Sil - Mitsubishi Electric MELSEC iQ-R Series Safety Manual

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.
Consequence
Minor
Marginal
Starting point for
risk reduction
Critical
Catastrophic
Definition of symbols:
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
Frequent
Moderate
Infrequent
(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
(target failure measure for consecutive or high demand mode of operation
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.
SIL Low demand mode of operation
-5
4 10
-4
3 10
-3
2 10
-2
1 10
(For details, refer to IEC61508-1: 2010)
*1 For the low and high demand modes of operation, refer to IEC61508: 2010.
Exposure Avoidance
Possible
Not likely
Rare
Possible
Frequent
Not likely
Rare
Possible
Frequent
Not likely
Rare
Possible
Frequent
Not likely
Definition
The probability of unwanted event occurrence is relatively high and repeatedly occurs.
The probability of unwanted event occurrence is relatively low and infrequently occurs.
The probability of unwanted event occurrence is extremely low and unlikely occurs.
*1
-4
 PFDavg < 10
-3
 PFDavg < 10
-2
 PFDavg < 10
-1
 PFDavg < 10
Demand rate
Relatively Very
high low
Low
a -
-
a -
1
1
2 a
3 2 1
3
4 2
b 4 3
*1
) indicating failure rates when safety functions are
High demand mode of operation
-9 -8
 PFH < 10
10
-8 -7
 PFH < 10
10
-7 -6
 PFH < 10
10
-6 -5
 PFH < 10
10
3 RISK ASSESSMENT AND SAFETY LEVEL
*1
) and PFH
*1
21
3.2 SIL
3