The zener diode barrier works by diverting potentially dangerous energy to ground before it can reach the hazardous area.
The zener diodes limit the fault voltage to the hazardous area. There are two such diodes for redundancy. The series resistor limits current to the hazardous area, and is considered an infallible component. The arrow in Figure shows the resultant path if excess current enters the barrier as a result of excess voltage input from the instrument. Since a zener barrier is powered by the loop and it has a current limiting resistor, it has a voltage drop across it. This barrier, in addition to protecting against dangerous energy entering the hazardous area, must allow the loop to function normally.