A method of detecting faults in a traction motor installation having a plurality of AC traction motors powered from a common power source, each of the traction motors being connected by associated motor cables to a cable junction and the common power source being connected to the cable junction by main cables, includes disconnecting the power source from the main cables and performing initial short circuit to ground, short circuit between phases and leakage to ground tests on the traction motors through the main cables. When a fault is detected by the initial tests, at least one detailed test is performed by applying a predetermined voltage to the main cables and sensing current flow in the motor cables connected to each of the motors. The detailed test is one of a detail short circuit to ground test, a detail short circuit phase to phase test, and a detail leakage to ground test.