A reusable control module mountable to a power-cartridge-operated interrupting module which includes a normally closed switch in shunt with a fuse. The control module houses a control circuit which ignites the power cartridge to open the switch. The control module includes two conductive shells threaded together to enclose the circuit, thereby providing a Faraday cage for the circuit and shielding the circuit from the environment. A current transformer is mounted in an exterior pocket in one of the shells over a mounting terminal thereon which extends out of the pocket to act as a single-turn primary for the transformer. The output of the transformer is connected to an input of the circuit. Circuit boards of the circuit are mounted within the shells, in part by a plastic plug, one end of which passes through one of the shells coaxially with a mounting neck. The other end of the plug has slots which engage and hold one edge of the boards. The plug contains a conductor connected to the output of the circuit. When the modules are moutned together via the neck, the conductor at the one end of the plug is electrically connected to the power cartridge and the shells can now carry the current in a protected circuit to the switch and the fuse. The plug is located on the axis of rotation of the shells when the shells are threaded together so that its holding and signal-carrying functions are not compromised.