The power and telephone feeds to your house can be either aerial or underground. Most people think underground is better from a lightning standpoint. Buried underground, it will not be hit directly, but if a nearby tree is hit, the amount of energy coupled through the conductive ground medium can be almost equal to a direct hit. By being underground, the shielding effect to the wires is not great. The buried depth does little when compared to the depth low frequency strike energy penetrates. When you consider the cost of underground utilities, these and the aesthetics must be weighed.

Solid copper wire/strap and copper clad steel rods, makes copper the most commonly used earthing material. 

Your below grade ground system should be made with the same material throughout. Mixing of materials, like galvanized rods with bare copper radials, will create a battery action and the zinc of the galvanized rods will become sacrificial, dissolving into the soil. This leaves bare steel to rust and not provide an optimum connection to earth. (Note: when wet, rust can conduct, bit not very well.) Using stainless rods in order to prevent corrosion will not provide the best conductivity. Since stainless wire will be required to interconnect the rods, the resistance of the system will increase. An all aluminum ground system should only be considered in very acid soil conditions and even then it should be chemically tested for other attacking soil compounds.

Joints between copper radials and copper clad rods should be made by exothermic welds or by using joint compounds in high compression clamps. Solder connections, even torched silver solder connections will not last as long as the above. An exothermic weld is created when a graphite mold around the connection is filled with copper oxide and aluminum powders. An additional starter powder ignites the exothermic process. The resultant molten copper is deposited into the lower mold cavity where it burns away any oxides and creates a larger fused connection. The larger cross sectional bond decreases the resistance and increases the surface area, reducing the inductance of the joint. Since the materials are all the same, the connection will last as long as the rest of the grounding material. High pressure clamps provide a meshing of copper to copper since the material is soft (malleable). The use of joint compounds further enhances the weather tightness of the bond.

The high pressure will need to come from another material stronger than copper.

If you find a rock layer is making the ground rod insertion tough and you can’ t remove the rod to start over a few feet away, the best idea is to cut off the rod and connect it to the system. A rock layer will hold water and salts so the conductivity above should be good. Making more connections to areas of higher conductivity will reduce the overall impedance of the ground system (resistance and inductive reactance).

The ground system has a resistance and an inductance value. (It has capacitance too!) The amount and location of the inductance can choke off the effectiveness of radials. When a radial is in poorly conductive soil such as buried in a dry, sandy layer, the radial inductance can be calculated as being in air (a very poor conductor).

When the radial runs in highly conductive moist soil (or doped soil), the inductance of the wire is shunted by the soil’ s conductivity, making it unimportant.

Since copper strap has lower inductance than wire, it is recommended for the radial run. The strap’ s extra surface area reduces the inductance and the sharp edges allow for a high E field concentration forcing more charge into the soil. Short multi-point (like bobbed wire) type grounding systems have been tried and have not been as effective as the sharp edge of copper strap for ground rod interconnecting material or for radial runs

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