The Turck MZB series of barriers are based on a simple principle. Each channel is fitted with two stages of pulse tested Zener Diodes and an infallible terminating resistor.

In the event of an electrical fault in the non-hazardous (controls side) of the barrier, such as an overvoltage, the Zener diodes limit the voltage available to the hazardous area circuits and a series resistance limits the current. There is also a fuse designed into the circuit that will rupture in the event of a continuous application of excess energy being applied to the barrier that is outside the design criteria for the loop.
TURCK Shunt Diode Safety Barrier, MZB Series, Positive Supply, 2-Channel, NCNR