18 www.megger.com5 I = E/R = 2,300/ (10 + 13) = 100 amperesThe metallic body in the earth is often referred to as an electrode even though it may be a water-pipe system, buried strips or plates, or wires. Such combinations of metallic bodies are called a grid. The earth resistance we’re concerned with is the resistance to current from the electrode into the surrounding earth.To appreciate why earth resistance must be low, you need only use Ohm’s Law: E = R x I where E is volts; R, the resistance in ohms; and I, the current in amperes. Assume that you have a 4000-V supply (2300 V to ground) with a resistance of 13 Ω (see Fig. 7). Now, assume that an exposed wire in this system touches a motor frame that is connected to a grounding system which has a 10-ohm resistance to earth.By Ohm’s Law, there will be a current of 100 A5 through the fault (from the motor frame to the earth). If you happen to touch the motor frame and are grounded solidly to earth, (by standing in a puddle) you could be subjected to 1000 V (10 Ω x 100 A).As you’ll note from point 2 in the following, this may be more than enough to kill you instantly. If, however, the earth resistance is less than 1 Ω, the shock you’d get would be under 100 V (1 x 100) and you’d probably live to correct the fault.Fig. 7: Example of an electrical circuit with too high an earth resistance
