The safety function is activated if the electrical 4 to 20 mA

setpoint signal is set to 0 mA. This 0 mA signal might be forced

as direct logic solver/DCS output or by an additional electrical

open/close switch within the 4 to 20 mA setpoint line.

As reaction on the 0 mA signal the electronics, firmware &

position measurement is deactivated and bypassed, and the

mechanical pneumatic system of the positioner will be

depressurized into the fail-safe position.

The return spring of the attached pneumatic actuator will move

the valve to a safe end position (OPEN or CLOSED) subsequently.

The final safe state is achieved if the valve is located in the safe end position.

The time taken to reach the expected safe end position at the

process valve is only partly determined by the positioner; it is

also dependent on the attached actuator and valve assembly and

further external conditions, therefore the correct safety function

as expected within the final application must be checked during

installation & commissioning and the end user is responsible to

validate whether the safe state is achieved in the expected

direction and within the expected time frame.