EARTH-RESISTANCE METER

PROBLEM TO BE SOLVED: To inform the erroneous connection with a commercial power source to an user in the measurement of the earth-resistance by a two-electrode method, by giving the alarm when the commercial power source is detected by a voltage detecting means which detects the voltage to be applied to a third measuring terminal. SOLUTION: In the case where the earth-resistance is measured by the two-electrode method, the measuring terminals P, C are short-circuited by a switch 13, and the measuring terminals E, C are respectively connected to an earthing body to be measured, and with an earthing line of the commercial power source, to measure the earth-resistance Rx. When the measurement terminal C is connected to a hot line for the commercial power source by an error, the electric current flowing in a measuring loop reaching the measuring terminal C through the measuring terminal C, a thermistor 6, an oscillating circuit 3, a transformer 1, the measuring terminal E and the earth-resistances Rx, Ro, is increased. Whereby the thermistor 6 generates the heat, the resistance value is increased, the electric current of the measuring loop is reduced, and the oscillating circuit 3 or the like is protected from AC 100 V. Further the AC 100 V is divided 8, 9, a FET 11 is switched ON by the voltage of a partial voltage point B, and an alarming sound is output from a buzzer 12.