Unbalanced voltage at the aircraft connector due to a broken neutral would add a 400 Hz voltage to the 28 VDC interlock voltage measured at the ground power unit. If the injected AC voltage exceeds approx. 20 VAC the output is disconnected. This type of error is recognised as an interlock failure by the ground power unit. Since the disconnection is a reaction against a heavy unbalanced voltage at the aircraft connector, it means that this type of supervision does not protect sensitive equipment in the aircraft. This option protects personnel as well as the aircraft in case of a broken neutral.