Rockwell Automation 20C-DG01 PowerFlex 700H AC Drive Safe Torque Off Option - Manual (Page 11)

Rockwell Automation Publication 20C-UM001B-EN-P - June 2011 11
Overview Chapter 1
System Performance Level
and Safety Integrity Level
(SIL) For safety-related control systems, Performance Level (PL), according to
EN 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 EN ISO 13849-1, IEC 61508, and EN 62061 standards for complete
information on requirements for PL and SIL determination.
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).