Bonding issue upon upgrading fusebox.

rattlehead85 · Mar 7, 2013

Was at a 3rd floor flat today on a TNC-S system fed from a communal intake using 16mm t&e. The job was to replace a light and wire in a fan to the bathroom. Carrying out the tests required to complete a MEIC i found the main switch at the fuse board was burnt out with some minor damage to the supply cable.
The board can be replaced and cable re terminated but the issue i have is the bonding to the gas meter which is in the flat is only 6mm2 and run to the board within the flat and not back to the intake source position.
The 6mm2 earth feeding the flat is clearly being used as a combined bonding and earthing conductor. The water bond is sized to 10mm2 and bonded back at the intake position.
There is no feasible way to get a 10mm2 gas bond back to the intake point as this would involve major disruption to decor and extensive damage.
I have checked that the existing gas bond is integral and shows no signs of deterioration or thermal damage.
Can i feasibly leave this, change the board and note on the EIC that the bonding although integral is not to current regs or do i have no option bar destroying the property in question.
I have carried out extended tests of all circuits as the board needs replacing and the results bar the gas bond are within BS7671 constraints.

Engineer54 · Mar 8, 2013

As far as the bonding cables go, both water and the gas bonding cables should be connected between the service pipes as they enter each flat, and the flats MET (which is an installation EMT) not to the buildings intake position (MET).

As this is a TNC-S (PME) installation, a minimum 10mm main earthing conductor should be run in with the 16mm T&E. I'd personally, never even think about running/using T&E as a sub-main cable to supply flats, even for TN-S systems, it just smacks of amateurism to me!! ...lol!!

rattlehead85 · Mar 8, 2013

Engineer54 said:
As far as the bonding cables go, both water and the gas bonding cables should be connected between the service pipes as they enter each flat, and the flats MET (which is an installation EMT) not to the buildings intake position (MET).

As this is a TNC-S (PME) installation, a minimum 10mm main earthing conductor should be run in with the 16mm T&E. I'd personally, never even think about running/using T&E as a sub-main cable to supply flats, even for TN-S systems, it just smacks of amateurism to me!! ...lol!!

Most of the old style houses that have been split into flats with a communal main head are done like this, especially
the old terraced style street properties.
It leaves you very few options when it comes to remedial and repair works. The existing bonding is continuos and shows no deterioration or signs of overheating.

matt.smallbone · Mar 8, 2013

You could try using the adiabatic equation to see what the minimum earthing conductor has to actually be and make a note of it on the test sheet. That might be a way around it?

rattlehead85 · Mar 8, 2013

matt.smallbone said:
You could try using the adiabatic equation to see what the minimum earthing conductor has to actually be and make a note of it on the test sheet. That might be a way around it?

Yep the earthing conductor works out fine using the adiabatic. It was just the fact the gas was 6mm. I have done a EIC and noted the gas as 6mm and verified for integrity and no thermal damage.

odawire · Mar 8, 2013

I know loads of old flats are supplied by t+e but its not really acceptable unless its buried more than 50mm all the way which it usually isnt

Engineer54 · Mar 8, 2013

Is the water service bonded within the flat and taken to the flat's MET?? Or are you referring to the water pipe bonding as it enters the building?? If the latter, there should also be a water bonding connection within the flat to the flats MET as previously stated!!

R&M · Mar 8, 2013

Where PME conditions apply, Regulation 544.1.1 requires the main bonding conductors to be selected in accordance with the neutral conductor of the supply.
The above requirement of Regulation 544.1.1 also applies to the earthing conductor of the installation. This is because the earthing conductor also performs the function of a main bonding conductor. The cross-sectional area requirements are given in Regulation 542.3.1 of BS 7671, which states that ‘Every earthing conductor shall comply with Section 543 and, where PME conditions apply, shall meet the requirements of Regulation 544.1.1 for the cross-sectional area of a main bonding conductor...’ That is to say, the csa of the earthing conductor must be not less than the minimum required for its function as an earthing conductor or for its function as a main bonding conductor, whichever is the greater.*

Part of TABLE 54.8 of BS 7671 Minimum cross-sectional area of the main protective bonding conductor in relation to the neutral of the supply

Copper equivalent cross-sectional area of the supply neutral conductor	Minimum copper equivalent cross-sectional area of the main protective bonding conductor
35 mm[SUP]2[/SUP] or less	10 mm[SUP]2[/SUP]

[TABLE="align: left"]
[TR]
[TD]
[/TD]
[/TR]
[/TABLE]
* These data should be used as a guide only
Source: ESC, BS7671

Regards
R&M

Engineer54 · Mar 8, 2013

Well that's the very long winded way of stating the same as has already been stated!! lol!!

