Requirements For Sensors And Actuators - Siemens SIMATIC ET 200S Installation And Operating Manual

Wiring and Fitting Modules

4.5 Requirements for Sensors and Actuators

4.5 Requirements for Sensors and Actuators
4.5
General Requirements for Sensors and Actuators
Note the following important information for safety-related use of sensors and actuators:
Warning
The use of sensors and actuators is outside of our sphere of influence. We have equipped
our electronics with such safety engineering features as to leave 85% of the maximum
permissible probability of hazardous faults for sensors and actuators up to you (This
corresponds to the recommended load division in safety engineering between sensing
devices, actuating devices, and electronic switching for input, processing, and output).
Note, therefore, that instrumentation with sensors and accouters bears a considerable safety
responsibility. Consider, too, that sensors and actuators do not generally withstand proof-test
intervals of 10 years (the interval for an external function test according to IEC 61508)
without considerable loss of safety.
The probability of hazardous faults and the rate of occurrence of hazardous faults of a safety
function must comply with an upper limit determined by a safety integrity level (SIL). You will
find a listing of values achieved by F-modules under "Fail-Safe Performance Characteristics"
in the technical specifications for F-modules in Chapter 9.
To achieve SIL3 (AK6/Category 4), suitably qualified sensors are necessary.
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ET 200S Distributed I/O System - Fail-Safe Modules
Hardware Installation and Operating Manual, 07/2005, A5E00103686-04