Abstract: There is disclosed a barrier in which two or more Zener diodes are mounted in parallel in an encapsulated module between the leads from an input/output terminal located in a safe area to or input/output terminal in a hazardous area such as an environment having an explosive mixture of gases or dust, and a current-limiting resistor in series with an interconnected end of said diodes, wherein the circuit elements in the encapsulated module are connected individually at each end to one of a series of input/output pins extending from said module whereby they can be individually tested and the pins are adapted to be interconnected selectively externally of said module to complete the barrier circuit.