@ An improved switch for a high-voltage device in which ignition of a power cartridge moves an insulative piston located in a conductive member away therefrom and into a passageway in an insulative liner. The piston moves a contact through a passageway and away from the conductive member to break an electrical interconnection between the conductive member and the movable contact, thereby opening the switch. The switch includes an insulative housing engageably surrounding, holding and fixing the relative positions of the conductive member and the liner. An interiorly relieved extension is formed at and about one end of the liner, and an exteriorly relieved region is formed in and about one end of the conductive member, whereby the region may be telescoped into and conformally engage the inside of the extension so that the conductive member and liner partially overlap along a tortuous interface so as to increase the interrupting capability of the switch by enhancing the isolation between the power cartridge and the passageway.