Bonding RSJ question.

[fatboyslim]

Hi all,

I would like some advice on a scenario currently at my workplace. I am replacing a Sub main DB and replacing the wiring to sockets/lights in one of our storage buildings.
My question is regarding bonding. The main bonding is 25mm, which from the sub mains EMT, my plan was to bond the nearest RSJ with 25mm cable.
However, a separate metal container (Two sockets and a light) is fed from this storage building. Currently, the metal container has no bonding and I'm sure a metal lump sitting directly on the earth would need bonding.
The problem is the submain in the storage building is over 140 metres away from the metal container. Could I bond the nearest RSJ of the storage building to the metal container which is 40 metres away, as the RSJ structure is bonded near the sub main? This would save 70 metres of cable.
Or just take the bonding cable back all the way to the sub main EMT.

I'm trying to avoid TT'ing the metal container as the ground is extremely hard and a lot of forklift traffic in that area. However, if it needs doing there's always a way!!

Thanks for any advice guys.

 

[LastManOnline]

Hi all,

Currently, the metal container has no bonding and I'm sure a metal lump sitting directly on the earth would need bonding.

Not necessarily.If there is no extraneous metalwork entering the container then in principle there is nothing to bond.
It's no different from the metal sheds commonly erected nowadays.If there are no metalic services entering the shed (such as a water pipe) then there is nothing to bond.

 

[Flanders]

Not necessarily.If there is no extraneous metalwork entering the container then in principle there is nothing to bond.
It's no different from the metal sheds commonly erected nowadays.If there are no metalic services entering the shed (such as a water pipe) then there is nothing to bond.

Is the metal container itself not an extraneous conductive part ?

 

[LastManOnline]

Is the metal container itself not an extraneous conductive part ?

I would say yes,technically, as it can "introduce a potential ", if it's sitting directly on earth.
But it differs from the conventional extraneous part in that it is introducing a potential to itself rather than other unrelated metalwork.It simultaneousely serves the purpose of been both extraneous and been the actual installation itself.These are normally 2 separate entities.
I treat these situations as I would any other metallic object or appliance sitting directly on earth (a genny)

 

[pc1966]

I would say yes,technically, as it can "introduce a potential ", if it's sitting directly on earth.
But it differs from the conventional extraneous part in that it is introducing a potential to itself rather than other unrelated metalwork.It simultaneousely serves the purpose of been both extraneous and been the actual installation itself.These are normally 2 separate entities.

But if the metalwork is not bonded to the CPC system they risk being at differing potentials, say should the CPC be on a faulted-PEN segment of the supply network.

So while all structural metal work might be at the same potential, any class I equipment may be at another. So bonding is still needed.

 

[LastManOnline]

So bonding is still needed.

Correct.I always bond the metalwork to the CPC .I understood from the OP that he wondered about running a separate bonding cable to the metallic structure.

 

[fatboyslim]

Thanks for the replies.
So no need for main bonding, just classing the metal container as an exposed conductive part. So link the metal container to the earthing system.
The metal container is supplied with 3 core 1.5mm SWA, so just what size conductor to link the metal container to the earthing system? 1.5mm sufficient?

 

[LastManOnline]

Thanks for the replies.
So no need for main bonding, just classing the metal container as an exposed conductive part. So link the metal container to the earthing system.
The metal container is supplied with 3 core 1.5mm SWA, so just what size conductor to link the metal container to the earthing system? 1.5mm sufficient?

Yes.Perfectly fine from an electrical point of view.But your regs might require it to be a little bigger (4mm?) due to mechanical protection considerations."pc1966" is better placed than me to a swer that.

 

[mainline]

Hi all,

I would like some advice on a scenario currently at my workplace. I am replacing a Sub main DB and replacing the wiring to sockets/lights in one of our storage buildings.
My question is regarding bonding. The main bonding is 25mm, which from the sub mains EMT, my plan was to bond the nearest RSJ with 25mm cable.
However, a separate metal container (Two sockets and a light) is fed from this storage building. Currently, the metal container has no bonding and I'm sure a metal lump sitting directly on the earth would need bonding.
The problem is the submain in the storage building is over 140 metres away from the metal container. Could I bond the nearest RSJ of the storage building to the metal container which is 40 metres away, as the RSJ structure is bonded near the sub main? This would save 70 metres of cable.
Or just take the bonding cable back all the way to the sub main EMT.

I'm trying to avoid TT'ing the metal container as the ground is extremely hard and a lot of forklift traffic in that area. However, if it needs doing there's always a way!!

Thanks for any advice guys.

I would class a metal shipping container the same as a caravan and NOT export a PME earth to it, but TT it.

 

[fatboyslim]

I would class a metal shipping container the same as a caravan and NOT export a PME earth to it, but TT it.

Sorry, I forgot to mention is a TN-S earthing system.

 

[mainline]

Sorry, I forgot to mention is a TN-S earthing system.

As long as you can guarantee that it is TN-S and isn’t taken from a TN-C network.

 

[fatboyslim]

As long as you can guarantee that it is TN-S and isn’t taken from a TN-C network.

Yeah definitely TN-S. We have two separate private LV transformers, as the site is spilt into two substations.
Recently had taps changed on both as voltage was around 250-260v now sitting comfortably at 230-240v. Some extra savings in the company's pocket!! Shame I don't benefit from seeing them in my payslip

 

[davesparks]

Could I bond the nearest RSJ of the storage building to the metal container which is 40 metres away, as the RSJ structure is bonded near the sub main? This would save 70 metres of cable.

Yes, subject to the steel framework being suitably continuous, not likely to be removed etc, see reg 543.2.1(vii) and reg 543.2.6

 

[pc1966]

Correct.I always bond the metalwork to the CPC .I understood from the OP that he wondered about running a separate bonding cable to the metallic structure.

My apologies, I was not really thinking of what the OP actually asked about.

 

[pc1966]

You could consider this as supplementary bonding so minimum would be 4mm copper.

If you are sure it is only ever going to be TN-S (so no PME conditions to satisfy) and only ever going to be fed via 1.5mm or so cable (so 4mm well below adiabatic for any supply OCPD) that should be sufficient.

There might be other considerations like lightning protection, mechanical strength, or potential increases in supply capacity that mean 10mm or more might be appropriate, but I guess not just now.

 

[fatboyslim]

Thanks for all the advice guys. Much appreciated

 

[narkedatninety]

Is the metal container itself not an extraneous conductive part ?

Very unlikey to introduce an outside potential into the system.

 

[narkedatninety]

But if the metalwork is not bonded to the CPC system they risk being at differing potentials, say should the CPC be on a faulted-PEN segment of the supply network.

So while all structural metal work might be at the same potential, any class I equipment may be at another. So bonding is still needed.

Not neccessarily. Bonding it may mean you are actually introducing a fault-path. We test to ascertain whether it is extraneous and whether it is likely to introduce an outside potential into the system. If the test proves otherwise, we do not bond.