Earthing SWA at both ends

[davesparks]

Agreed,does anyone still run a separate tail from a socket earthing terminal to a metal wall box ?

Yes, it's how I was taught as an apprentice and I'm a stubborn bugger.

 

[davesparks]

No, you should never take an accessory off without the circuit being isolated. If It's isolated then there's nothing that can bite you therefore there is no need for the fly lead

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?

 

[Dave OCD]

Really ?:laugh3:

 

[Leesparkykent]

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?

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

 

[shanky887614]

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?

you do this

 

[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.

 

[GEOFF S]

Personally, it depends upon the use of SWA as to whether the armour needs earthing, its good practice to earth it in my opinion.

What happens if the basic swa insulation fails, is there any supplementary insulation? The bedding of SWA is PVC, are the live parts of this cable any worse than that of T/E. Is there any supplementary insulation between cpc and basic insulation in T/E?

Cheers

 

[telectrix]

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.

but once the faceplate fixing screws are fitted, it then becomes "exposed".

 

[stevethesparks]

Personally, it depends upon the use of SWA as to whether the armour needs earthing, its good practice to earth it in my opinion.

I would say its imperative to earth the armour of an SWA.

What happens if the basic swa insulation fails, is there any supplementary insulation? The bedding of SWA is PVC, are the live parts of this cable any worse than that of T/E. Is there any supplementary insulation between cpc and basic insulation in T/E?
Cheers.

T&E and SWA can't be compared, either are chosen to suit the design of the installation

 

[GEOFF S]

I would say its imperative to earth the armour of an SWA.

T&E and SWA can't be compared, either are chosen to suit the design of the installation

I think your missing my point, look at the definition of an exposed conductive part, it states basic insulation, swa has basic and supplementary insulation.

Does the armour need connecting to earth, in many instances but not all.

Cheers

 

[davesparks]

I think your missing my point, look at the definition of an exposed conductive part, it states basic insulation, swa has basic and supplementary insulation.

Does the armour need connecting to earth, in many instances but not all.

Cheers

It always needs connecting to the earthing system of the installation it is fed from.
One purpose of the armour is to connect any metal object penetrating the cable with earth before it makes contact with any live conductor.

 

[stevethesparks]

It always needs connecting to the earthing system of the installation it is fed from.
One purpose of the armour is to connect any metal object penetrating the cable with earth before it makes contact with any live conductor.

Precisely

 

[GEOFF S]

It always needs connecting to the earthing system of the installation it is fed from.
One purpose of the armour is to connect any metal object penetrating the cable with earth before it makes contact with any live conductor.

Okay, lets say we use swa instead of T/E surface mounted, there is no requirement for an earthed sheath, just because its there, where in the regs states it must be earthed, Reg numbers?

If the basic insulation fails Dave, will the armour become live or will the supplementary insulation prevent this. So how does it meet the definition of an exposed conductive part, any Regulation numbers would be good?

Now if the cable is buried, thats a different matter.

So surface mounted, into plastice enclosure via plastic gland, boot over end of cable protecting ends of cable within enclosure. What are the Regulations requiring the armour to be earthed?

Cheers

 

[davesparks]

Okay, lets say we use swa instead of T/E surface mounted, there is no requirement for an earthed sheath, just because its there, where in the regs states it must be earthed, Reg numbers?

If the basic insulation fails Dave, will the armour become live or will the supplementary insulation prevent this. So how does it meet the definition of an exposed conductive part, any Regulation numbers would be good?

Now if the cable is buried, thats a different matter.

So surface mounted, into plastice enclosure via plastic gland, boot over end of cable protecting ends of cable within enclosure. What are the Regulations requiring the armour to be earthed?

Cheers

What supplementary insulation? The conductors are insulated, they are then bound together with tapes or an extruded bedding, armour laid over that with an outer PVC serving.
There is only one insulation layer.

It is not an exposed conductive part, I have never suggested it is! But the cable is designed to be installed with the armour earthed, if it is not then it is not being installed as it is intended.

The cable is designed, tested and approved for use under the British standards referenced in bs7671. That approval will have been given with the armour earthed as it was designed to be.

 

[Dave OCD]

No at the very least on smaller sizes of SWA there is definitely a solid insulating sheath external to the insulated cores !
So IMO that is insulated AND sheathed.

 

[davesparks]

No at the very least on smaller sizes of SWA there is definitely a solid insulating sheath external to the insulated cores !
So IMO that is insulated AND sheathed.

Where?
The sheath is not insulation on standard swa

 

[Dave OCD]

Well I've stripped hundreds of SWA ends and the material under the armour is just like the sheath of flex, and it's not conductive is it ?!

 

[davesparks]

Well I've stripped hundreds of SWA ends and the material under the armour is just like the sheath of flex, and it's not conductive is it ?!

The bedding? It's carries no insulation rating whatsoever, it's purpose is to give the cable shape and prevent the armour coming in to contact with the insulation of the cores.
You say it is just like the sheath of flex, which particular flex are you referring to? I can't think of any flex which has a sheath that soft. It's irrelevant anyway as the sheath of flex is also a sheath and not insulation

 

[Dave OCD]

Well we must be using versions of SWA from different universes Dave. :confused5: I'm thinking 1.5/2.5/4/6mm 3core , and that sheath is as tough as flex sheath.

 

[davesparks]

Well we must be using versions of SWA from different universes Dave. :confused5: I'm thinking 1.5/2.5/4/6mm 3core , and that sheath is as tough as flex sheath.

Yes the sheath is as tough as some flexes, less tough than some and more tough than others. It does not make it insulation though!

It is not tested or rated as insulation and so from a technical standpoint it cannot fulfill the role of insulation. If you carried out voltage withstand testing on the sheath as of it was insulation it would fail!

 

[Dave OCD]

I'm not so sure.:wink5:

 

[Leesparkykent]

The bedding is Generally made out of polyvinyl chloride (pvc) which is the same as the Sheath.

 

[Dave OCD]

Precisely !

 

[GEOFF S]

What supplementary insulation? The conductors are insulated, they are then bound together with tapes or an extruded bedding, armour laid over that with an outer PVC serving.
There is only one insulation layer.

It is not an exposed conductive part, I have never suggested it is! But the cable is designed to be installed with the armour earthed, if it is not then it is not being installed as it is intended.

The cable is designed, tested and approved for use under the British standards referenced in bs7671. That approval will have been given with the armour earthed as it was designed to be.

The bedding? It's carries no insulation rating whatsoever, it's purpose is to give the cable shape and prevent the armour coming in to contact with the insulation of the cores.
You say it is just like the sheath of flex, which particular flex are you referring to? I can't think of any flex which has a sheath that soft. It's irrelevant anyway as the sheath of flex is also a sheath and not insulation

Dave the Extruded bedding compound used by cables to BS5467 is PVC, are you suggesting this has no insulating properties?

So there is no requiremnet within BS7671, i have various Manufacturers data, where are you finding reference to the armour requiring earthing, which manufacturer?

Cheers

 

[davesparks]

Dave the Extruded bedding compound used by cables to BS5467 is PVC, are you suggesting this has no insulating properties?

So there is no requiremnet within BS7671, i have various Manufacturers data, where are you finding reference to the armour requiring earthing, which manufacturer?

Cheers

Yes it is sometimes PVC, it can also be a string and tape but this is less common these days. but it is not tested or rated for any voltage withstand.

 

[Leesparkykent]

Yes it is sometimes PVC, it can also be a string and tape but this is less common these days. but it is not tested or rated for any voltage withstand.

What PVC?