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.

Unbalanced voltage at the aircraft plug due to a broken neutral would add a 400 Hz voltage to the 28VDC interlock voltage measured at the GPU. The AXA2200 GPU supervises the interlock signal and disconnects the output in case that the injected AC voltage exceeds 20 V approx. This type of error is recognised as an interlock failure by the GPU. Since the disconnection is a reaction against a heavy unbalanced voltage at the aircraft plug, this means that this type of supervision does not protect sensitive equipment in the aircraft.
Therefore, AXA offers a supervision of a rupture of the neutral conductor. This option is based on the supervision of an injected 50/60 Hz current in the neutral conductor.

Please refer to the sketch below.

                                                                   Fig 1. Sketch showing the Functional Principle

Measuring the current in the z-wire does the supervision. If the GPU is provided with two outputs, the supervision can be made at both outputs.

The supervision can be disabled at the I/O module.