Main Protective Bonding Conductors

[jaypp]

Hi all.
Could someone with better understanding help clear this up for me please;
In the Regs (544) there appears to be no value of resistance for the above and also states that if copper, need not exceed csa of 25mm2.
In GN3 it talks about using a value of 0.05 ohms for verification of bonding conductors that can't be seen.
If I had 100m of 10mm2 Earth visible for its entire length am I correct it would comply(assuming no other factors apply), and that if not visible, an estimated reading in the area of 0.18 ohms would mean that it wouldn't comply as per GN3?

Above is just example to show I've made a bit of an effort!!

Thanks in advance

 

[SirKit Breaker]

You are correct matey. Main bonding conductors must not exceed 0.05ohms. So you would need to increase the CSA until you got under this value. When you tick the box to confirm connection of bonding, you are also ticking got say it is compliant, and you have used the correct method to confirm that it is out of spec.

Cheers..........Howard

 

[D Skelton]

0.05 does not refer to the resistance of the bonding conductor, it refers to the resistance of the bonding connection, and even then, it is guidance only. You size your bonding conductor as you normally would, ie not less than half the required size of the main earthing conductor or table 54.8 depending on whether or not the system is TN-S or PME.

During periodic inspetion 0.05 ohms can be used as a figure to determine the effectiveness of the main bonding. Like I said, it is guidance only.

 

[jaypp]

Cheers Howard. Was 'hypothetical' question really, but gives me a better understanding in the unlikely event I come across a conductor of that length!!!

 

[telectrix]

deleted.

 

[jaypp]

0.05 does not refer to the resistance of the bonding conductor, it refers to the resistance of the bonding connection, and even then, it is guidance only. You size your bonding conductor as you normally would, ie not less than half the required size of the main earthing conductor or table 54.8 depending on whether or not the system is TN-S or PME.

Much appreciated DS. That's cleared up the reason GN3 gives a value where connections not visible; because can't check it's mechanically sound. So would expect it to be no greater than calculated conductor resistance + 0.05 ohms.
That's what I was looking for, makes perfect sense really!!. Thanks

 

[SirKit Breaker]

Mr Skelton has given you a very good answer, with a bit more depth to it than mine. But you had got your head around the basics to start with.

Cheers................Howard

 

[SirKit Breaker]

Cheers Howard. Was 'hypothetical' question really, but gives me a better understanding in the unlikely event I come across a conductor of that length!!!

You may not come across a cable of that length, but you may come across and out of spec reading, so its worth knowing what the reading should be. And as Mr Skelton points out well, it is the resistance of the connections that are important, and it is usually these that give problems. The correct way to test a main bonding conductor, is to disconnect it at the CU and test using test method 2, from the disconnected end, to the pipework adjacent to the clamped connection, this verifies the cable and its connection. GN3 covers it well.

Cheers...............Howard

 

[jaypp]

Thanks gentlemen for your advice and explanations. much appreciated.

 

[D Skelton]

It's threads like this and the comments and advice contained within that give me faith in this forum once again. No sabre rattling, no mud slinging, just a plain, straight up and well written question with the opportunity for others to offer advice.

These threads are getting few and far between, but as long as they keep coming, I still have a reason to stick around