Mitsubishi Electric MELSEC iQ-R Series - Page 27

3 RISK ASSESSMENT AND SAFETY LEVEL
3.3 PL
25
3
Selecting performance level
The following shows the relationship between PL and category, average diagnostics coverage (DCavg), and mean time to
dangerous failure (MTTFd). Configure the safety system by selecting diagnostics methods to ensure achievement of the
safety system PLr for the target machines.
(For details, refer to ISO13849-1: 2006)
■Common Cause Failure (CCF)
The CCF is a failure of multiple components caused by a single event. This includes environmental factors such as
temperature and EMC, design failure, and software bags.
Quantify safety measures for CCF based on the Annexed table F, ISO13849-1: 2006. Add points specified in the table, if
techniques and measures specified in the table are observed. Consider the measure for CCF of the safety system sufficient if
it eventually earns 65 or more points.
Relationship between PL and SIL
As described in the following, the relationship between PL and the probability of dangerous failure per hour (PFHd) is
specified in ISO13849-1: 2006.
Based on the results, relationship between PL and SIL is specified in ISO13849-1: 2006
For relationship between the category and SIL, refer to IEC62061: 2012.
PL Probability of dangerous failure per hour (PFHd) (1/h)
a10
-5
 PFHd < 10
-4
b3  10
-6
 PFHd < 10
-5
c10
-6
 PFHd < 3  10
-6
d10
-7
 PFHd < 10
-6
e10
-8
 PFHd < 10
-7
PL SIL
High demand modes of operation/consecutive operation
aN/A
b1
c1
d2
e3
PL
a
b
c
d
e
: MTTFd=Low
: MTTFd=Medium
: MTTFd=High
Category 4
DCavg=High
Category 3
DCavg=Medium
Category 3
DCavg=Low
Category 2
DCavg=Medium
Category 2
DCavg=Low
Category 1
DCavg=None
Category B
DCavg=None