Mitsubishi Electric MELSEC iQ-R Series - Sil

3 RISK ASSESSMENT AND SAFETY LEVEL
3.2 SIL
21
3
3.2 SIL
SIL and target failure measure (PFDavg/PFH)
SIL is standardized by IEC61508: 2010 and used in IEC62061: 2012, as an index showing the safety level. The following risk
graph method can select a SIL.
Definition of symbols:
(For details, refer to IEC61508-5:2010)
To achieve the selected SIL, a safety system shall be established according to the requirements for each SIL of the IEC61508:
2010. For example, the following table shows PFDavg (target failure measure for low demand mode of operation
*1
) and PFH
(target failure measure for consecutive or high demand mode of operation
*1
) indicating failure rates when safety functions are
nullified are specified according to the SIL level. Page 27 Calculation of the target failure measure (PFDavg/PFH) shows the
calculation method for PFDavg/PFH when the safety programmable controller is used. For details on the requirements for
each SIL, refer to SIL IEC61508: 2010.
(For details, refer to IEC61508-1: 2010)
*1 For the low and high demand modes of operation, refer to IEC61508: 2010.
Symbol Definition
, a No safety requirements
b Not sufficient with a single safety-related system.
1, 2, 3, 4 Safety integrity level
Stands for SIL1, SIL2, SIL3 and SIL4 respectively.
Probability of risk event
occurrence
Definition
Frequent The probability of unwanted event occurrence is relatively high and repeatedly occurs.
Moderate The probability of unwanted event occurrence is relatively low and infrequently occurs.
Infrequent The probability of unwanted event occurrence is extremely low and unlikely occurs.
SIL Low demand mode of operation
*1
High demand mode of operation
*1
410
-5
 PFDavg < 10
-4
10
-9
 PFH < 10
-8
310
-4
 PFDavg < 10
-3
10
-8
 PFH < 10
-7
210
-3
 PFDavg < 10
-2
10
-7
 PFH < 10
-6
110
-2
 PFDavg < 10
-1
10
-6
 PFH < 10
-5
a
1
2
3
4
b
a
1
2
3
4
-
a
-
-
1
2
3
Starting point for
risk reduction
Minor
Marginal
Possible
Rare
Frequent
Rare
Rare
Frequent
Frequent
Not likely
Possible
Not likely
Possible
Not likely
Possible
Not likely
Relatively
high
Critical
Catastrophic
Low
Demand rateConsequence Exposure Avoidance
Very
low