Main Protective Bonding (Domestic CU Change, TN-S supply)

[Tuttle]

I've gone around in circles several times on this and I'm still not completely sure what the answer is ... generally speaking, when changing a domestic CU in a house that has a TN-S supply and 6mm[SUP]2[/SUP] main bonding (which is in good condition) is it necessary to upgrade the main bonding to 10mm[SUP]2[/SUP]?

I know that 544.1.1 says that, for non PME supplies, main protective bonding conductors must have a cross-sectional area of not less than half that required for the earthing conductor (and not less than 6mm[SUP]2[/SUP]), and 543.1.1 says that the required cross-section area for the earthing conductor may be calculated (using the adiabatic equation) or selected according to 543.1.4.

If you select according to 543.1.4 you basically end up with a 16mm earthing conductor and 10mm main bonding conductors, but if you calculate using the adiabatic equation and measured prospective fault current then I think that the required earthing conductor normally ends up being less than 12mm, meaning that 6mm main bonding is ok.

What I am a little concerned about is that the On-Site Guide section 4.4 states that the minimum csa for main bonding conductors (for 16mm[SUP]2[/SUP] or 25mm[SUP]2[/SUP] tails) is 10mm[SUP]2[/SUP]. It doesn't say, as far as I can see, that a smaller csa can be used in any situation, it just states that 10mm[SUP]2[/SUP] is the minimum. Am I right in thinking that Table 4.4 in the OSG is just based on 543.1.4 and/or is intended to give a minimum csa that will be suitable for any installation that falls within the scope of the OSG (section 1.1), i.e. worst case minimum (if so I wish that they would make that clear in the notes rather than stating that the values are minimums)

So what I really want to get to the bottom of is ... is it acceptable to size the earthing conductor and main bonding conductors according to measured Ze/PFC (for existing installations), is it ok to disregard the "minimum csa" stated in section 4.4 of the OSG when calculating using the adiabatic equation, and if so what is the justification for this, and basically is it generally ok to leave 6mm main bonding in place on TN-S installations when changing the CU?

Also, has anyone here talked to any of the Part P scheme providers about this and got their view on it?

 

[Deleted member 26818]

There are a number of areas where the OSG and the Regulations differ.
I don't know why this is.
I would ignore the recommendations in the OSG, some of which if followed would actually result in non-compliance with BS7671.

 

[rich.250]

OSG after all is a guide.
As spin is saying,
Follow BS7671

 

[Tuttle]

Thanks guys.

So I'm thinking that if the existing 6mm main bonding has been in place for a long time, is in good condition, is correctly installed and the supply is TN-S and the measured PFC is not huge, i.e. the adiabatic equation gives an earthing conductor of no bigger than 12mm[SUP]2[/SUP], then the 6mm main bonding can be considered "adequate" and doesn't need to be upgraded.

Does that sound reasonable? Anyone disagree with that?

I'd be interested to know what everyone else generally does in this situation.

 

[telectrix]

exactly what you have just said. ^^^^