Cable Tray cross bonding?

[davelerave]

cable tray is an exposed conductive part afaik


commercial ceiling grids can't be bonded effectively although they may be suspended from structural steel which has a main bond

 

[Sintra]

Bonding between each length of tray is not required if using metal jointing strips with roofing nuts and bolts although IMO it is good practice to use the copper link straps. I would not go to the extreme of bonding with cable and lugs unless it was specified by the designer.

 

[caddy]

Taken form GN8 Earthing and bonding

"Cable tray and Cable Basket

A cable tray or cable basket where used as a support and cable management system has to be considered in the contaxt of earthing and bonding. In other words, are such sytems, Where consisting of metal and plastic coated metal, exposed-conductive-parts or extraneous-conductive-parts and consequently do they require earthing and bonding?

Addressing, first, the question of earthing and weather the cable tray or basket should be earthed, electrical equipment such as cables mounted on a metallic support system will normally be equivalent to either a class I construction (for example copper sheathed, mineral insulated cables without an overall PVC covering) or a class II equivalent construction (for example PVC insulated and sheathed cable).

Exposed-Conductive-parts of cables, such as the copper sheath of a mineral insulated cable, are required to be connected to the MET of the installation bya CPC designed to conduct earth fault currents. The cable tray or basket which the mineral insulatedcable is attached to, or may be in contact with, is not itself an exposed-conductive-part and therefore it does not require earthing. To do so would only server to distribute further any touch voltage resulting from an eath fault on an item of equipment to which the cable is connected.

A cable complying with the appropiate standard having a non-metallic sheath or a non metallic enclosure is deemed to provide satisfactory protection against both direct and indirect contact, as does an item of class II equipment. Class II equipment is constructed such that any insulation fault in a 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."

 

[davelerave]

Taken form GN8 Earthing and bonding

"Cable tray and Cable Basket

A cable tray or cable basket where used as a support and cable management system has to be considered in the contaxt of earthing and bonding. In other words, are such sytems, Where consisting of metal and plastic coated metal, exposed-conductive-parts or extraneous-conductive-parts and consequently do they require earthing and bonding?

Addressing, first, the question of earthing and weather the cable tray or basket should be earthed, electrical equipment such as cables mounted on a metallic support system will normally be equivalent to either a class I construction (for example copper sheathed, mineral insulated cables without an overall PVC covering) or a class II equivalent construction (for example PVC insulated and sheathed cable).

Exposed-Conductive-parts of cables, such as the copper sheath of a mineral insulated cable, are required to be connected to the MET of the installation bya CPC designed to conduct earth fault currents. The cable tray or basket which the mineral insulatedcable is attached to, or may be in contact with, is not itself an exposed-conductive-part and therefore it does not require earthing. To do so would only server to distribute further any touch voltage resulting from an eath fault on an item of equipment to which the cable is connected.

A cable complying with the appropiate standard having a non-metallic sheath or a non metallic enclosure is deemed to provide satisfactory protection against both direct and indirect contact, as does an item of class II equipment. Class II equipment is constructed such that any insulation fault in a 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."

you're right it's not exposed -it won't become live if basic insulation fails( unlike trunking)
and it may not be extraneous

but carrying pvc/pvc cables it could become live if a cable was damaged

 

[caddy]

you're right it's not exposed -it won't become live if basic insulation fails( unlike trunking)
and it may not be extraneous

but carrying pvc/pvc cables it could become live if a cable was damaged

In which case you would need to ignore the GN8 advice on earthing and bonding including that of indiscriminatly earthing everything could lead to an increased risk by sending fault currents through even more metal work and carry on and earth the tray / basket..

Im not saying you must not earth it in all cases but some people seem to have a mind set that if its metal it needs earthing / bonding which is not the case.

 

[Crosswire]

"A cable complying with the appropiate standard having a non-metallic sheath or a non metallic enclosure is deemed to provide satisfactory protection against both direct and indirect contact, as does an item of class II equipment. Class II equipment is constructed such that any insulation fault in a 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."

This doesn't make any sense.

Firstly,

"A cable complying with the appropiate standard having a non-metallic sheath or a non metallic enclosure is deemed to provide satisfactory protection against both direct and indirect contact, as does an item of class II equipment"

OK fair enough, twin and earth cable is similar to Class II equipment


"Class II equipment is constructed such that any insulation fault in a cable cannot result in a fault current flowing into any conductive parts with which the equipment may be in contact"



EH? So if the cable insulation fails for whatever reason, it is impossible for the metal cable tray that it is cable tied to to become live? WTF?!!

I'm pretty certain that if a line voltage is applied to a metal cable tray, then that cable tray will become live. Cos it's made of metal.


"Hence the metal cable tray or basket need not be earthed."


