Test Case Design by Means of the CTE XL

An electric shutter having an armature operable to an unattracted position for allowing closing of the shutter blades. The armature is biased by a pressure-applying spring toward the attracted position by a greater force than a force applied biasing it away from the attracted position. The pressure-applying spring force is rendered ineffective after the shutter blades have been opened thereby avoiding premature closing of the shutter in the event that the energized electromagnet cannot hold the armature in an attracted position.