On 02/07/2020 11:53, John Rumm wrote:
On 02/07/2020 08:11, wrote:
On 01/07/2020 13:50, John Rumm wrote:
On 30/06/2020 22:25, wrote:

I have a sub-main to a remote socket with a TT earth and,
currently, the SWA braid is terminated on a plastic enclosure. I
need to change the enclosure to metal so have to find a way to
terminate the SWA braid without connecting it to the metal
enclosure. This must be a standard problem with a standard
solution, but it's not one I've had to deal with before so I
don't know the "standard solution" :-(

The standard in the past would have been what you have - an
insulated CU at the TT end. With a move to "non combustible" (e.g.
metal) CUs for all things that is more difficult.

Answers I can think of: 1 terminate on a plastic box attached to
the metal enclosure (not ideal in this application),

Yup that is commonly done - an adjacent plastic box etc

e.g.

https://www.tlc-direct.co.uk/Products/LB9025.html

2 cut-off and insulate the braid and use a standard nylon gland
(loss of some mechanical protection, but I could clip the cable
to the metal enclosure), 3

It would work.

There are actually "insulated" SWA glands available that maintain
the IP and mechanical protection:

https://www.etscablecomponents.com/c...b348-b348.html

Â*(no idea on price or a supplier though)

I think the problem with those is that it would still be possible to
Â*touch both earths, although they would solve the isolation problem.

That question arises from time to time with SWA and exported TN-C-S
earthing, since even with the insulating boot fitted to normal SWA
glands, one can in theory still touch the earth without the need of a
tool since the boot can be moved by hand.

By insulating the braid and using a regular IP66 nylon gland there is
no possibility of touching both earths. The cable isn't restrained as
well as when the braid is clamped but in my case that isn't an
issue.

Indeed, and plenty adequate in the circumstances by the sounds of it.


isolate the head end of the SWA and connect the remote end to the
TT earth (easier to do because it comes through a plastic
enclosure, but it doesn't "feel" like the right approach)

That is fine if you have RCD protection at the origin of the
submain - then a fault to the TT earth, would still disconnect the
power. It would not be practical if the submain is fed only from a
Fuse/MCB, since you are unlikely to have a low enough earth loop
impedance to reliably trip the circuit protection under fault
conditions.

Yes, but that would result in an RCD at each end of the sub-main.
Much simpler (on reflection) just to isolate the braid at the remote
end.

Yup, there are practical advantages to not having a RCD at the head end.
The 18th edition requirements to RCD protect all "unprotected" cables
though can make it harder to implement in a compliant way when there is
a run of T&E feeding the start of the SWA.


In my case the T&E runs in a floor void (and terminates to SWA under the
floor, accessible by lifting a labelled board) so, I believe, does not
need RCD protection.