Tray and basket bonding

[Buzz1980]

Hi all
Regarding bonding of cable tray and basket. Is basket required to be bonded if only data cables installed on it ? plus bonding cable tray, best method i use it 6mm brass nuts and bolts with earthing straps. But brass nuts and bolts can be expensive, will roofing bolts and nuts be suitable, can’t think why they wouldn’t as they bolt the tray together but on jobs I’ve been on before spec has stated all tray joints bonded with earth braids and brass nuts and bolts. Any views much appreciated ??

 

[Ian1981]

Do you mean bonded or earthing , as there’s a world of difference between the two?
I normally don’t do either

 

[Buzz1980]

Well I’m tearing this as a exposed conductive part as on the tray are not swa or cables with earthed metallic sheathing so in the event of a damaged cable tray could become live ?

 

[Spoon]

Have a look at this

You would you need to bond cable tray or basket, if just data cables are installed on it?

 

[Buzz1980]

Treating I mean not tearing ?

 

[Ian1981]

My personal view is that it’s only forms part of the electrical installation and therefore is an exposed conductive part of it’s being used as the cpc. It can’t be by definition an extraneous conductive part and form part of the electrical installation, I treat it as mearly a cable management system used for swa, data , fire and insulated and sheathed pvc cables offering the same degree of protection as double insulation.

 

[Strima]

In telecoms they bond everything, don't stand still for too long or you'll end up with some green and yellow up the wrong'un...

 

[Ian1981]

In telecoms they bond everything, don't stand still for too long or you'll end up with some green and yellow up the wrong'un...

So if the main protective bonding conductor requires to be say 35mm, they actually use 35mm to bond it with?
Or is it just ‘earthed’ ?

 

[Buzz1980]

Thanks for all replies. So I have watched the video and I am thinking the tray is a exposed conductive part as if the cables become damaged then the tray can become live. Same why we earth the SWA glands, cable inner sheath has two layers of insulation but still classed as a exposed conductive part the gland. I am also thinking that the basket will not be a exposed conductive part or extraneous conductive part as the basket is not buried or going outside the building. Plus if data cable is damaged then the basket will not become live as only data cables on basket. Any thoughts on my points ??

 

[Ian1981]

If the tray is used to carry cables and is not used as a protective conductor two scenarios exist:

1. The cable tray carries metallic sheathed cables, such as bare micc. - In the event of a fault on the circuit the fault path will be the metallic sheath of the cable and therefore the tray is not an exposed-conductive-part and does not require earthing. If the tray was to be connected to the MET, under fault conditions the tray would only serve to distribute further any touch voltage.

2. The cable tray carries cables with a non-metallic sheath. - In this case the cables are deemed to provide the same basic and fault protection as class II equipment (see Regulation 412.2.4.1) and as such, in the event of an insulation fault in the cable a fault current cannot flow in a conductive part and hence the tray does not require earthing.

In either of the scenarios above the tray is not required to be earthed and in some cases earthing could increase the shock risk under fault conditions.

From guidance note8 but ultimately it’s down to the designer of the electrical installation to decide if it’s required.

I personally do not earth cable basket or tray directly, tho they maybe earthed indirectly by being fixed to say bonded steel work.

 

[PEG]

Any designer,could mitigate the ordering of a skip full of brass fixings,by suggesting some testing of the finished tray/basket. ?

 

[Strima]

So if the main protective bonding conductor requires to be say 35mm, they actually use 35mm to bond it with?
Or is it just ‘earthed’ ?

All the tray and racks tend to be bolted together creating a mesh so normally you may only see a little bit of bonding where containment might no fully meet I.e. a fire break or change from tray to basket.

IIRC BS6701 covers the requirements for earthing and bonding for telecoms but I can’t be bothered to look at the minute. It does give cable sizes etc for various size racks and power systems.

 

[UKPN]

Do you mean bonded or earthing , as there’s a world of difference between the two?
I normally don’t do either

Well, there is a statement. Are you an Electrician?

 

[timbobelfast]

Is there a specific Client spec for the site? Normally this will give guidance on what is expected. Normal practice would be to bond to local earth bar.

Brass nuts and bolts are not as expensive as you may think, well M6 size anyway. I usually cringe when I see a roofing nut and bolt use in earth tags!!

 

[i=p/u]

Bonded, as exposed conductive part. I as the inspector decides.

 

[loz2754]

Cable tray can only be classed as an exposed conductive part if it is being used as a CPC. If it was an exposed conductive part it would need to be earthed, not bonded. The cables will be insulated and sheathed, so they're adequately mechanically protected.

Cable tray can only be classed as an extraneous conductive part if it brings in a potential, usually earth potential. This can be ascertained easily by measuring the resistance from the MET to the tray and, if the result is more than 22k ohms, it does not need to be bonded.
So, in most cases, cable tray does not need to be earthed or bonded.

 

[i=p/u]

If it becomes live, that's fine , everyone touch here ...

 

[Ian1981]

Well, there is a statement. Are you an Electrician?

What do you think?
My profile is open to view , unlike certain other members who randomly comment.

 

[i=p/u]

surely is part of the electrical system and requires bonding. Where did you learn this 18th edition????

 

[UKPN]

Cable tray can only be classed as an exposed conductive part if it is being used as a CPC. If it was an exposed conductive part it would need to be earthed, not bonded. The cables will be insulated and sheathed, so they're adequately mechanically protected.

Cable tray can only be classed as an extraneous conductive part if it brings in a potential, usually earth potential. This can be ascertained easily by measuring the resistance from the MET to the tray and, if the result is more than 22k ohms, it does not need to be bonded.
So, in most cases, cable tray does not need to be earthed or bonded.

22k? isnt that from one of the many "on site guides"?