Rockwell Automation Publication 2094-RM002B-EN-P - May 2012 11
Safety Concept Chapter 1
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 EN 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, EN 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).
PFD and PFH Data
These PFD and PFH calculations are based on the equations from Part 6 of
EN 61508 and show worst-case values.
This table provides data for a 20-year proof test interval and demonstrates the
worst-case effect of various configuration changes on the data.
Table 2 - PFD and PFH for 20-year Proof Test Interval
Attribute Value
PFH [1e-9] 4.09
PFD [1e-4] 3.90
SFF % 99.5
To Next Page
Loading...
