A broken neutral conductor in the 400 Hz cable combined with an unbalanced aircraft load could lead to a destructive phase-neutral voltage in the aircraft and to hazardous voltages between the aircraft chassis and the ground level. The NCR option secures that a broken neutral will be detected instantaneously.

                         NCR Board                                     Internal Wiring Diagram

                       Wiring diagram for Neutral Conductor Rupture Supervision

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. The NCR option protects personnel as well as the aircraft in case of a broken neutral.