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"?

 

[i=p/u]

What if a cable gets damaged on the tray, what if the tray is within persons reach and could be touched every day by people walking by?

 

[MJPD29]

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

Are you? Or are you a meter fitter UKPN?

 

[pc1966]

It is one of those "it depends" situations. For data cables alone then you don't need to bond it as no risk of shock.

However, there is always the chance it gets used for some 3-core cable, etc, in the future!

I always bond the tray, partly as it was always done under the EEBADS, and partly for EMC reasons as well (telecoms background). On one case it really mattered as a tree came down in high winds, a 240V cable going outside was wrenched out and shorting to the bonded tray was what tripped the OCPD (not RCD'd)!

You don't need to use brass fixings as the roofing bolts are fairly good conductors in the first place and the majority of the current flows between the crimped lug and the tray, with the bolt simply holding the two together. As mentioned above, in most cases the mechanical assembly is sufficient bonding, and the wires are needed only to link between air-separated section and ultimately to the MET via whatever CPC is available for it.

Being a bit paranoid I usually have two connections to the installation's CPC just in case one gets broken.

 

[pc1966]

Here is a photo of the pulled cable I mentioned:


And close up of the OCPD trip results:

 

[Rockingit]

Interesting that this discussion has just popped up today as I've been mulling it over in my head as I've got my annual inspection tomorrow - commercial with a bunch of tray, mix of inside and outside, some making contact with wet pipe work, some not, some being attached to the same lengths of strut that panels are also mounted on. I've 'belt and braces' bonded the lot back to the MET, wondering if it might become a discussion with the engineer.

 

[pc1966]

My view has always been that if you can't be sure that it is not in contact with anything that could become a hazard, best to bond it.

In fact, even if it is all just screwed to brickwork, etc, if the supply is TN-C-S you might find it is below 22k to true Earth and so might be considered an "extraneous conductive part" as it could give enough of a shock w.r.t. some class I appliance's CPC-bonded metalwork to fall off a ladder, etc.

 

[Rockingit]

My view has always been that if you can't be sure that it is not in contact with anything that could become a hazard, best to bond it.

In fact, even if it is all just screwed to brickwork, etc, if the supply is TN-C-S you might find it is below 22k to true Earth and so might be considered an "extraneous conductive part" as it could give enough of a shock w.r.t. some class I appliance's CPC-bonded metalwork to fall of ladder, etc.

I've actually found (admittedly it was central London) a final circuit with a Zs of 0.17 - with a disconnected PME! So that totally vindicates your point here.

 

[Ian1981]

My view has always been that if you can't be sure that it is not in contact with anything that could become a hazard, best to bond it.

In fact, even if it is all just screwed to brickwork, etc, if the supply is TN-C-S you might find it is below 22k to true Earth and so might be considered an "extraneous conductive part" as it could give enough of a shock w.r.t. some class I appliance's CPC-bonded metalwork to fall off a ladder, etc.

Earth it or bond it?
Bonding something because your not sure is no reason to bond anything.
It’s an exposed conductive part so earth it it , if it’s not then don’t, it’s extraneous then bond it, it’s not extraneous then don’t.
lumping a quite possibly large protective bonding conductor on to metal work because your not sure is not a valid reason to do so.

 

[pc1966]

Earth it or bond it?
Bonding something because your not sure is no reason to bond anything.

It is an exposed conductive part.

But it might not be in any way dangerous unless it also has the ability to introduce a voltage. So that means either:

• It can come in contact with a power source above 50V AC, etc.
• It can connect to the true Earth and you are on a TN-C-S supply

In the original discussion the talk was of data cables, and for Ethernet (even PoE) they do not meet the first point. But that might not continue in the future. That is the uncertainty, and while it is often argued we cannot engineer for future changes, my expression of "not sure" comes down to this aspect.

It’s an exposed conductive part so earth it it , if it’s not then don’t, it’s extraneous then bond it, it’s not extraneous then don’t.
lumping a quite possibly large protective bonding conductor on to metal work because your not sure is not a valid reason to do so.

The argument about bond/Earth could run and run and often does:

• Are you wiring back to the MET? Then its a CPC.
• But it is extraneous and back to the MET? Then it is a main protective bond
• Are you linking it to nearby metalwork such as data racks? Then it is supplementary bonding.

Most likely it will be 4mm cable (from 544.2.1) unless the whole TN-C-S aspect kicks in and whatever cable try structure really is able to divert significant current in the event of an open PEN then its Table 54.8 which probably means the usual 10mm.

Though unless the tray assembly is in contact with major parts (structural steel, service pipes, external cable work) it won't event see such currents, so a quick check of imeadance to MET before bonding could save you going down the big girth route if, say, it is > 100 ohms.