JPH05288790

PURPOSE:To improve reliability of a system by connecting a power supply for applying an AC voltage a cross a current sensor coil, a relay for turning on or off voltage, and a resistor for measuring current which flows and then judging presence or absence of disconnection according to a phase difference of voltage which appears in the resistor. CONSTITUTION:A current sensor coil 1 is connected to an overhead earth wire, current which is proportional to current which flows in the overhead earth wire is allowed to flow to the coil 1, and voltage at both edges of the resistor 2 is measured by a measuring device 3, thus measuring current of the overhead earth wire. When detecting disconnection, a disconnection detection part 7 closes the switch of a relay 4 and applies a voltage of an AC power supply 6 to the coil 1 through a resistor 5. When the coil 1 is not disconnected, the phase of voltage which is applied to both edges of the coil 1 differs from that of current which flows. However, no phase difference is generated between voltage and current. The phase difference between the voltage and current is measured by a detection part 7, thus judging presence or absence of disconnection. A frequency of the AC power supply 6 which is not integer-multiples of the commercial frequency is used.