An open frame chassis has a top opening and a bottom opening permitting ambient airflow. A plurality of modules, each enclosing electrical components in thermal contact with a heat sink area of their corresponding module, can each be inserted in the chassis. Ambient air may flow from the bottom opening across the heat sink area of each module to the top opening to passively cool the modules and electrical components. Key pins guide the modules into place and prevent incorrect insertion of a different type of electrical module not corresponding to the electrical connection of the chassis for that slot. Guide pins on corners of the modules mate with guide holes in the chassis to secure the module to the chassis and decrease vibration. Both sides of the chassis have side openings through which the fins of the modules in the end slots of the chassis may be exposed.