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Enabling Switch with an Integrated Safety Controller and a POINT Guard I/O Safety Module Safety Function Application Technique
The Enabling Switch Safety Function can be modeled as shown.
Enabling
Switch
Channel A
Enabling
Switch
Channel B
Subsystem 1
Functional Safety Data Required for Determining the Performance Level of Electromechanical Devices
Because these devices are electromechanical devices, the functional safety data that are required for Performance Level calculation include
the following:
•
Mean Time to Failure, dangerous (MTTFd)
•
Diagnostic Coverage (DCavg)
•
Common Cause Failure (CCF)
The functional safety evaluations of the electromechanical devices 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 that are provided for the contactors along with the estimated frequency of use, entered
during the creation of the SISTEMA project.
The DCavg (99%) for the contactors is selected from the Output Device table of ISO 13849-1 Annex E, Direct Monitoring.
The CCF value is generated by using the scoring process that is outlined in Annex F of ISO 13849-1. The complete CCF scoring process must
be performed when actually implementing an application. A minimum score of 65 must be achieved.
The enabling switch has one mechanical actuator. To account for this single point of failure, Subsystem 2 (fault exclusion) is included in the
SISTEMA calculation. Note that a fault exclusion is allowed in safety functions up to and including PLd, per annex D.8 of ISO 13849-2 2012
When an application includes two-channel, single-actuator mechanical safeguarding devices such as interlocks or mechanical emergency
stop devices such as E-stops or cable pull switches, fault exclusion for single- actuator failure must be taken into account and, when
required, applied when determining the Performance Level.
Exclusion of the possible fault of the single actuator failing to switch the two channels properly is not allowed. Therefore, single types of
electromechanical devices are limited to a maximum Performance Level of d. The Performance Level required (PLr) in safety function
application techniques is PLd. Redundancy of safeguarding switches is required to achieve Performance Level e.
14
Input
Fault
1734-IB8S
Exclusion
Subsystem 2
Subsystem 3
Rockwell Automation Publication SAFETY-AT125C-EN-P - February 2022
Logic
GuardLogix
Safety
1734-OB8S
Controller
Subsystem 4
Subsystem 5
Output
100S-C
K1
100S-C
K2
Subsystem 6
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