Main bonding

[Deleted member 9648]

Evening all,
Have kind of over thought this one until I'm not sure any more! Input welcome.
Have been installing a supply to a machine, and gathering the necessaries for the EIC today I noted that the bonding is undersized. It's a 400a incomer with 185mm tails into an MCCB panel DB. TNCS....and main bonding to water,gas and steel is in 25mm. Table 54.8 gives a minimum CSA of 50mm where the supply neutral is over 150mm. Where I've started to doubt my initial assessment is that I've taken the tails as the benchmark for the supply neutral size....or should it be the DNO incomer neutral CSA which I cannot confirm? Been a long day and I'm not thinking straight.
It matters because it is an installation newly constructed 5 years ago, and a recent EICR by another contractor didn't flag any issues with bonding. Don't want to look a dick and flag up a non-issue.

 

[Leesparkykent]

Its the DNO neutral CSA you need not the tails.

 

[Deleted member 9648]

Its the DNO neutral CSA you need not the tails.

Fat chance of finding that out!

 

[Leesparkykent]

For a 400A supply it will probably be 185mm-300mm wavecon depending on conductor material. It would obviously be handy if you could get the DNO to confirm but taking an educated guess from the age of the install and size of the supply the 25mm bonding will probably be undersized.

 

[Adam W]

I looked into this recently for similar reasons. I found a reg stating the bonding should be no less than half the size of the CPC, and that there is no requirement to install bonding cables any larger than 25mm².
I don't have the book with me.

 

[Leesparkykent]

I looked into this recently for similar reasons. I found a reg stating the bonding should be no less than half the size of the CPC, and that there is no requirement to install bonding cables any larger than 25mm².
I don't have the book with me.

Except where PME conditions apply.

 

[jackhammerJIM]

I looked into this recently for similar reasons. I found a reg stating the bonding should be no less than half the size of the CPC, and that there is no requirement to install bonding cables any larger than 25mm².
I don't have the book with me.

Except where the supply is PME .


You beat me to it LEE

 

[Adam W]

Where's the like button gone?

 

[Deleted member 9648]

Thanks chaps, EIC will go to the company tomorrow with a note that main bonding is undersized. It seems odd though that a large and otherwise well designed recent install would have such a fundamental error.

 

[davesparks]

Unless the supply is PME then the 25mm is the correct size. A supply of that size is reasonably unlikely to be connected to a PME network I should have thought.

How have you confirmed that it is TNCS? Presumably the previous paperwork?


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[Deleted member 9648]

It is TNCS. An earthing bar exits the side of the head on the RHS and must be linked to the neutral. PFC and PEFC are the same, and previous EICR shows TNCS

 

[davesparks]

It is TNCS. An earthing bar exits the side of the head on the RHS and must be linked to the neutral. PFC and PEFC are the same, and previous EICR shows TNCS

Ok, but is it PME?
I think it is more likely that it is going to be fed from its own fuse way at the substation/feeder pillar than from a PME distributing main

 

[Deleted member 9648]

Ok, but is it PME?
I think it is more likely that it is going to be fed from its own fuse way at the substation/feeder pillar than from a PME distributing main

If you read the definition of PME in the BYB it begins...."An earthing arrangement,found in TNCS systems......"
As far as I'm concerned that tells me that if it is TNCS then PME requirements apply.

 

[Deleted member 26818]

In the UK, I believe ESQR requires all distributed TN-C-S supplies to be PME.

 

[davesparks]

If you read the definition of PME in the BYB it begins...."An earthing arrangement,found in TNCS systems......"
As far as I'm concerned that tells me that if it is TNCS then PME requirements apply.

Then I would suggest you do a little bit more research and actually learn about this before making a report which could cost your customer a lot of money to fix something which may not need fixing.

The other common arrangement for TNCS is PNB, this is used where a single service is fed from a transformer or fuse way at a feeder pillar.