According to the U.S. Department of Labor, earth resistance values are a measure of how resistant the earth is to electrical current. The scientific unit used to measure electrical resistance is the ohm.
Earth resistance values are critical in designing grounding systems. As soil is generally a poor conductor of electricity when compared with metal, its function in grounding systems is critical. For exposed metal components, soil prevents dangerous electrical discharge in the event of insulation failure. It is also useful for limiting the accumulation of static electricity. In grounding systems, the ground wire's function as a conducting pathway is to carry excess current into the soil in the event of an electrical fault. Moisture levels, temperature and dissolved salts affect the soil's conductivity. This means that the soil's level of resistance may change from season to season. Ice is a poor conductor of electricity, so its presence must be accounted for on sites that experience colder winters. However, while soil is usually a poor conductor of electricity, its variability means it can act as a conductor of electrical current. In these cases, known as single wire earth return power transmission systems, the typical grounding system design is not necessary.