Leaving 6mm bonding conductors

[vxwestie]

Been called out to an existing customer today where a 30amp Wylex fuse carrier had burnt out. After sorting that out i proceeded to talk to the client and ended up with the job of replacing the fuseboxes with MCB board. (currently 2 x 8way wylex units)

After noting that the tails were ok and it was going to easy enough to replace the main earthing conductor (PME) the problem of the bonding conductors arrose.

The property is a late 1970's very high spec "designer" property, three split levels and 5 beds. BUT the consumer unit is on the top level and the Water/oil incoming supply is on the lowest level. (probably a 50-60m run from the consumer unit). The whole building is pre-cast concrete, walls/floors and most ceilings, most of the existing cabling is in ducts/conduit and with cpc in the lighting circuit im happy very little else will have to be touched. HOWEVER the only direct cabling run for any 10mm earth wire would be 50-60m run of surface mounted trunking, ceiling mounted (currently fancy coving, glass panels, hardwood decor) around doors, stairwells down walls with numourous direction changes etc, etc. ... with the high spec of the decor etc its something neither myself or the elderly couple are keen on and will probably cease the job.

As an NICEIC approved contractor I should probably know the answer to this, but having never contiplated leaving small bonding before ive never considered it.... What regs will i break? what harm? would there be with leaving the 6mm conductors in place? Can it just be a matter of a deviation noted on the completion cert? "upgrade of bonding conductors not practicable" ????
I maybe able to pick up a new 10mm bond on the water closer and easier (but not at incoming posistion), but with the oil so far away it isnt going to be easy.

Whats people's opinions?

 

[Noshocks]

I am an NICEIC AP as well and on one of my assesments I asked the area engineer the same question, he said if the 6mm bonds are in place then there was no need to change them, although prefereable to do so where possible, up till this point i had always upgraded them and still do if practicable, however some houses it just cant be done without ripping floors up and doing chases.

 

[Des 56]

Changed my mind about the esc,it may only be for eicrs,I will have a look


Here is what i found
quote ESC

Can I carry out an alteration or addition to an existing installation that has inadequate earthing and/or bonding arrangements?

No. Any alteration or addition to an existing installation must comply with the current edition of BS 7671 (including any amendments). This requires any earthing and bonding arrangements upon which the alteration or addition relies for safety to be adequate.
Regulation number(s)

• 132.16
• 633.2
• 411.3.1.1

Now all you need is to interpret the word adequate

 

[yellowvanman]

As long as the existing 6mm is not damaged and showing signs of overheating then its OK. It specifically mentions this in the Periodic Inspection document from the ESC.

 

[Noshocks]

BS7671 open to interpretation, now thats a first!!

 

[Paul.M]

Test the 6mm bonding conductors and see if they meet current regs. If not scratch your head on how the hell you can replace them without doing damage? Btw 6mm to 10mm won't make that much difference. Good luck.

 

[Guest123]

Quote ESC......

"Where a designer finds, in an existing installation where PME conditions apply, that the main bonding conductors have a csa of say 6 mm2 rather than 10 mm2, the designer may, having carefully considered all the circumstances, conclude that the deficiency does not pose a significant risk to the users.

For example, if there is no evidence of overheating of existing main bonding conductors and terminations, it may be reasonable to assume that the existing arrangements are adequate for any network circulating current (unless the main bonding conductors were only recently installed). However, the deficiency must be recorded, together with the reasons why the installer has concluded that the deficiency does not pose a significant risk to the users.
It should be noted that the non-correction of departures from the requirements of BS 7671 is not an option in a new installation or for the rewire of an existing installation".

 

[morph]

adiabatic?? not sure what device you do the sum on though, ie would it be the main fuse

 

[yellowvanman]

adiabatic?? not sure what device you do the sum on though, ie would it be the main fuse

Adiabatic is for the protective conductor of a circuit, if you take the main feed then that tells you what the main earth should be and if not TNC-S the bonding can be half the size of the main earth.

As this install is TNC-S then bonding should be 10mm minimum, if new build or re-wire.

Its my understanding that for TNC-S in certain fault conditions on the supply side there is in the region of 40A flowing around the earth conductors, hence the 10mm size. Not sure how they come to this figure but was told that once by Scottish & Southern engineer, or thasts how I understood it!

 

[tony mc]

You can't use the adiabatic to size main bonding conductors and with it being PME you would be going off table 54.8 anyway!

Lennys post explains it very well !