Is galv conduit containing insulated and sheathed cables (e.g. T&E) still an exposed-conductive part?

[happysteve]

Eh up

As per the title, "Is galv conduit containing insulated and sheathed cables (e.g. T&E) still an exposed-conductive part?"

Clearly, if it contained unsheathed singles, it obviously is.

If it's not an exposed-conductive part (and it's not extraneous in the case I'm thinking of) then it doesn't require earthing.

My gut instinct is that it's still an exposed-conductive part, but section 10.11 of GN8 - which is about cable tray, not conduit - got me thinking. That section (which refers to tray), states:

"A cable complying with the appropriate standard having a non-metallic sheath or a non-metallic enclosure is deemed to provide satisfactory basic protection and fault protection, as does an item of Class II equipment (Regulation 412.2.4.1 refers). Class II equipment is constructed such that any insulation fault in the cable cannot result in a fault current flowing into any conductive parts with which the equipment may be in contact. Hence, the metal cable tray or basket need not be earthed."

On the other hand, as insulated and sheathed cables are drawn into galv conduit, they may experience damage to their sheath which could not be spotted by inspection (they shouldn't get damaged if you do it carefully, but sometimes it happens). Hence my gut feeling of treating it as an exposed-conductive part. Which means an extra 4mm conductor for earthing it back to the MET (trivial to do).

Any thoughts, one way or the other?

 

[Deleted account]

I see the dilemma it just states cable tray and basket. No mention of conduit.

Rather than take 4mm back to the board, earth it locally using final circuit conductors cpc.
but that brings another dilemma, if you break into the T&E to expose the cpc in order to earth conduit you’ve exposed the basic insulation resulting in the need to bond.

We’ve just entered a paradox.

 

[cliffed]

Interesting,using conduit for T/E,seen it so many times,the conduit connects to a metal socket outlet,then it will be earthed.
years ago this method would not be used,it’s the same old story,fast fixing.
Never like the idea of mixing different cabling installs,myself.

 

[telectrix]

I see the dilemma it just states cable tray and basked. No mention of conduit.

Rather than take 4mm back to the board, earth it locally using final circuit conductors cpc.
but that brings another dilemma, if you break into the T&E to expose the cpc in order to earth conduit you’ve exposed the basic insulation resulting in the need to bond.

We’ve just entered a paradox.

just earth conduit at source , then flylead from cpc at end of circuit.

 

[westward10]

The 15th Ed allowed for steel conduit, where used for the mechanical protection of cables having a non-metallic sheath to be absent of indirect contact protection (earth). This disappeared in the 16th although it was still permitted provided the length of conduit did not exceed 150mm and was concealed. Not looked in 17th or 18th.

 

[davesparks]

Why is the T&E being installed in steel conduit? Is it decorative or there for mechanical protection?

If its there for mechanical protection then I would definately earth it as this implies there is risk of mechanical damage which could result in the conduit being crushed or penetrated damaging the cable inside.

If its just decorative then maybe not worry about earthing it.

 

[happysteve]

Rather than take 4mm back to the board, earth it locally using final circuit conductors cpc.
but that brings another dilemma, if you break into the T&E to expose the cpc in order to earth conduit you’ve exposed the basic insulation resulting in the need to bond.

I didn't want to introduce any unnecessary joints, and in this case it's trivial to run an extra 4mm G/Y to the MET.

Interesting,using conduit for T/E,seen it so many times,the conduit connects to a metal socket outlet,then it will be earthed.
years ago this method would not be used,it’s the same old story,fast fixing.
Never like the idea of mixing different cabling installs,myself.

In this case, the conduit is used purely for mechanical protection of the cables, outside. It's running new circuits for a new kitchen round the outside of a house (6mm cooker and 2x2.5mm for a kitchen sockets RFC; lighting already in place). Often this would be done in PVC conduit (mostly for UV protection of the cables, and looks, as well as a bit of mechanical protection) but the client wasn't convinced PVC would offer enough mechanical protection, so galv it is. The conduit won't be connected directly to the back of the socket (for a start, there's more than one circuit). I've installed galv conduit a few times, and have always used singles up to now.

just earth conduit at source , then flylead from cpc at end of circuit.

