PURPOSE: To confirm disconnection trouble, by detecting such a state that the change in output voltage due to a zero phase current at the time of trouble is larger than detection sensitivity wherein a max. value is setting and the ratio of said max. value and the change value of output voltage is present between two setting values.

CONSTITUTION: The max. change value among change values of data due to a zero phase current inputted to an input port PI on the basis of the control of CPU is successively and succeedingly selected. By the disconnection trouble and the other state change of a distribution line, the max. value among the change values of data due to the zero phase current is selected. When this value is larger than the detection senitivity of setting and succeeds for a set time while the ratio of the max. value Vod among the change values of data due to the zero phase current and the change value Vd of data due to the load current of distribution line is larger than a first setting value, by operation control of CPU, it can be confirmed that the distribution line max. in the change values of data due to the zero phase current has disconnection trouble.