Main bonding in large industrial building

[sterandall]

hi, just looking for some help on how to install the main bonding on a large industrial building.
Firstly the supply neutral to the building is over 150mm so am i correct thinking i need to use 50mm bonding conductors for water etc?
Secondly the MET is about as fair away as possible on this site, there are MCCB boards all over the place with a 70+ earth to them and tray mounted earth bars with 50+mm earth to them, as long as i verify these are still connected can my bonding conductors come off of these?
I also need to install the bonding in another building on the same site for structural steel, water and hot water, the supply at this building is 25mm with a 70mm earth, what size bonding conductor do i need to use here? and can i just take it from the distribution board?

Any help and i will be very grateful, as ive never worked on a site this size and am pretty much lost in regards to what i can and cant do,

thanks

 

[Deleted member 26818]

What's the earthing system, and is it the same throughout the site?

 

[sterandall]

im unsure at the minute, il try and find out for tomorrow morning. I know theres 8 11kv transformers on site, dont know if that tells you anything?

 

[malcolmsanford]

Is the building metal frame?

 

[Deleted member 26818]

Well, if the earthing is TN-S, then the bonding conductor need not exceed 25mm².
However if TN-C-S then 50mm² would be correct.

 

[sterandall]

Is the building metal frame?

yeah it is

 

[sterandall]

Well, if the earthing is TN-S, then the bonding conductor need not exceed 25mm².
However if TN-C-S then 50mm² would be correct.

Thanks, what about the other seperate building on the site with the 25mm²?

 

[Deleted member 26818]

Thanks, what about the other seperate building on the site with the 25mm²?

Sorry forgot about that.
I don't understand why the earth is so much bigger than the supply, is it a typo?
If it is a typo, and the supply is 125mm², then both systems would require 35mm² bonding conductors.
If it isn't a typo, and the supply is only 25mm², then with TN-C-S, you would only require 10mm² for bonding.
If TN-S, you would have to verify the correct size earth is being used, one would normally expect the earth conductor to be only 16mm², with 10mm² bonding.
If the 70mm² earth is correct, you would need 35mm² bonding.

 

[sterandall]

nope, its not a typo, 25mm supply with a 70mm earth im wondering why aswel.

 

[Deleted member 26818]

Seems very odd.

 

[Knobhead]

If it’s an industrial site you can near guarantee it will be TN-S.
As to the 70mm earth I don’t know why. The company if first worked for 16 & 70mm was always on stock, anything else had to be ordered, could be a similar situation. It’s what they’d got so it got used.

 

[sterandall]

does anyone know if i can connect the bonding conductors to the tray mounted earth bars/MCCB board terminals? And if so would i need to trace the cables back to the MET and fit a 'safety electrical connection do not remove' tag at each connection? Everything ive read always say to take them to the MET but the actual METs are about as far away as possible on this site from all the stuff that needs bonding

 

[johnboy6083]

I would connect a seperate earth bar to the earth terminal at the panelboards, and create an Earth marshalling terminal. This is very common on industrial sites ive worked on, and its the way that we bond the pumping stations and treatment works.

 

[johnboy6083]

malc mentioned about steel frame earlier. He is hinting at the fact that it is permitted to use any extraneous metalwork as bonding or indeed CPC, provided precautuions are taken against its removal, and it is of a suitable CSA, which 90% of steelwork on a building will be.

 

[sterandall]

Does this mean for say a ducting system i could take a bonding conductor from the structural steel to bond the ducting or did i misunderstand? sorry about all the questions but im pretty new to working in places like this and im quite interested in it.

 

[johnboy6083]

yes you can. It must be labelled as usual, and of course the steel must be bonded to the main earth to the building. I would guess that it will be, but its worth checking.

 

[sterandall]

yes you can. It must be labelled as usual, and of course the steel must be bonded to the main earth to the building. I would guess that it will be, but its worth checking.

i never knew that, that makes life a hell of a lot easier now though thanks, the structural steel is bonded in about 240mm. When you say precautions must be taken against its removal is that just as simple as labeling it?

 

[johnboy6083]

the bonding connnection should be labelled with a label not less than that of a BS951 clamp. other precautions are things like not connecting to steel that is likely to be removedand maybe enusring that removal of any cable in the building by non sparks requires the permission of the electrical engineer.

 

[Engineer54]

If the building is all structural steel and has been bonded back to the MET at origin, you can safely say that the whole of the steelwork is one big bonding conductor that is interconnected by cross beams and sections etc!! haha!! Just try isolating all those parallel earth paths, ...no chance!! The chances are, that the structural steel of these buildings is also forming part, if not all of the earthing for the site too.

As you say, ...you can just link the steelwork to the system or service you want to bond out. In reality, the chances are it's already bonded by fixings etc,, but you will still need to make purpose made bond connections, to satisfy all the various requirements you have these day's...

Be interesting to know what sort of Ze and/or Zs values you get at the buildings incoming supply point??

 

[Knobhead]

When bonding the steel work use clamps for the connection or weld a bolt on, don’t drill structural steel. If you weld a bolt then put it inside the web out of harms way.