Adjustable flux transfer shunt trip actuator and electric power switch incorporating same

A flux transfer shunt trip actuator for an electric power switch comprising:

• a magnetically permeable frame incorporating a seat;
• a magnetically permeable plunger mounted within said magnetically permeable frame for movement between a retracted position with a first end seated against said seat and an actuated position with said magnetically permeable plunger spaced from said seat;
• a biasing spring biasing said magnetically permeable plunger toward said actuated position;
• at least one permanent magnet generating a permanent magnet force sufficient to overcome said biasing spring to hold said magnetically permeable plunger in said retracted position seated against said seat;
• a coil generating an electromagnetic force when energized bucking said permanent magnet force so that said biasing spring moves said magnetically permeable plunger to said actuated position;
• an adjustment device adjusting preloading on said biasing spring without adjusting position or length of said magnetically permeable plunger;
• and
• wherein said magnetically permeable frame comprises a magnetically permeable U-shaped member having a bight and a pair of legs spaced apart and extending from said bight, said at least one permanent magnet being mounted between said pair of legs and defining a gap, said magnetically permeable frame further including a magnetically permeable seat member projecting from said bight toward said gap and terminating in a free end forming said seat, said first end of said magnetically permeable plunger extending into said gap when seated against said seat in said retracted position, said biasing spring extending along said seat member and having a first end bearing against said first end of said magnetically permeable plunger, and said adjustment device including an adjustment member, and a mount mounting said adjustment member for movement relative to said bight toward and away from said magnetically permeable plunger, said adjusting member bearing against said biasing spring to adjust preload on said biasing spring.