Earthing SWA at both ends

[Dave OCD]

But a circuit is not isolated until it has been proven dead with a suitable tester. So how do you get the tester in to the lightswitch without taking it off the backbox?

I thought we were talking sockets Dave, on a decorative metal light switch I'd always connect a link between the front and box, just habit.

 

[trev]

But a circuit is not isolated until it has been proven dead with a suitable tester. So how do you get the tester in to the lightswitch without taking it off the backbox?

Find the correct MCB (by a process of elimination if necessary, we've all been there) turn it off and then test. You've taken reasonable precautions

 

[shanky887614]

Find the correct MCB (by a process of elimination if necessary, we've all been there) turn it off and then test. You've taken reasonable precautions

ok trev what do you do when you have removed all the fuses and your snips go bang.

sorry mrs ive just chopped your neibors lighting cable lol

funny as **** when you see someone do it

 

[Dave OCD]

With lights I always switch them on before isolating, I know it's not 100% bombproof but the light/s going off should help tell you you turned the right MCB off. [Or other OCPD as applicable]. :biggrin5:

 

[trev]

ok trev what do you do when you have removed all the fuses and your snips go bang.

sorry mrs ive just chopped your neibors lighting cable lol

funny as **** when you see someone do it

Your snips should never go bang because you should always follow the safe isolation procedure. If you're in an upstairs flat then I appreciate that you could chop the wrong cable and make them bang but......

 

[shanky887614]

Your snips should never go bang because you should always follow the safe isolation procedure. If you're in an upstairs flat then I appreciate that you could chop the wrong cable and make them bang but......

it wasn't in this case thats what made it amusing, someone was stealing there lecky (terrice)

 

[davesparks]

By identifying the circuit and carrying out the safe isolation procedure.

Yes, the safe isolation procedure, which requires access to the terminals. Those terminals must be assumed to be live and treated as such until tested to prove dead. So the switch has to come off the wall whilst in a condition where it must be assumed to be live.

 

[stevethesparks]

I think its good practice to install the fly-lead, I still do it. I have mentioned before, it may be the householder that loosens / removes an outlet for decorating purposes, and then screws it back again afterwards without isolating. Wrong on their part, but a most likely scenario.

 

[Deleted member 26818]

Back to my question.
If a metal back box is deemed to be an exposed-conductive part, then why not the armour on an SWA cable?

 

[Leesparkykent]

Back to my question.
If a metal back box is deemed to be an exposed-conductive part, then why not the armour on an SWA cable?

Surley that's obvious?

 

[Deleted member 26818]

Of course it's obvious, the metal back box cannot normally be touched, without the use of a tool and the removal of earthed screws, whereas the armour can be touched simply by sliding a gland boot down. Something that can occur without human intervention.

 

[ImpededLoop]

Back to my question.
If a metal back box is deemed to be an exposed-conductive part, then why not the armour on an SWA cable?

I would say that it's because there are two layers of insulation between the conductors and the metallic part (insulation and sheathing inside the swa).

Same as putting cables on cable tray.

 

[Deleted member 26818]

Would that not just make it an exposed-conductive part which encloses a double insulated cable?
We are required to earth exposed-conductive parts.
Normally a metal back box is not exposed, as it is buried in a wall.
Tools would be required to expose it.
The armour of an SWA cable is also not normally exposed as it is sheathed in PVC and is covered by a boot at each end.
However the boot can be removed without the use of a tool, and can slide down the cable exposing both the armour and the gland simply due to vibration or even just gravity alone.
Metal back boxes are provided with a terminal and SWA glands a banjo, both to be used to connect an earth conductor.

How is it that the one which requires the use of tools to expose, is considered as being an exposed-conductive part, whereas the other is not?

 

[Richard Burns]

SWA armour is an exposed conductive part. (At least according to GN8)
The deeming of the flush metal back boxes as exposed conductive parts is just silly, but thems the rules!

 

[davesparks]

One of the main functions of the armour is to connect any metal object which penetrates the cable to the earthing system of the installation which contains the protective device for that cable.

If you didn't connect the armour to earth at one end (specifically supply end if it feeds a building with a seperate earthing system) then it would not perform this function.