Maybe it NEED not be earthed, as a piece of twin and earth is regarded as Class II equipment (
????) , but to me that is sailing close to the wind. As I've SEEN a ceiling grid become live as a result of a fault , I am gonna ignore the advice from GN8 and I will earth both the tray and the grid.

 

[davelerave]

yep technically they may be correct -but they haven't factored in damage etc. to pvc/pvc cables on a tray-if a tray goes live you can be sure the sparkies at fault then!!

have you checked continuity on the grid to make sure it's suitable for earthing?

is it suspended from concrete or purlins?

any cables draped on it?

 

[Crosswire]

yep technically they may be correct -but they haven't factored in damage etc. to pvc/pvc cables on a tray-if a tray goes live you can be sure the sparkies at fault then!!

have you checked continuity on the grid to make sure it's suitable for earthing?

is it suspended from concrete or purlins?

any cables draped on it?

Grid not gone in yet, but will be suspended from concrete.

Won't be draping any cables on it but the final flexes to the lights may well be in contact with it.

 

[davelerave]

i would be cautious about earthing the grid

afaik it's not normally done

NICEIC Technical Manual - Voltimum UK - Electrical Installation Products and Contracting

prob no requirement to earth/ bond grid -but pvc/pvc cables shouldn't be draped on it creating a potential hazard

but you already know that

 

[Crosswire]

i would be cautious about earthing the grid

afaik it's not normally done

NICEIC Technical Manual - Voltimum UK - Electrical Installation Products and Contracting

prob no requirement to earth/ bond grid -but pvc/pvc cables shouldn't be draped on it creating a potential hazard

but you already know that

Hmmm interesting document.

Cables going on tray to a Klik box, in ceiling, just final flexes to lights may touch the grid, for the reasons descibed in the document.

Just to bring up a point made there, E.G. that flexes should be 'as short as possible' . Is there any maximum length allowed? Or is it up to designer?

 

[mogga]

Cross You seem to be clutching at straws determined to "Prove" your boss wrong even though all the advice on forum saing Not to bond it every regulation is open to interpritation why not phone NICEIC on monday and ask them their opinion ??

Solong as you can prove the electrical continuity of the tray there is no need to bond every joint

The 15th had everry bit of metal earthed including windowframes This was removed from 16th 17th edd I susspect if you have seen bonds to grid ect ect ect it may have been a 15th edd instalation

 

[Crosswire]

Cross You seem to be clutching at straws determined to "Prove" your boss wrong even though all the advice on forum saing Not to bond it every regulation is open to interpritation why not phone NICEIC on monday and ask them their opinion ??

Solong as you can prove the electrical continuity of the tray there is no need to bond every joint

The 15th had everry bit of metal earthed including windowframes This was removed from 16th 17th edd I susspect if you have seen bonds to grid ect ect ect it may have been a 15th edd instalation

1) Not clutching at straws, I accept that I will not need to bond every joint of the tray which was the original question.

2) Didn't know about 15th edition , as I am 16th/17th which would explain why I have seen it in installations. Thanks for telling me.

3)Boss told me to earth ceiling grid and tray, as he is originally a 15th Ed spark, which would explain why he said it. But as the prevailing opinion seems to be that the ceiling grid does not need earthing, just find it strange as I've seen someone get a belt off one. Just didn't want it to happen again.

4)Also find strange that GN8 regards tray as forming part of ClassII equipment. Just trying to use a bit of common sense instead of unquestioningly accepting everything I read.

5) "why not phone NICEIC on monday and ask them their opinion ?? "

Because I was trying to have a debate from multiple electricians with different experiences, and so draw upon a wider knowledge base, which so far has been very useful and informative. I could phone the ECA (who I'm with) helpline at any time.

 

[caddy]

4)Also find strange that GN8 regards tray as forming part of ClassII equipment. Just trying to use a bit of common sense instead of unquestioningly accepting everything I read.

Its refering more to PVC/PVC cables being ClassII, but yes they are attached to the tray. But then that would be no different to a piece of ClassII equipment that has metal parts forming part or all of the enclosure.

At the end of the day every instance will be different and its upto the installer / designer to decide if it requires earthing or not. Simple seciotns of tray run on a wall containing SWA, PVC/PVC may not specifically need earthing. However Tray runing around a unit connected to the structual steel will be erthed via its contact anyway and if the ends of the tray are close to the boards then you may well decide to earth them.

If a couple of sections of tray are used simply to span a gap would you really run an supplemety bonding cable from them all the way back to the boards especially as there may not be a specific need to.

I was trying to have a debate from multiple electricians with different experiences, and so draw upon a wider knowledge base, which so far has been very useful and informative.

Well that is what forums are for hopefully its cleared up some of the questions you had but i suspect its left you with more