A method for detecting an isolation fault in an isolation resistance between a positive and negative high-voltage bus rail and a vehicle chassis on a high-voltage propulsion side of a high-voltage system of an electric or hybrid-electric vehicle. The high-voltage system is split into a battery side and propulsion side by means of two high-voltage contactors located in the positive and negative high-voltage bus rails. The propulsion side includes first and second capacitors connected in series across the positive and negative bus rails, a common junction of the first and second capacitors connected to the chassis. The method supplies a low-voltage output to the positive and negative bus rails to charge the capacitors; and determines, based on charging current, voltage level or energy level of the capacitors, whether an isolation fault is present between the positive and/or negative high-voltage bus rail and the vehicle chassis on the propulsion side.