Hold on. Terminal 3 is the negative terminal of the instrument, but it's where the voltage should be positive.
Did you see the manual?
The Zener barrier is there to reduce the available current to the output... not shorting to ground.
The Zener barrier itself, with the ia rating, is made to work safely with component faults inside it too, hence why the schematic shows two Zener diodes (in the manual, not the barrier itself).