Cable Tray cross bonding?

[Crosswire]

I am running a load of cable tray at the minute. I said it required cross bonding with 4mm cable, but my boss said it wasn't required as we are using metal joiners...........


Who is right?

 

[Deleted member 9648]

'Cross bonding' ????........you may need to link joints in the tray with a copper link to ensure earth continuity if the joints are not electrically continuous ,but 'cross bonding' ?..is this the same as the ancient-mythical practice of wasting ones time linking boiler pipes together etc.?

 

[Crosswire]

'Cross bonding' ????........you may need to link joints in the tray with a copper link to ensure earth continuity if the joints are not electrically continuous ,but 'cross bonding' ?..is this the same as the ancient-mythical practice of wasting ones time linking boiler pipes together etc.?

Errr......not sure.

What I mean is to link from one length of tray to the next length of tray with a bit of 4mm green and yellow single core cable with a crimped lug at each end, bolted to both lengths of tray with a 4mm pan head brass bolt, and secured to the tray with a 4mm nut, with the bolt passing through the hole of the lug.

I am considering this in the event that the metal cable tray joiner is not sufficient to maintain electrical continuity between the two abutting lengths of tray.

My question is, is the steel cable tray joiner sufficient to maintain electrical continuity, or will I need to use another method E.G. the copper link you suggest, or the method described above?

Cheers!

 

[bonnie lad]

i have put miles of tray in on jobs, only ever used tray joints for continuity, bond final ends of tray onto DB,s or panels with 4/6mm earth bond with lugs. no need to cross bond to other services.

 

[Crosswire]

i have put miles of tray in on jobs, only ever used tray joints for continuity, bond final ends of tray onto DB,s or panels with 4/6mm earth bond with lugs. no need to cross bond to other services.

No, not to other services, just from one tray to another............

Although they will be connected to other services at the MET, of course...



Anyhow, thanks for the reply, will stick to tray joints unless I hear otherwise

 

[ian.settle1]

Can also depend upon what is specified by the client.

Place where I work for a FM company the client specify's that every joint on trunking and tray has a earth link across every joint, they also all specify that where different runs of tray cross or are next to each other they are also bonded together as well as bonded to the Distribution Board and the earth tape that runs around the building.

 

[lamb]

Cable tray (or other steel containment) containing only sheathed cables, even if bare MICC, is not an exposed conductive part of an electrical installation. Therefore it is not required to be connected to the MET by a cpc.
If it is earthed this would be down to the job spec or working practices.

However, if likely to introduce earth potential from one building or installation to another, main equipotential bonding would be required at point of entry.

This is a specific topic covered in IEE Guidance Note 8 : Earthing and Bonding.

 

[Sparky Ninja]

Can also depend on the location. I had installed tray on a rooftop and it was necessary to bond to the existing lightning protection, as it was at a different potential, even though it was bonded at the origin.

 

[Crosswire]

Cable tray (or other steel containment) containing only sheathed cables, even if bare MICC, is not an exposed conductive part of an electrical installation. Therefore it is not required to be connected to the MET by a cpc.
If it is earthed this would be down to the job spec or working practices.

However, if likely to introduce earth potential from one building or installation to another, main equipotential bonding would be required at point of entry.

This is a specific topic covered in IEE Guidance Note 8 : Earthing and Bonding.

Well definitely gonna earth it because , although not an exposed conductive part, I certainly consider it an extraneous conductive part.

I know it doesn't fall under the bonding requirements of 411.3.1.2, but this and the ceiling grid will be earthed, in the event that they may become live under fault conditions.

Not got guidance notes 8 (got 1 and 3), so can't look it up

 

[caddy]

Well definitely gonna earth it because , although not an exposed conductive part, I certainly consider it an extraneous conductive part.

I know it doesn't fall under the bonding requirements of 411.3.1.2, but this and the ceiling grid will be earthed, in the event that they may become live under fault conditions.

Not got guidance notes 8 (got 1 and 3), so can't look it up

Unless the cable tray comes out of the ground how could you define it as an Extraneous conductive part if it cant introduce an earth potential. Of course if the cable tray can introduce an earth potential then it would need to be bonded accordingly. there is also no requirement to earth a ceiling grid, since ceiling grids are push fit you would have to link every length of grid together to ensure contiuity, just as you suggest doing with the cable tray.

Sometimes bonding everything metal you can get your hands on can lead to an increased risk of a shock.

 

[mogga]

just another bit to the Argument who is going to pay for this cross bonding If your boss has said NOT to do it and you go and do it will he pay both your time and material for doing it??

 

[Crosswire]

Unless the cable tray comes out of the ground how could you define it as an Extraneous conductive part if it cant introduce an earth potential. Of course if the cable tray can introduce an earth potential then it would need to be bonded accordingly. there is also no requirement to earth a ceiling grid, since ceiling grids are push fit you would have to link every length of grid together to ensure contiuity, just as you suggest doing with the cable tray.

Sometimes bonding everything metal you can get your hands on can lead to an increased risk of a shock.

Ok you are right, it is not an extraeneous conductive part, but surely it IS an exposed conductive part:

"Conductive part of equipment which can be touched and which is not normally live but which can become live when basic insulation fails"

Also I remember once on a job another sparks had stripped out lights simply by cutting the flex off at the light and leaving it dangling still plugged into the klik rose. When the circuit was turned back on one of the flexes was touching the ceiling grid which resulted in the whole ceiling grid becoming live!

We found this out when owner of the building touched the grid.

Had the ceiling grid been earthed this would not have happened. If it managed to become live via the push fit connections , then there is continuity, and by the same logic it can become earthed without copper links.

Every bone in my body is telling me to earth it - I can't see how this could be wrong

 

[Crosswire]

just another bit to the Argument who is going to pay for this cross bonding If your boss has said NOT to do it and you go and do it will he pay both your time and material for doing it??

He has said not to make the links, but to earth the tray at the end. If I can make a convincing argument he will change his mind.

Also I can just tell the apprentice to do it when he is not there

 

[chris gray]

why would you consider bonding every link? thats going to take you for ever and for what? youll gain continuity by your roofers/links or what ever means of connection you make between the tray.
each to their own but a supplementary bond at the start would suffice.

 

[Crosswire]

why would you consider bonding every link? thats going to take you for ever and for what? youll gain continuity by your roofers/links or what ever means of connection you make between the tray.
each to their own but a supplementary bond at the start would suffice.

Ok fair 'nuff. That was what my boss said. Just that I've seen lengths of tray linked with supplementary bonding in the past, and copper links do exist. Just wondered if they were required.

 

[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