ABB TZIDC / TZIDC-200 Digital Positioner Safety function

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.

The positioner safety function is declared as Type A element

with HFT=0 according to IEC 61508 capable to be used for SIL

Functional Safety Functions up to SIL 2.

According to IEC 61508-2. 7.4.7.4 a useful lifetime, based on

experience should be assumed.

The components of the positioner do not contain any specific

components with a reduced useful lifetime that are contributing

to the dangerous undetected failure rates.

Because the positioner is a moving masses system supplied by

instrumented air the final device usage and environmental

operating conditions (e.g., no/many open-close cycles,

good/bad supply air quality) will have an impact on the usefullifetime.

Therefore, the useful lifetime should be evaluated by the end

user according to the final plant experience data.

The Periodic Proof-Test methods within chapter Periodic Proof

Test and Maintenance on page 9 are suggested to validate the

correct safety function.

When final plant experience, proof testing and related field

monitoring data indicates a limiting useful lifetime by related

operating conditions, then the plant experience-based lifetime

must be used and considered on the failure rates, proof-test

intervals, and replacement intervals.