What actually decides the size of a driven earth rod?. Sure I know it's written in the Regs in most countries, as to the size, but do you stop at that?. Surely soil types vary from area to area, take for instance between Rotorua and here in Ashburton. (Rotorua has a very sulphuric soil), here the soil is very stony. How do you get good contact?.
Depend upon surface area and type of soil. PH value, acid or sulphuric.
Auckland is mostly clay and volcanic rock.
We have a quarry out Bethells which has an overhead twin 70mm² earth to get the earth resistance low enough, below 10 Ohms.
Upnorth in Kaitaia where i have done a few wiring jobs they have additional to the 1.8m earth pin a 15 meters of 25mm² bare strip earth laid in the ground which is mostly sand. To do it properly overthere with the backfilling, peat and clean topsoil is added to make a good contact area for the strip earth wire. Regards Ray
The product of rotation, excitation and flux produces electricty.
As far as domestic is concerned, 4 ft. is the norm here. As earth rods are only required on a TT service, which means having one or more main RCDs anyway (or the older, now-obsolete voltage ELCB), the loop impedance is seldom if ever going to be problem.
There appears to have been extensive research done on this issue. While I do not know the original sources, the information is summarised in the "American Electricians' Handbook," sections 8.146- 8.148.
The data presented show very little change by increasing the diameter of the rod.
The depth of the rod has a tremendous effect up to about a four foot depth (This is where a tangent to the curve is approximately at 45 degrees). By ten foot depth, the curve is essentially horizontal; that is, increasing the depth has very little effect.
Finally, multiple ground rods show very little effect, until the rods are spaced (again) at least four foot apart.
In general, our codes here ask for a 5/8" rod, driven 8 ft. into the earth, and at least six feet between rods.
In summary, yes, the type and condition of the earth matter; it's just that there isn't much you can do to change things.
Thanks for your input guys, to this rather strange topic. Ray, I certainly agree with your comments about the use of bare copper underground to effect a good contact, with the ground. John, the use of more than one electrode here, is more or less a rarity here, mainly because of the method that Ray explained above, however, it used to be mentioned in both the 1961 and the 1976 Electrical Regs, under the Part pertaining to testing of Earthing Electrode resistance, that during the test, the rods were required to be at least 6ft apart, to prevent the resistance areas of the 2 rods crossing and mucking up the readings. Briselec raises a very good point, if an electrode has a certain resistance upon it's installation, caused by good damp earth surrounding it, what then happens, should the water table then drop (as it has drastically done in a few parts of the South Island here), does that then render the earthing system useless?.
Yes Kiwi and real maths they were. None of your wimpy no-one can fail Maths as they teach these days. Hell, we didn't even have calculators, and to a degree I'm glad of that. Mmmm, working things out in your head. Just look at what happens when there is a power failure in a supermarket these days. All that "education" for nothing.