Ground Meter

a device for measuring earth resistance. Ground meters and measuring methods may be simple (ammeter-voltmeter methods) or complex, if used in precision measurements (compensation or bridge measurements). If the ammeter-voltmeter method is used in measurements (Figure l,a), the resistance of the ground conductor A is determined according to the formula r_g = V/I, where V is the

Figure 1. Diagrams of the measurement of earth resistance by the ammeter-voltmeter method (a) and the compensation method (b): (E) current source, (T_r) current transformer,(A) ground, (P) probe, (B) auxiliary ground,(R) rheostat, (T_i) insulating transformer, (I) ground current

voltage between A and the probe P in the zone of zero potential and I is the current in the grounding conductor. If the compensation method is used (Figure 1,b), the voltmeter V∽ is connected across the secondary winding of transformer T_i. A sliding contact of the rheostat R is then moved until the equality U₁ = U₂ is achieved. When this condition is reached, no current flows in the windings of T_i, (V∽ reads zero), and the earth resistance can be read on the scale of the rheostat, since r_g = R_a.