Earth fault detecting device, and electricity storage system

An earth fault detecting device comprising:

• an impedance element to which a predetermined alternating-current (AC) voltage is applied;
• a capacitor connected between an output terminal of the impedance element and an electricity storage unit mounted in a vehicle, the electricity storage unit being electrically insulated from a chassis of the vehicle;
• an earth fault determiner that determines whether an earth fault is present in the electricity storage unit, based on a detection value, the detection value being based on a peak-to-peak value of a voltage at a node between the impedance element and the capacitor;
• a filter that removes a high-frequency component from the voltage at the node to adjust an offset of the voltage;
• and
• an analog-to-digital (AD) converter that converts an analog voltage received from the filter into a digital value and outputs the digital value to the earth fault determiner, wherein the earth fault determiner
• determines that the earth fault occurs, when the detection value is less than or equal to a first reference value and more than or equal to a second reference value, the first reference value being set based on the peak-to-peak value of the voltage at the node when an insulation resistance between the electricity storage unit and the chassis of the vehicle is a minimum allowable value, and the second reference value being set based on the peak-to-peak value of the voltage at the node when a complete earth fault occurs between the electricity storage unit and the chassis of the vehicle,
• calculates an average value of interval integrated values, over a unit period, of an AC voltage at the node, an average value of a maximum value and a minimum value of the AC voltage over the unit period, or a moving average value of the AC voltage over a predetermined period, and
• determines that the earth fault occurs, when the detection value is less than or equal to the first reference value and more than or equal to the second reference value and the average value is substantially equal to a center value of an input voltage range of the AD converter.