An insulating rotary diaphragm for a vacuum interrupter (VI) electrical switch. The insulating diaphragm is designed for use in underground or pad-mounted VI switches where an external lever is rotated by a line worker to manually open the switch. A torsional insulating rod is coupled between a switch actuator and the external lever, and the diaphragm maintains constant contact with the insulating rod and an outer housing when the lever and rod are rotated, thus ensuring adequate isolation between the actuator and the lever. The diaphragm deforms torsionally when the lever and rod are rotated. This configuration allows the actuator to be at medium voltage, eliminates the need for a translational insulating rod between the medium voltage switch components and the lever, and thereby reduces the overall length of the VI switch.