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Rockwell Automation Publication SAFETY-AT134B-EN-P - November 2015
Safety Function: Cable Pull Switch with a Configurable Safety Relay
The Lifeline 4 cable pull switch safety function can be modeled as follows.
Lifeline4 cable pull switches are considered complimentary safety devices by the relevant standards. As such, they are not a
substitute for safeguarding measures, nor can they impair the effective operations of any safeguarding measures.
Due to the single mechanical actuator of the cable pull switch, a fault exclusion must be considered. In most instances the
fault exclusion required for electromechanical devices with a single mechanical actuator, such as a typical tongue interlock,
limits the safety function in which they are included to a maximum Performance Level of PLd.
Calculation of the 440C-CR30 relay subsystem is straightforward. Its relevant safety data is automatically entered into
SISTEMA when it is selected from the Rockwell Automation SISTEMA library.
The calculation for the Lifeline 4 cable pull switch input subsystem, and the 100S contactor output subsystem is different.
Because these are electro-mechanical devices, the Lifeline 4 cable pull switch and safety contactor data includes the
following:
•
Mean Time to Failure, dangerous (MTTFd)
•
Diagnostic Coverage (DCavg)
•
Common Cause Failure (CCF)
•
Electro-mechanical devices' functional safety evaluations include the following:
•
How frequently they are operated
•
Whether they are effectively monitored for faults
•
Whether they are properly specified and installed
•
SISTEMA calculates the MTTFd by using B10d data provided for the contactors along with the estimated
frequency of use, entered during the creation of the SISTEMA project. In this application, the estimated annual
number of contactor operations is 17520 per year (the Lifeline 4 cable switch is initiated once per hour, plus the
E-stop is initiated once per hour, 24 hours per day, 365 days a year).
The DCavg (99%) for the contactors is selected from the Output Device table of ISO 13849-1 Annex E, Direct
Monitoring.
The DCavg (99%) for the E-stop is selected from the Input Device table of ISO 13849-1 Annex E, Cross Monitoring.
Input
Logic
Output
Cable Pull Switch 1
S1
Cable Pull Switch 2
S2
Subsystem 1
Subsystem 2
Subsystem 3
440C-CR30
Relay
100S-C
K1
100S-C
K2
Subsystem 4
Fault Exclusion
Fault
Exclusion
