Main bonding questions

[Mikegh]

Probably a basic question

In an installation with pipework or cable ladder that's not extraneous as such but is connected to earthed metal

What's the logic for not main bonding it as the metalwork can surely introduce a potential in a fault situation?

 

[GBDamo]

Probably a basic question

In an installation with pipework or cable ladder that's not extraneous as such but is connected to earthed metal

What's the logic for not main bonding it as the metalwork can surely introduce a potential in a fault situation?

The earthed metal would surely be bonded if extraneous, no?

 

[mainline]

Probably a basic question

In an installation with pipework or cable ladder that's not extraneous as such but is connected to earthed metal

What's the logic for not main bonding it as the metalwork can surely introduce a potential in a fault situation?

It's either extraneous or it isn't.
Have you measured it ?

 

[Pretty Mouth]

Probably a basic question

In an installation with pipework or cable ladder that's not extraneous as such but is connected to earthed metal

What's the logic for not main bonding it as the metalwork can surely introduce a potential in a fault situation?

By earthed metal, I assume you mean metal that is connected to the MET?

For example: plastic incoming supply pipework, metal pipework to boiler, boiler connected to MET via CPC.

The metal pipework is not an extraneous part, so not introducing an extraneous potential. It is however connected to the MET via the boiler and CPC. All that bonding would do is connect it to the MET again, achieving the same thing.

 

[Mikegh]

Yes I mean incidentally earthed via exposed metal

 

[Mikegh]

By earthed metal, I assume you mean metal that is connected to the MET?

For example: plastic incoming supply pipework, metal pipework to boiler, boiler connected to MET via CPC.

The metal pipework is not an extraneous part, so not introducing an extraneous potential. It is however connected to the MET via the boiler and CPC. All that bonding would do is connect it to the MET again, achieving the same thing

Will it achieve the same result in the event of a fault?

The CPC connected to boiler or a main vond connected to the metalwork that's not extraneous

 

[Midwest]

I always found this article helpful, with regards cable tray;

Does a metallic cable tray require earthing or bonding?

In short the answer is 'depends' but the answer could be ‘earthing’, ‘bonding’ or ‘neither’ depending on the circumstances. Read more on our blog to find out more information.

www.tradeskills4u.co.uk

 

[ipf]

Probably a basic question

In an installation with pipework or cable ladder that's not extraneous as such but is connected to earthed metal

What's the logic for not main bonding it as the metalwork can surely introduce a potential in a fault situation?

If metalwork is directly connected to earthed metal, what would be the point?
Am I missing something, here?

 

[mainline]

If metalwork is directly connected to earthed metal, what would be the point?
Am I missing something, here?

I'll say the same as I did in #3

Without an actual measurement, we don't know if it's extraneous or not.

 

[Pretty Mouth]

Will it achieve the same result in the event of a fault?

The CPC connected to boiler or a main vond connected to the metalwork that's not extraneous

I believe so. Here's how I see it:

When a direct short occurs between L and earthed metalwork, a circuit is completed via the CPC, back to the MET, and back to the transformer via whatever earthing system is in place, and possibly through bonded extraneous parts.

Like any circuit, there will be voltage drop throughout the fault loop, with the highest potential for accessible metalwork being at the site of the fault, and the lowest being at the MET.

In my example above, the metal pipework is already connected to the MET via the boiler CPC, and is not taking a potential from anywhere else (it's not an extraneous part), so it can't be at a lower potential than the MET. Connecting the pipework back to the MET with bonding would not bring the potential of the pipework any higher than it already is.

 

[ipf]

I'll say the same as I did in #3

Without an actual measurement, we don't know if it's extraneous or not.

I'm just presuming 'earthed metal' to be part of the earthing system. If something needs bonding, it's not.

 

[mainline]

I'm just presuming 'earthed metal' to be part of the earthing system. If something needs bonding, it's not.

I don't really know if they meant earthed as in to the Met or Earthed as in Terra Firma, which is why I asked the question.

 

[ipf]

I don't really know if they meant earthed as in to the Met or Earthed as in Terra Firma, which is why I asked the question.

Yeah, plenty of folk don't know the difference between earthing and bonding, though.

 

[Mikegh]

It's either extraneous or it isn't.
Have you measured it ?

Not extraneous
I'm just trying to understand the shock risk between the pipework that is earthed by the boiler and another piece of exposed metal if it becomes live temporarily

So you've got 1 hand on the pipework and another on say a piece of live exposed metal

 

[mainline]

So you've got 1 hand on the pipework and another on say a piece of live exposed metal

If you have one hand on a live piece of metal and another on a pipe that's earthed via the boiler, then you're going to get one of the worst types of shock you can have, one that's across your chest.