Mitsubishi Electric MELSEC-QS Series Safety Application Manual page 29

3
RISK ASSESSMENT AND SAFETY LEVEL
Figure 3.5 shows relationship between Category, DCavg (average diagnostic coverage),
and MTTFd (mean time to dangerous failure) of each channel and the resulting PL.
PL
a
b
c
d
e
Category B
DC
Figure 3.5 Relationship between Category, DCavg, and MTTFd of each channel and the resulting PL
Refer to the following summary of safety category requirements.
The PL of a safety-related part in control system must be higher than or equal to the PLr of
the safety function.
PL
Category 1 Category 2 Category 2
=None DC =None DC =Low DC
avg avg avg avg
: MTTFd=Low
: MTTFd=Medium
: MTTFd=High
Table 3.2 MTTFd (mean time to dangerous failure)
Level
Low
Medium
High
Table 3.3 DCavg (average diagnostic coverage)
Level
None
Low
Medium
High
PLr
Category 3 Category 3
=Medium DC =Low DC =Medium DC
avg avg
Range
3 years MTTFd 10 years
10 years MTTFd 30 years
30 years MTTFd 100 years
Range
DCavg 60%
60% DCavg 90%
90% DCavg 99%
99% DCavg
Category 4
=High
avg
(Referred to ISO13849-1.)
3
3.3 PL
1
2
3
4
5
6
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