Abstract: An apparatus for creating a isothermal cavity about the terminal pin connector panel of an input/output module and a spark gap of least impedance to earth ground potential at the terminal pins to protect the circuitry of the module. In the preferred embodiment, the structure forming the isothermal cavity and spark gap are functionally and physically interrelated into a simple and compact design easily receivable in a card cage format. The structure forming the isothermal cavity preferably surrounds all the sides of the terminal pin connector panel except the side providing access to the pin connectors. In this manner, free and continuous access to the pin connectors is maintained even while the module is in the card cage. Additionally, a puller section is incorporated into the apparatus for easy manipulation of the module.