I'm only going to connect at source, if I'm honest. I could take a 4mm G/Y from one of the back boxes (cooker isolator or first socket) back to the final end BESA box of the run (outside) as well, but I don't really see the need if it's earthed at source. Happy to be convinced otherwise, though.

Why is the T&E being installed in steel conduit? Is it decorative or there for mechanical protection?

If its there for mechanical protection then I would definately earth it as this implies there is risk of mechanical damage which could result in the conduit being crushed or penetrated damaging the cable inside.

If its just decorative then maybe not worry about earthing it.

Thanks, this reply appeared while I was typing the rest.
It's for mechanical protection, but not in the "traditional" sense - it's not likely to suffer impact damage, it's more to prevent other people messing with circuits on the outside of a house. As far as I can see, the only risk of damage is at the point where you draw the cables in, which will be minimal.

Plus, it was an interesting technical thing to ponder.

Thanks everyone for your replies so far (particularly @westward10 for the historical perspective).

 

[Ian1981]

I think it’s a judgement call for the designer/installer to satisfy themselves that if the conduit is not earthed, then the conduit is very unlikely to become live due to external influences or damage to the internal wiring system Within.
[automerge]1571570403[/automerge]
Tho to add we do not treat swa the same, we earth the armouring as it’s classed as an exposed conductive part.
There’s a layer of insulation’bedding’ that covers the single insulated conductors that may not be classed the same as sheathing on t&e but nevertheless it’s similar but we wouldn’t dream of not earthing the armour, even if glanded into a plastic enclosure.

 

[telectrix]

earth it and bang in a couple of rods, connected to the conduit with jubilee clips. get the Zs right down then.

 

[Dustydazzler]

My preference (not actually looked into the regs book) would be the conduit is earthed over its entirety.
Either by means of it being 1 long continues length earthed at the start
Or
Connected to a cpc at each junction box via the back box

 

[Timbo]

And don't forget the SPD, due to the risk of lightning strikes on the conduit...... ?

 

[ipf]

There is a situation of such which occurs in domestic rewiring, isn't there.
In the main, existing (old) conduits in walls being used for rewiring switch drops, using T+E or sheathed singles. It was usually old 5/8 conduit. Being enclosed in the wall, I can't see any problem not earthing, as long as due care is taken, whilst installing.....and assuming there is enough space in the conduit. If it's for mechanical protection, earth it.

 

[Megawatt]

Eh up

As per the title, "Is galv conduit containing insulated and sheathed cables (e.g. T&E) still an exposed-conductive part?"

Clearly, if it contained unsheathed singles, it obviously is.

If it's not an exposed-conductive part (and it's not extraneous in the case I'm thinking of) then it doesn't require earthing.

My gut instinct is that it's still an exposed-conductive part, but section 10.11 of GN8 - which is about cable tray, not conduit - got me thinking. That section (which refers to tray), states:

"A cable complying with the appropriate standard having a non-metallic sheath or a non-metallic enclosure is deemed to provide satisfactory basic protection and fault protection, as does an item of Class II equipment (Regulation 412.2.4.1 refers). Class II equipment is constructed such that any insulation fault in the cable cannot result in a fault current flowing into any conductive parts with which the equipment may be in contact. Hence, the metal cable tray or basket need not be earthed."

On the other hand, as insulated and sheathed cables are drawn into galv conduit, they may experience damage to their sheath which could not be spotted by inspection (they shouldn't get damaged if you do it carefully, but sometimes it happens). Hence my gut feeling of treating it as an exposed-conductive part. Which means an extra 4mm conductor for earthing it back to the MET (trivial to do).

Any thoughts, one way or the other?

If it’s a hard pull with a lot of NM sheath cables I will meg all cables to test the insulation to make sure nothing got damaged

 

[Megawatt]

If it’s a hard pull with a lot of NM sheath cables I will meg all cables to test the insulation to make sure nothing got damaged

If you run galvanized conduit from metal enclosures to metal enclosures it does not have to be grounded