CSA of main earthing conductor for TT

davezawadi (David Stone):

I gave the reg number Z. 2.5 with mechanical protection is permitted. 543.1.1, at least in my book, why are you saying 4mm? Grumpy, Andy and I seem to prefer (option) to use 10mm anyway. Not that it matters.

Hello Dave,

                    all contractors around here use 16mm2 for earth rods at TT installations. I suppose that it is readily available, off the van, and it has strength against bashing and chemical attack. Considering its important safety role I use it as well. I have never seen 4.0mm2 or 2.5mm2 used to an earth rod ever. I did once use 10mm2 though. Considering that the run is normally a shortish one its cost is not too great.


So, according to 543.1.1 we can size a protective conductor by calculation (see 543.1.3)


or


Select it from table 54.7 (see 543.1.4).


Further the reg. says that if the protective conductor is


(iii) not an integral part of a cable.


OR


(iv) not formed of conduit, ducting or trunking,


OR


(v) not contained in an enclosure formed by a wiring system.


then the cross sectional area shall not be less than 2.5mm2 copper equivalent if protected against mechanical damage, or 4.0mm2 copper equivalent if mechanical protection is not provided.


Most earth rods have the top exposed in the garden, along with the cable, so to my mind that requires at least 4.0mm2 as it is not protected against bashing. If we bury the cable in an earth pit that is extra work and expense. It also has to be at least 16mm2 copper in size if buried. (Table 54.1).


Z.






 

Sparkingchip:

There are some very involved answers to a simple question.


Table 54.1 says it is 2.5 mm2 provided the cable has a sheath and is protected against mechanical damage.


16 mm2 if it has a sheath, but is not protected against mechanical damage.


25 mm2 if it does not have a sheath and is not protected against mechanical damage.


Andy B.

Table 54.1 applies to a BURIED earthing conductor. Grumpy was interested in an earthing conductor to a rod which was NOT buried.


Z.

Grumpy:

Thanks both. So, are we saying that if you can't do the sums then you have to use 16mm? There isn't a definitive statement in the BBB like there was in the OSG?

Mind you he only "presumed" the main earth was 6mm instead of 16mm so am I to assume he couldn't find one? Surely that might have been worthy of a comment, as might the presence of three electric showers, disconnection times of 0.4 and 5 seconds . . . I'm beginning to wonder if I've wasted my dollar!!

Are those the disconnection times entered on the certificate when he has condemned the earthing?