The zener safety barrier was invented in the 1950s by Bob Redding to prevent electrical sparks causing explosions in the petrochemical industry. Despite the safety benefits it brought, it required much perseverance on his part before the industry accepted it.
A zener barrier contains three zeners connected in parallel, a fuse connecting the zeners to the safe area and a resistor to connect them to the hazardous area. Three diodes are used to provide redundancy so that even if two of them fail (two countable faults) the barrier will continue to provide protection.
The maximum output voltage is clamped by the zeners and the maximum current is limited by the resistor. The fuse protects against very large currents which would cause all three zeners to fail open circuit. All the components are used within 2/3 of their specified maximum rating.
A zener barrier must be connected to a high integrity safety ground to be effective and the output parameters of the barrier must be compatible with the input parameters of the device it is protecting.