TT system main bonding CSA?

[Falcon Flyer]

Rewiring an older TT terraced property. There is currently 6mm earthing from the rod, and also as main bonding to gas and water. I (was!) confident that 6mm main bonding met the requirements of BS7671, but a fellow Part P installer insists that I need to upgrade the main bonding to 10mm, but that 6mm for the earth rod is satisfactory.

So, please could someone tell me the minimum CSA for the main bonding, and, if it is meant to be 10mm, how come the earth rod only needs 6mm? A reference or page number to the big book would also be helpful.

Many thanks

Falcon Flyer

 

[Sintra]

Page 167 BGB.

 

[Farmelectrics]

Have you taken a ze reading

 

[davesparks]

Earthing conductor should be sized using the adiabatic equation.
Bonding should be sized relative to the size of the earthing conductor.
Neither should be less than 6mm.

 

[Deleted member 9648]

Your fellow Part P man is confused...minimum 10mm bonding only applies to PME systems 544.1.1.

 

[Falcon Flyer]

Thanks for all the prompt replies chaps. Ze was 180 ohms, but to be fair it was a very wet day.
My memory isn't what it used to be, but I distinctly remember that the CSA for main bonding should be minimum of 50% of the main incomer, but never less than 6mm. As the main rod is 6mm I couldn't see any practical requirement to change the main bonding to 10mm.

 

[Farmelectrics]

All ok then

 

[davesparks]

Ze of 180 ohms on a wet day?

That's appalling, you should be more concerned about getting a decent set of earth electrodes installed an connected.

Did you also measure Ra?

 

[Farmelectrics]

What reading would you be looking for Dave its under the max value as it is

 

[trev]

180 ohms on a wet day and you're happy with that?
Good grief man! What's going to happen during the summer when the soil around what's probably the twig you've installed dries out?

 

[Farmelectrics]

Ok trev point taken but how low a reading would you expect few months back on a tt system I recorded 120 ohms on test sheet and the company I worked for we're happy with that

 

[davesparks]

What reading would you be looking for Dave its under the max value as it is

What max value? There is no max value stated in the regulations, where did you find one?

Id be looking for less than 20ohms as a minimum but will always aim for the lowest possible.

I always start with 2x 8'x5/8" earth rods spaced at least 10' apart and make an assessment based on the Ra achieved with them.

 

[davesparks]

Looking at the op again he is rewiring a terraced property so could easily install a rod at the front and back of the property for a start.

 

[Farmelectrics]

Tt max value is 200 ohms

 

[trev]

Well it was directed at Falcon Flyer but if you and your employer are happy with what could easily be an unstable reading then fine.
It wouldn't be acceptable to me on one of my jobs though mate