S but moved away from it?
I have in moments of irritation / dark humour suggested that it is because it makes it easier for the DNOs as their workers no longer need to be able to count any higher than four .. though there are some very capable folk in DNO land, so that is probably unfair to judge all by the actions of a few.
In reality the CPC came almost for free in the era of underground lead sheathed cables and sweated (wiped solder) joints, and once the hessian serving had rotted off or been eaten by mice, the lead sheath also served as as a kilometer length distributed electrode. Overhead 2 wire service was always TT from day one, but the lead water pipe serving as the electrode was miles long was quite capable of blowing the low current service fuses of the day even though they were the old hot wire type.
The lead sheath was never seen as a credible current carrying conductor, so was never used as the neutral, and the TNS earth Zs is allowed to be slightly higher because of that.
OF course under tension or in heaving ground, it is possible the lead cracks all round and the cores stay connected, and so an 'off earth' fault is possible, and not always spotted straight away.
The modern cable dispensed with the lead sheath some time ago, and those cables are now failing and being replaced, in many cases by aluminium clad cable where the outer is as good a conductor as the cores, and so is used as the neutral. The problem is that in turn that aluminium jacket rots off quite fast once water gets in, hence the open PEN problems.
It would be perfectly possible to make a modern cable with 4 segmented cores inside a metal jacket instead of 3, but there is some economy in not having 2 conductors at (well, nearly) the same voltage. It is essentially about cost, and the fact that the open PEN fault is rare enough, like car crashes, to be acceptable.
