Resistance to current through an earth electrode actually has three components.
- Resistance of the electrode itself and connections to it.
2.Contact resistance between the electrode and the soil adjacent to it.
- Resistance of the surrounding earth.
Rods, pipes, masses of metal, structures, and other devices are commonly used for earth connections. These are usually of sufficient size or cross-section that their resistance is a negligible part of the total resistance.
Electrode-Earth Contact Resistance:
This is much less than you might think. If the electrode is free from paint or grease, and the earth is packed firmly, contact resistance is negligible. Rust on an iron electrode has little or no effect; the iron oxide is readily soaked with water and has less resistance than most soils.
But if an iron pipe has rusted through, the part below the break is not effective as a part of the earth electrode.
Resistance of Surrounding Earth:
An electrode driven into earth of uniform resistivity radiates current in all directions. Think of the electrode as being surrounded by shells of earth, all of equal thickness (see Fig.). The earth shell nearest the electrode naturally has the smallest surface area and so offers the greatest resistance. The next earth shell is somewhat larger in area and offers less resistance.
Finally, a distance from the electrode will be reached where inclusion of additional earth shells does not add significantly to the resistance of the earth surrounding the electrode. It is this critical volume of soil that determines the effectiveness of the ground electrode and which therefore must be effectively measured in order to make this determination.
Ground testing is distinct when compared to more familiar forms of electrical measurement, in that it is a volumetric measurement and cannot be treated as a “point” property.
Generally, the resistance of the surrounding earth will be the largest of the three components making up the resistance of a ground connection.