Siemens SIMATIC ET 200AL, System Manual

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Digital I/O module F-DI 4+F-DQ 2x24VDC/2A, 4xM12 (6ES7146-5FF00-0BA0)
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Equipment Manual, V1.0, 05/2021, A5E51082342-AA
Applications of the F-I/O module
5
The PLC system has an extremely low failure rate and therefore contributes only a small part
of the total probability of dangerous failure of an automated system. The probability of
dangerous failures of the sensors and actuators is typically far larger than the probability of
dangerous failure per hour (PFH)/probability of dangerous failure on demand (PFD) of the PLC
system. Estimates for quantitative safety properties like PFD and PFH are calculated using the
following methods:
•
standard data tables
• calculation methods according to international standards which include the following
standards:
– IEC 61508
– ISO 13849
– IEC 61131-6
The values are based on the following assumptions:
•
continuous full nominal load
• nominal voltage
• equipment ambient temperature of 55 °C
Faults in the wiring system can also be a substantial contributor to failures.
To achieve a targeted level of safety performance for each safety function, perform these
actions:
•
Choose an appropriate architecture.
• Choose sensors and actuators that are appropriately rated.
• Provide a safety program that meets the requirements of the safety function.
• Provide diagnostics and proof-tests to maintain the ratings of the sensors and actuators.
• Use wiring installation practices, diagnostics, and proof-tests to assure wiring integrity.
• Generate and control operating and maintenance procedures for the lifetime of the
installation.
The ET 200AL, ET 200pro, ET 200SP, and S7-1200/1500 Fail-Safe systems provide a high level
of internal diagnostic coverage. Diagnostic coverage of your external circuits, sensors, and
actuators depends on your design choices using features of the PLC system and other
measures.
The PFH/PFD of each fail-safe PLC component is stated assuming there is no field proof-test
within the lifetime of the product. Sensors and actuators typically require regular proof-tests
to maintain an expected level of safety performance.
The reaction time of each safety function depends on the reaction time of each component,
including the sensor, the PLC system, and the actuator. "Maximum response time (Page 83)"
gives further information on delay times through the PLC components. You must choose PLC