Cause : Line phase failure in main circuit comes when power unit, the DC link voltage ripple has exceeded the permissible limit value due to a line phase has failed, the 3 line phases are inadmissibly unsymmetrical or the fuse of a phase of a main circuit has ruptured.

- check the main circuit fuses.
- Check whether a single-phase load is distorting the line voltages.
- check the motor feeder cables.