TT systems

[sparky40]

Hello All,
I am trying to find out what size conductor should be used between an 'earth rod' and the consumer unit. The only information I can find in the OSG is that it should be 2.5mm if protected and not buried, and 4mm if un-protected ( or protected against corrosion only ) If buried then I know the size increases and can go up to 25mm.

Is a bit of 2.5mm big enough ?? Can somebody please explain why this cable need not be of a larger c.s.a ?

Cheers in advance for any input.
p.s I have a tenner bet on this, I quoted the 2.5 and 4mm to my mate, as the cable in question is not buried. He reckons it should be at least 10mm !

 

[slippyr4]

the resistance of the rod/mud junction is so high that 2.5mm^2 is plenty low enough R

 

[securespark]

You what?

 

[sparky40]

Sorry, still cannot get my head around this one, can anyone explain why the conductor size is so small please ?
Thanks for any help in advance people.

 

[securespark]

the resistance of the rod/mud junction is so high that 2.5mm^2 is plenty low enough R

Still waiting for explanation of this!!

 

[fastJedi]

Because ... even the best earth rod installation still has a relatively high resistance .... therefore the max current is going to be relatively small.

PS: I used 10mm on mine cus it looked better!!!

 

[securespark]

I understand what you're getting at...

The bit I don't get is this "rod/mud junction" business.

 

[Big_Spark]

The cable from the Rob to your main earth terminal must comply with the Regs in all respects. Therefore it must comply with Section 54 of BS7671.

I would direct your attention to Reg 547-02-01, now this deals with MAIN EQUIPOTENTIAL CONDUCTORS not main earthing conductors, however it states that these must not be less than 6mm².

Now if you refer to table 54A you will see there that the minimum size is 16mm² for any earthing conductor buried in the ground. Now if you want to know how you should earth a TT system, then I would refer you to 542-02 as this is a requirement as stipulated in 542-01-04.

Basically what it boils down to is that the main conductor, the link from the rod to the earth terminal, should not be less than half the csa of the main phase conductor.

 

[Big_Spark]

Because ... even the best earth rod installation still has a relatively high resistance .... therefore the max current is going to be relatively small.

PS: I used 10mm on mine cus it looked better!!!

Not true, I have seen Earth Rod installations where the measured resistance of the Rod was less than 1 Ohm. I regularly see installations where the resistance of the rods used for lightning protection are below 5 Ohms.

The big problem is that most people do not install the rods properly, they buy a sinbgle rod, drive into the ground and think this good enough.

 

[sparky40]

Thanks for the posts, but seeing as I have a tenner on it I am standing my ground, and saying that the cable only need be 2.5 or 4mm, depending on whether its protected or not, so long as it isn't buried.
I am not talking about main equipotential bonding.

 

[Softus]

The bit I don't get is this "rod/mud junction" business.

"rod" refers to the earth rod; "mud" refers to the surrounding ground; "junction" refers to the point(s) of contact between the rod and the surrounding ground.

 

[Big_Spark]

Thanks for the posts, but seeing as I have a tenner on it I am standing my ground, and saying that the cable only need be 2.5 or 4mm, depending on whether its protected or not, so long as it isn't buried.
I am not talking about main equipotential bonding.

I knew that if you read the first post I did, but surely the rod is for connection of the Main Bond, and even if this is not the case, then I would still suggest 6mm² as a bare minimum.

 

[Softus]

Er, I'm not an electrician, but I can't make any sense of the belief that the gauge of the conductor leading to the earth rod can be any less than the gauge used for the main bonding conductor.

 

[Spark123]

It depends on the supply, the purpose of the main equipotential bonds it to keep all ecps in the premises at about the same potential. The MET is connected to the electrode in an attempt to keep the equipotential bonding at earth potential. They get away with using a smaller a main earthing conductor in a TT system owing to the high external loop impedance and the use of RCDs.

 

[Softus]

That makes perfect sense. Thanks!