Quality earth rods are commonly made from either solid copper, stainless steel or copperbonded steel. Three types of earth rod are available, but the copperbonded steel cored rod is by far the most popular, due to its combination of strength, corrosion resistance and comparatively low cost.
Solid copper and stainless steel rods offer a very high level of corrosion resistance at the expense of lower strength and higher cost.
Diameter of rod
One common misconception is that the diameter of the rod has a drastic effect on lowering earth resistance. This is not true! As the graph shows, you only lower the resistance value by 9.5 per cent by doubling the diameter of the rod (which means increasing the weight and the cost of the rod by approximately 400 percent). Thus the rationale is: Use the most economical rod that soil conditions will allow you to drive. This is one of the ways to ensure that you don’t waste money on over-dimensioned rods.
Thread and shank diameters
Confusion often arises between thread and shank diameters for threaded rods. The thread rolling process, used by quality rod manufacturers, raises the surface of the rod so that thread diameter is greater than shank diameter. All threads are Unified National Coarse (UNC-2A).