Rockwell Automation Publication SAFETY-AT125C-EN-P - February 2022 13
Enabling Switch with an Integrated Safety Controller and a POINT Guard I/O Safety Module Safety Function Application Technique
Calculation of the Performance Level
When properly implemented, these safety functions can achieve a safety rating of category 3, Performance Level d (cat. 3, PLd), according to
ISO 13849-1: 2015, as calculated by using the Safety Integrity Software Tool for the Evaluation of Machine Applications (SISTEMA).
The SISTEMA file that is referenced in this safety function application technique is attached to this document.
The average frequency of a dangerous failure per hour (PFH) for electromechanical systems may be calculated differently based on the
version of ISO 13849 supported by SISTEMA. ISO 13849-1:2015, which changed the maximum MTTFd from 100
…2500 years, is supported
starting in version 2.0.3 of SISTEMA. As a result, the same SISTEMA data file that is opened in two different versions of SISTEMA can yield
different calculated results.
The functional safety specifications of the project call for a Performance Level of PLd (minimum) and a structure of
Cat. 3 (minimum). A PFHd of less than 1.0 E-06 for the overall safety function is required for PLd.
The GuardLogix 5570 controller subsystem uses 1.2% of PLe bandwidth. The Compact GuardLogix 5370 controller subsystem uses 1.5% of
PLe bandwidth.
Complete Safety Function PL Calculation Example
Assuming the use of a Compact GuardLogix 5370 controller, here are all of the subsystems shown together. If you choose a different
controller, you need different calculations.
IMPORTANT
To calculate the PL of your entire safety function, you must include the specific subsystems that you chose.
Depending on the devices you choose, the overall safety rating of your system will be different. An example
that describes how to calculate the safety rating for a complete safety function appears in the section titled
Complete Safety Function PL Calculation Example
.
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