Bonding to gas 6mm vs 10mm

[baldsparkies]

When bonding to gas 10mm seems to be the norm these days. I came across a pme install with a 60 amp cut out and 16mm tails with 10mm main cpc. The bond was in 6mm with R2 well under 0.5 ohms. So I ask the question, should this be upgraded to 10mm or will 6mm suffice, and if so why?
Would be interested in your thoughts guys,

 

[Notsosmart]

15th edition probably, my 1984 built house was wired in the same manner, no reason that you have to change it unless you make a major alteration then I would say that upgrading would be prudent (IMHO)

 

[Deleted member 9648]

Assume we are just talking 'normal' domestic etc in which case 6.0mm for a TT system, and 10mm for tns and tncs. You can leave 6.0mm if it is already installed as long as you are satisfied that it is sound and there is no sign of overheating.
Personally I feel that 10mm is over the top....6mm is of sufficient size to be mechanically strong and I have to say I have never seen any sign of heat damage to main bonds in 30yrs of contracting...and I've seen plenty of 2.5mm bonds on tncs systems.I honestly believe 10mm is an un-necessary cost burden and unjustified. Has anyone ever seen a main bond of any size damaged by heat caused by carrying a current ???

 

[Guest123]

Has anyone ever seen a main bond of any size damaged by heat caused by carrying a current ???

Nope.

 

[baldsparkies]

Thanks for the reply Notsosmart, but regulations aside, can anyone give a reason why the bonding in this instance would need to be upgraded. I am thinking in terms of a practical real life situation that would lead the installation to be better with a 10mm vs 6mm bond. By the way the board is fitted with
b type mcb's 60898 with 32amp being the highest rated. With all the facts present what conclusions would you come to bearing in mind that the regulations are for guidance purposes ie rule of thumb.
I am thinking of the potential for a fault current being generated that would deem the 6mm to be unsafe, and I cant see one. What do you reckon lads. Are people installing 10mm bonds for regs sake, or are they thinking about the decision.

 

[bugsy]

Reg's sake, 90% of reg's are over engineered

 

[baldsparkies]

Reg's sake, 90% of reg's are over engineered

So the big question remains, Why ARE we installing 10mm bonds especially where rcd protection is provided.

 

[pushrod]

According to the regs as i understand them for a pme/tn-c-s system even if the line conductor was 6mm and the main earth was 6mm then the protective bonding conductors should be 10mm seems strange to me.

 

[Jason Smith]

In both the OSG and EG to the building regs it shows the equis as 10 mil and the earth to the met as 16 mil. I have therfore always installed 16 mil to the earth rod as that made sense. I work on TT probaly 80% of the time as I live on an island. Nearly fell through the floor when my inspector told me for a TT you only need 2.5 from the cu to the rod. This wil of course carry enough juice to trip the rcds. Having had a closer look at the books, it cleary shows 16 and 10 mil protective conductors, but the bit down to the rod in the book is not labelled. never noticed. Must get the calculator out

 

[baldsparkies]

In both the OSG and EG to the building regs it shows the equis as 10 mil Must get the calculator out

Well said Jason,
But looking at the tables and working out the calcs, then applying practical real life scenario's ie the generated fault current, disconnection times, max permmited Zs for the overcurrent devices in the c/u and voltage PD's, will lead you back to the same conclusions.
Why are we installing 10mm.

 

[Sintra]

For TN-S or TT systems the csa of main protective bonding conductors must be a minimum of 6mm and not be less than half the csa of the main earthing conductor. 544.1.1
For a PME system the csa of the main bonding conductors must not be less than that given in Table 54.8 ie a 10mm protective bonding conductor for a neutral conductor of 35mm or less. 544.1.1

 

[andyb]

According to the regs as i understand them for a pme/tn-c-s system even if the line conductor was 6mm and the main earth was 6mm then the protective bonding conductors should be 10mm seems strange to me.

It's because with a pme supply, if you lose the neutral the earth will take the full load, and other than rcd protected circuits will carry on working.
With a t-n-s system the same fault will result in the circuit not working.