Abstract: The disclosure generally relates to a protective device for an electronic instrument wherein input and output lines to and from the instrument are always isolated from calibration input and output terminals. This is accomplished by placing reed switches on or closely adjacent the non-magnetic instrument front panel, the switches being arranged to connect one of the input lines and terminals to the instrument circuitry while disconnecting the others therefrom, this being accomplished under control of a magnet placed adjacent the switches during a calibration procedure. The switches are further arranged to short circuit the output test terminals in the unenergized switch state to prevent insertion of unwanted signal current or voltage in that state.