User guide

10 Rockwell Automation Publication 20C-UM001B-EN-P - June 2011
Chapter 1 Overview
PL d/Category 3
Performance Definition
To achieve PL d/category 3 according to EN ISO 13849-1, the safety-related
parts have to be designed such that:
• the safety-related parts of machine control systems and/or their protective
equipment, as well as their components, shall be designed, constructed,
selected, assembled, and combined in accordance with relevant standards
so that they can withstand expected conditions.
• well tried safety principles shall be applied.
• a single fault in any of its parts does not lead to a loss of safety function.
• some but not all faults will be detected.
• the accumulation of undetected faults can lead to loss of safety function.
• short circuits in the external wiring of the safety inputs is not one of the
faults that can be detected by the system, therefore, according to
EN ISO 13849-2, these cables must be installed so as to be protected
against external damage by cable ducting or armor.
• whenever reasonably practical a single fault shall be detected at or before
the next demand of the safety function.
• the average diagnostic coverage of the safety-related parts of the control
system shall be low to medium.
• the mean time to dangerous failure of each of the redundant channels shall
be medium to high.
Stop Category Definitions
The selection of a stop category for each stop function must be determined by a
risk assessment.
• Stop Category 0 is achieved with immediate removal of power to the actuator,
resulting in an uncontrolled coast to stop. See “Description of Operation”
Example 1 on page 17
.
• Stop Category 1 is achieved with power available to the machine actuators to
achieve the stop. Power is removed from the actuators when the stop is
achieved. See “Description of Operation” Example 2 on page 18
.
IMPORTANT
When designing the machine application, timing and distance should be
considered for a coast to stop (Stop Category 0 or Safe Torque Off). For
more information regarding stop categories, refer to EN 60204-1.