This is a phase sequence indicator. How to use: switch on the 3 switches. The left 3 lamps will light up, to indicate that 3 phases are available. The right lamp will light up to indicate a right rotating field. Switch off the 3 switches and switch the 2 change-over switches. Switch on the 3 switches again, now the left lamp will light up, to indicate a left rotating field. Switch off 1 phase. Now both field indicators will light up, and the phase lamp will be off. This will indicate that 1 phase is missing.
