Main bonding conductors connected to trunking.

[AdamKerr88]

Today i have been asked to give an independant opionion on electrical work undertaken by another electrician on a commercial property.

There has been work done to "improve the mains bonding arrangments".

The buildings supply is a 100a 3 Phase TN-C-S. Connection to this is by a 50mm2 single, crimped and bolted to the side of the cut-out connecting to a small domestic 8 way MET. There is then a 16mm2 cpc supplying the D.B which is fed from a 100x100 trunking between the supply and the D.B.

Before the work was carried out there was 2 x 10mm2 singles connecting the MET to the incomming gas service. He has now cut these back, crimped and screwed them to the side of the 100 x 100 trunking. Surely this has only made the situation worse ??

543.2.2 does say that a metalic cable management system can be used as a protective conductor but surely not for mains bonding as this must be connected directly to the MET?

This is by no means the only issue here, however i dont want to list "Protective bonding conductors not connected directly to the MET" if this is not actually incorrect.

Thanks

 

[Guest55]

youre right.
main bonds should be continuous and unbroken en route to the services.
only cpc's can use metal containment as a carrier.
i'd give it only a code 3 defect on an inspection to be fair though.

 

[daiplayer]

surely bonding should from services straight to met.
less than 0.05 ohms ? doubt it

 

[johnboy6083]

I doubt that the steel trunking has enough CSA to meet the requirements as a bonding conductor. It's common practice to use exposed metalwork as a bonding conductor. It's usually structural steelwork though, as that is very unlikely to be removed. I don't like the idea of using trunking as a bonding conductor, especially in a PME scenario, as if the supply neutral is lost, then the trunking would probably get warm, and possibly overheat the cables inside.

 

[Chr!s]

youre right.
main bonds should be continuous and unbroken en route to the services.
only cpc's can use metal containment as a carrier.
i'd give it only a code 3 defect on an inspection to be fair though.

He's not, Mains bond don't need to be continuous, old wive tale that one.

 

[Chr!s]

I doubt that the steel trunking has enough CSA to meet the requirements as a bonding conductor. It's common practice to use exposed metalwork as a bonding conductor. It's usually structural steelwork though, as that is very unlikely to be removed. I don't like the idea of using trunking as a bonding conductor, especially in a PME scenario, as if the supply neutral is lost, then the trunking would probably get warm, and possibly overheat the cables inside.

100 x 100 copper equivalent = 40.2mm and i doubt heat would be an issue in a PME scenario, no insulation.

 

[Guest55]

He's not, Mains bond don't need to be continuous, old wive tale that one.

i cant think of a single benefit from changing a continuous conductor into a conductor interupted by containment connections thus introducing more opportunities for a break in continuity.
its certainly not what i consider an "improvement in earthing arrangements"

 

[johnboy6083]

I agree that the lack of insulation will aid in heat dissapation, but if cables inside the trunking are at full capacity, and therefore at max operating temp, then I feel that a few installations worth of neutral current could cause a problem.

 

[AdamKerr88]

i cant think of a single benefit from changing a continuous conductor into a conductor interupted by containment connections thus introducing more opportunities for a break in continuity.
its certainly not what i consider an "improvement in earthing arrangements"

Thats exactly it ! Its only made it more likely to be broken at some point. Also the trunking is not connected to the MET other than its connection to a metal db.

 

[daiplayer]

He's not, Mains bond don't need to be continuous, old wive tale that one.

needs to be continous if one bonding conductor is used for two services .

 

[jumpin jax]

needs to be continous if one bonding conductor is used for two services .

That also i believe is advisory too

 

[Guest55]

Also the trunking is not connected to the MET other than its connection to a metal db.

well it is connected to the MET if the trunking forms part of the main bonding ?

Does the trunking earth fixing bolts have "safety electrical connection" tags that you find on pipe clamps ?
if not you can code it on that alone ;-)

 

[AdamKerr88]

well it is connected to the MET if the trunking forms part of the main bonding ?

Does the trunking earth fixing bolts have "safety electrical connection" tags that you find on pipe clamps ?
if not you can code it on that alone ;-)

Sorry was'nt too clear with that......

It connects to the trunking instead of the MET.

 

[Chr!s]

surely bonding should from services straight to met.
less than 0.05 ohms ? doubt it

Why?

i cant think of a single benefit from changing a continuous conductor into a conductor interupted by containment connections thus introducing more opportunities for a break in continuity.
its certainly not what i consider an "improvement in earthing arrangements"

In a commercial/Industrial scenario you will find many a connection through out the length of the Main Bonding

I agree that the lack of insulation will aid in heat dissapation, but if cables inside the trunking are at full capacity, and therefore at max operating temp, then I feel that a few installations worth of neutral current could cause a problem.

And you think these conductors will be operating at Max temp with a broken pen? Will all equipment be operating under fault conditions?

Thats exactly it ! Its only made it more likely to be broken at some point. Also the trunking is not connected to the MET other than its connection to a metal db.

Well, who ever is carrying out work on any installation needs to be competent to do so, you cant design a fortress.

needs to be continous if one bonding conductor is used for two services

Say's where

Does the trunking earth fixing bolts have "safety electrical connection" tags that you find on pipe clamps ?
if not you can code it on that alone ;-)

I suggest you read the reg on that one


Your the inspector, its for you to decide as to whether the installation is designed and installed correctly, Trunking can be used as Main Bonding, is it designed and installed correctly, that's your call.

 

[Engineer54]

And there was me thinking that metal trunking/tray containment systems always use at least 2 copper bonding links between bolted sections!!!!