Earth fault detection apparatus

An earth fault detection apparatus to be connected with a non-grounded battery and for calculating an insulation resistance of a system including the battery, the earth fault detection apparatus comprising:

• a capacitor configured to be operated as a flying capacitor;
• a switch group configured to switch between a first measurement path, a second measurement path, and a third measurement path, the first measurement path including the battery and the capacitor, the second measurement path including the battery, a negative-side insulation resistance that is an insulation resistance between a negative-side line of the battery and ground, and the capacitor, the third measurement path including the battery, a positive-side insulation resistance that is an insulation resistance between a positive-side line of the battery and the ground, and the capacitor;
• and
• a control unit configured to calculate a reference value based on each charging voltage of the capacitor in each of the first to third measurement paths, and configured to calculate the insulation resistance with reference to a predetermined conversion map, wherein
• the conversion map includes a conversion map corresponding to a capacitance of the capacitor, and
• the control unit is configured to estimate the capacitance of the capacitor, and refers to the conversion map corresponding to the capacitance of the capacitor that has been estimated.