Wylex 100A dp isolators in plastic enclosures.

don't forget, sheathed cables (e.g. T&E) are considered to have "double insulation".

Could we pick an example other than T&E please? (say a nice round 3-core flex - or singles in insulating conduit - or even the new Irish style of T&E).


I think there's an issue with T&E and providing protection by double or reinforced insulation. 412 allows protective conductors to be present, but requires them to be insulated as if they were live conductors. I think there's a good reason for this - which T&E illustrates nicely.


Say we had a breakdown of the basic insulation in a T&E cable - do we have protection from electric shock? Well yes and no. Yes, you're protected by the insulating sheath from getting a shock if you touch the cable itself. But what you're not protected from is getting a shock from something else in the system that's connected to the c.p.c. Remember that in T&E they're only one layer of basic insulation between the live conductors and the c.p.c. - so while there's protection from getting a shock from the cable, there's no protection from getting a shock as a result of a fault in the cable. Unless of course a fault to the c.p.c. triggers ADS. 99% of the time that's exactly what would happen of course - so no problem. But occasionally we don't have that luxury (like the supply to a TT system upstream of the first tier of RCDs) - then we really do need 412 to fully provide protection against shocks, so the last thing we want is just a single layer of basic insulation between live parts and protective conductors (or exposed-conductive-parts). Internationally almost all insulation sheathed cables have an insulated c.p.c. - thus providing a 2nd layer of insulation between live parts and protective conductors, so maintaining the usual double fault to danger approach. The UK's tradition of bare c.p.c.s is very unusual in that respect.


My preference when it comes to pre-RCD live conductors would be to keep protective conductors (and exposed-conductive-parts) well away (not in the same enclosure say), but occasionally that's not possible. Take for instance the supply to a touring caravan (where we can't make many assumptions about the 'shore' supply, especially when abroad) - so I must admit that the insulation on the c.p.c. of the flex as well as the line conductor does provide the required two-faults-to-danger.


Having T&E (with the c.p.c. connected to the installation's earthing system) on the supply side of the first RCD in a TT installation is something I wouldn't be at all happy with (regardless of the exact wording of 412.2.4.1 which I suspect was copied from IEC documents which in turn presume that all 'class II' cables have an insulated c.p.c. if they have a c.p.c. at all).



  - Andy.

don't forget, sheathed cables (e.g. T&E) are considered to have "double insulation".

Could we pick an example other than T&E please? (say a nice round 3-core flex - or singles in insulating conduit - or even the new Irish style of T&E).


I think there's an issue with T&E and providing protection by double or reinforced insulation. 412 allows protective conductors to be present, but requires them to be insulated as if they were live conductors. I think there's a good reason for this - which T&E illustrates nicely.


Say we had a breakdown of the basic insulation in a T&E cable - do we have protection from electric shock? Well yes and no. Yes, you're protected by the insulating sheath from getting a shock if you touch the cable itself. But what you're not protected from is getting a shock from something else in the system that's connected to the c.p.c. Remember that in T&E they're only one layer of basic insulation between the live conductors and the c.p.c. - so while there's protection from getting a shock from the cable, there's no protection from getting a shock as a result of a fault in the cable. Unless of course a fault to the c.p.c. triggers ADS. 99% of the time that's exactly what would happen of course - so no problem. But occasionally we don't have that luxury (like the supply to a TT system upstream of the first tier of RCDs) - then we really do need 412 to fully provide protection against shocks, so the last thing we want is just a single layer of basic insulation between live parts and protective conductors (or exposed-conductive-parts). Internationally almost all insulation sheathed cables have an insulated c.p.c. - thus providing a 2nd layer of insulation between live parts and protective conductors, so maintaining the usual double fault to danger approach. The UK's tradition of bare c.p.c.s is very unusual in that respect.


My preference when it comes to pre-RCD live conductors would be to keep protective conductors (and exposed-conductive-parts) well away (not in the same enclosure say), but occasionally that's not possible. Take for instance the supply to a touring caravan (where we can't make many assumptions about the 'shore' supply, especially when abroad) - so I must admit that the insulation on the c.p.c. of the flex as well as the line conductor does provide the required two-faults-to-danger.


Having T&E (with the c.p.c. connected to the installation's earthing system) on the supply side of the first RCD in a TT installation is something I wouldn't be at all happy with (regardless of the exact wording of 412.2.4.1 which I suspect was copied from IEC documents which in turn presume that all 'class II' cables have an insulated c.p.c. if they have a c.p.c. at all).



  - Andy.