Model MTL7728+ is an intrinsically safe, zone-powered, Zener-diode barrier for use in conventional fire detection systems for protection within hazardous areas. Each Zone contains two stages of pulse-tested Zener or forward- connected diodes and an \'infallible\' terminating resistor. In the event of an electrical fault in the safe area, the diodes limit the voltage that can reach the hazardous area and the resistor limits the current. A fuse protects the diodes, and the two stages of voltage limitation ensure continued safety if either stage should fail. No active output-current limiting circuits are employed. Model MTL7728+ is certified \'ia\' for all zones and \'IIC\' for all explosive atmospheres.