Rockwell Automation 21G PowerFlex 750 Series Safe Torque Off - Manual (Page 11)

Rockwell Automation Publication 750-UM002F-EN-P - July 2013 11
Safety Concept Chapter 2
Performance Level and Safety Integrity Level (SIL) CL3
For safety-related control systems, Performance Level (PL), according to ISO
13849-1, and SIL levels, according to IEC 61508 and EN 62061, include a rating
of the system’s ability to perform its safety functions. All of the safety-related
components of the control system must be included in both a risk assessment and
the determination of the achieved levels.
Refer to the ISO 13849-1, IEC 61508, and EN 62061 standards for complete
information on requirements for PL and SIL determination.
Functional Proof Tests The functional safety standards require that functional proof tests be performed
on the equipment used in the system. Proof tests are performed at user-defined
intervals and are dependent upon PFD and PFH values.
PFD and PFH Definitions Safety-related systems can be classified as operating in either a Low Demand
mode, or in a High Demand/Continuous mode.
• Low Demand mode: where the frequency of demands for operation made
on a safety-related system is no greater than one per year or no greater than
twice the proof-test frequency.
• High Demand/Continuous mode: where the frequency of demands for
operation made on a safety-related system is greater than once per year or
greater than twice the proof test interval.
The SIL value for a low demand safety-related system is directly related to order-
of-magnitude ranges of its average probability of failure to satisfactorily perform
its safety function on demand or, simply, average probability of failure on demand
(PFD). The SIL value for a High Demand/continuous mode safety-related
system is directly related to the probability of a dangerous failure occurring per
hour (PFH).
IMPORTANT Your specific application determines the time frame for the proof test interval.