Working principle of ground resistance meter

　　The ZC-8 ground resistance meter is made according to the principle of compensation method, with a hand-cranked AC generator as the power source. Its working principle is shown in the figure. In Figure (a), TA is a current transformer, F is a hand-cranked AC generator, Z is a mechanical rectifier or a phase-sensitive rectifier amplifier, S is a range conversion switch, G is a galvanometer, and Rs is a potentiometer. The meter has three ground terminals, namely, ground terminal E (terminal E is formed by short-circuiting the potential auxiliary terminal P2 and the current auxiliary terminal C2 inside the meter), potential terminal Py, and current terminal C). Each terminal is inserted into the ground through a probe at a specified distance to measure the soil resistance between the two terminals E and P). In order to expand the range, two groups of different shunt resistors R1~R3 ​​and R5~R8 are connected in the circuit to realize the three-speed shunt of the secondary current I2 of the current transformer and the galvanometer branch. The switching of the shunt resistor is completed by the range conversion switch S, corresponding to the three gears of the conversion switch, which are 0~1Ω. 1～10Ω and 10～100Ω.

　　Figure ZC-8 ground resistance measuring instrument working principle

　　The circuit in Figure (a) is simplified and drawn into a schematic diagram for actual measurement, as shown in Figure (b). In the figure, E' is the grounding body, P' is the potential grounding electrode, and C' is the current grounding electrode, which are connected to the E, P1, and C1 terminals respectively and inserted into the soil at a distance of not less than 20m and 40m from the grounding body.

　　Assuming that the hand-cranked AC generator F outputs AC power at a certain moment, and its left end is at a high potential, then at this moment, the current J passes through the primary side of the current transformer → terminal E → grounding body E' → earth → current grounding electrode C' → terminal C1, and then returns to the right end of the hand-cranked AC generator, forming a closed loop. The voltage drop formed on the grounding resistance Rx of E' is IRx, and the voltage drop IRx drops sharply as the distance from the E' pole increases, and drops to zero at the P' pole. Similarly, a voltage drop will also occur between the two electrodes P' and C', and its value is IRc. The potential distribution is shown in Figure (b).

　　The secondary current of the current transformer is KI (K is the transformer ratio: I2/I1), and the voltage drop of this current through the potentiometer point s is KIRs. By adjusting the active contact W of the potentiometer, the galvanometer indication is zero. At this time, the potential between points P' and s is zero, which is

　　It can be seen from formula (8-2) that the measured grounding resistance Rx can be determined by the transformation ratio K of the current transformer and the resistance R of the potentiometer, and has nothing to do with the resistance R of the current grounding electrode C'. The method of measuring grounding resistance using the above principle is called compensation method.

　　It should be pointed out that the current grounding electrode C' is absolutely necessary to form the path of the grounding current. If there is only one electrode, the measurement result will inevitably include the grounding resistance of the grounding body E', which is obviously incorrect. It should also be pointed out that AC is generally used to measure the grounding resistance. This is because the conductivity of the soil mainly depends on the action of underground electrolytes. If DC is used, it will cause chemical polarization, which will seriously distort the measurement results.