... is normal, A, B and C indicators will light up constantly; when the three phases are abnormal, ... supply trips, the tripping lamp of A will light up 2) Status indicator lamp of reserve power supply ... are normal, A, B and C indicators will light up constantly; when the three phases are abnormal, ... supply trips, the tripping lamp of B will light up 3) Regulating potentiometer: Used to regulate switch-off ...