R&M · Mar 8, 2013

Engineer54 said:
Well that's the very long winded way of stating the same as has already been stated!! lol!!

lol...... I tried to be precise.

Regulations are boring. Not my fault.

Regards
R&M

rattlehead85 · Mar 8, 2013

Engineer54 said:
Is the water service bonded within the flat and taken to the flat's MET?? Or are you referring to the water pipe bonding as it enters the building?? If the latter, there should also be a water bonding connection within the flat to the flats MET as previously stated!!

The water main stopcocks to each flat are located below the intake cupboard in the communal hall. These are all bonded using 16mm2 earth running back to the m.e.t. next to the flat switch fuses. The gas meters are located in each flat and the one i am working on has a 6mm2 bond running back to the consumer unit which i have verified for continuity.

Engineer54 · Mar 8, 2013

rattlehead85 said:
The water main stopcocks to each flat are located below the intake cupboard in the communal hall. These are all bonded using 16mm2 earth running back to the m.e.t. next to the flat switch fuses. The gas meters are located in each flat and the one i am working on has a 6mm2 bond running back to the consumer unit which i have verified for continuity.

That's all well and good, but the water service pipe as it enters each and every flat (which should really have a stop-cock anyway), needs to be bonded back to the flats MET!!
What i'm saying here, is that each flat is to be considered as a separate installation, and bonded accordingly!!

oldtimer · Mar 8, 2013

I think you have answered your own question if the board is not getting upgraded then its leave alone if it is getting upgraded and its a TNCS then it 10mm to water and gas end of story. So explain it to the customer.

How many times a week is this question asked ?

ses · Mar 8, 2013

Welcome to reality
Its just not always practical to upgrade bonding in the real world.
Some of the buildings in london i work in it would be impossible running bonding back to met
If your upgrading the board your improving the installation and making it safer
Make a note on cert as you said and move on

HappyHippyDad · Mar 11, 2013

Best Practise Guide No2 - Consumer Unit replacement in domestic and similar premises says the following on Page 5 (note 3)

"A 6mm main protective bonding conductor size could be deemed adequate where the minimum size required by regulation 544.1.1 of BS7671 is 10mm if the bonding conductors have been in place for a significant time and show no signs of thermal damage."

So it looks like you're ok as long as you meet those 2 criteria.

Also worth looking at note 2 on the same page (5) which mentions certain criteria for using a 6mm main earthing conductor.

http://www.esc.org.uk/fileadmin/user_upload/documents/industry/best_practice/BPG6_09.pdf

rattlehead85 · Mar 11, 2013

happyhippydad said:
Best Practise Guide No2 - Consumer Unit replacement in domestic and similar premises says the following on Page 5 (note 3)

"A 6mm main protective bonding conductor size could be deemed adequate where the minimum size required by regulation 544.1.1 of BS7671 is 10mm if the bonding conductors have been in place for a significant time and show no signs of thermal damage."

So it looks like you're ok as long as you meet those 2 criteria.

Also worth looking at note 2 on the same page (5) which mentions certain criteria for using a 6mm main earthing conductor.

http://www.esc.org.uk/fileadmin/user_upload/documents/industry/best_practice/BPG6_09.pdf

I had totally overlooked using BPG 2 which clearly points out what is permissible on a board change.
I was correct with regards to checking the bonding for thermal damage and corrosion if less than 10mm for continued use. I had also confirmed the adequacy of the earthing conductor by applying the adiabatic. On existing installations where a board change is needed i will from now on make reference in the 'extent of works' box that the change has been completed in accordance with BPG 2.and will note any departures found in the relevant box on the EIC.

rattlehead85 · Mar 11, 2013

Engineer54 said:
As far as the bonding cables go, both water and the gas bonding cables should be connected between the service pipes as they enter each flat, and the flats MET (which is an installation EMT) not to the buildings intake position (MET).

As this is a TNC-S (PME) installation, a minimum 10mm main earthing conductor should be run in with the 16mm T&E. I'd personally, never even think about running/using T&E as a sub-main cable to supply flats, even for TN-S systems, it just smacks of amateurism to me!! ...lol!!

I appreciate your view on this mate. If this was a new installation i also wouldn't consider using a t&e cable for the main supply and would use the sizing tables for the required earthing over the use of the adiabatic, however, this is an existing installation with very limited to no possibility for upgrade.
As a thought, as the flats have separate supplies fed from individual switch fuses at the intake and the service belonging to each flat are bonded to these wouldn't this be classed as satisfactory as the M.E.T within the switch fuse could be classed as part of that installation only?