A first voltage sensor measures a primary voltage between a first terminal of a tested device and electrical ground when a first switch and a second switch are in various on states or off states. In a test state either the first switch or the second switch is in an on state and in reference state both the first switch and the second switch are in on states. An observed leakage resistance is estimated based on the measured primary and secondary voltages of the test state. A reference leakage resistance is based on the measured primary and secondary voltages of the reference state. A test circuit has failed if the observed leakage resistance differs from the reference leakage resistance by more than a threshold amount.