30 www.megger.comDead Earth Method: When using a four-terminal instrument, P1 and C1 terminals connect to the earth electrode under test; P2 and C2 terminals connect to an all-metallic water-pipe system. With a three-terminal instrument, connect X to the earth electrode, P and C to the pipe system (Fig. 12). If the water system is extensive (covering a large area), its resistance should only be a fraction of an ohm. You can then take the instrument reading as being the resistance of the electrode under test.The dead earth method is the simplest way to make an earth resistance test. With this method, resistance of two electrodes in series is measured — the driven rod and the water system. But there are three important limitations:1. The waterpipe system must be extensive enough to have a negligible resistance.2. The waterpipe system must be metallic throughout, without any insulating couplings or flanges.3. The earth electrode under test must be far enough away from the water-pipe system to be outside its sphere of influence.In some locations, your earth electrode may be so close to the water-pipe system that you cannot separate the two by the required distance for measurement by the two-terminal method. Under these circumstances, if conditions 1 and 2 above are met, you can connect to the water-pipe system and obtain a suitable earth electrode. As a precaution against any possible future changes in the resistance of the water-pipe system, however, you should also install an earth electrode.Due to the many uncertainties associated with this method of testing, it should be considered using as a “last resort.”Effects of Different Reference Probe LocationsNow, you may ask: if the right location for probe P is always 62 percent of the distance between the earth electrode and C, why bother with all the tests at other locations for P? Why not just drive P in at the 62 percent distance and assume that the measured resistance is the correct earth resistance? The following should help answer these questions.
