Protective conductor in T&E

Spectric · Nov 26, 2020

Hi all

Been in electrical/electronics for many years and am now semi retired but keeping my hand in, but is it just me, my age or what but back in the eighties / ninties nothing seemed to change that much and yes there was far less choice in fittings. Having spent more time in industrial than domestic and always needing answers I prefer radial circuits over rings but in industrial running singles I would use the same CSA for the CPC as the other conductors so why in 2.5 and 4.0mm T&E is the CPC only 1.5mm?

To disconnect a 32amp protective device requires a fault current greater than a 1.5mm can handle so is the reliance on the RCD, ie in theory the RCD will disconnect before the full fault current flows? Seems like relying on the unknown.

Dustydazzler · Nov 26, 2020

It’s all to do with speed of disconnection of supply

to destroy a piece of 1.5csa copper conductor you would have to run 100s of amps down it for quite a while

pc1966 · Nov 26, 2020

The 1.5mm CPC meets the adiabatic limit for the sort of OCPD those circuits should be designed for (assuming the end of cable Zs meets the disconnection limits).

The IET's on-site guide has a handy table of standard circuits (Table 7.1(i) in the book) and that give you the length limits for combinations of cable and OCPD. What is also useful is it also tells you why the limit was reached.

For example, for 4mm/1.5mm T&E and 32A MCB/RCBO radial the limit for type B no RCD is 28m with "zs" showing you it fails to meet disconnection times. But for type C/D breakers and no RCD it is not permitted and the "ad" tells you it exceeds the adiabatic limit. In the RCD cases the limit is 43m and that is down to voltage drop.

Personally I would design for the no-RCD case and treat the RCD only for additional protection (i.e. I assume the thermal/magnetic trip or a fuse is far more reliable than RCD electronics when lives might depend on things).