Sil Definition 3d5z72
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Definition of Safety Integrity Levels and the Influence of Assumptions, Methods and Principles Used H. Schäbe TÜV InterTraffic, Am Grauen Stein, 51105 Köln,
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1
Introduction
• Methods for derivation of Safety Integrity Levels • Minimum Endogeneous Mortality (MEM) • As Low As Reasonably Practicable (ALARP), • Globalement Aussi Équivalent (GAME), • IEC 61508 risk graph Study of different principles and comparison
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1
Safety Integrity Level Definition – IEC 61508 • Risk Graph fromIEC 61508 W3
C1 P1
F1
Starting point for risk reduction C2 estimation
P1 P2
F1
C3
F2
C4
C = Consequence risk parameter
W1
a
-
-
b
P2
F2
W2
a c
b
a
d
c
b
e
d
c
f
e
d
g
f
e
h
g
f
a, b, c, d, e, f, g, h represent the necessary minimum risk reduction. The link between the necessary minimum risk reduction and the safety integrity level is shown in the table.
Necessary minimum risk reduction
Safety integrity level
-
No safety requirements
a
No special safety requirements 1 2 3 4 An E/E/PE SRS is not sufficient
F = Frequency and exposure time risk parameter P = Possibility of avoiding hazard risk parameter W = Probability of the unwanted occurrence a, b, c ... h = Estimates of the required risk reduction for the SRSs
b, c d e, f g h
3
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Safety Integrity Level Definition – IEC 61508 Risk parameter Consequence (C)
Classification C1 C2 C3 C4
Minor injury Serious permanent injury to one or more persons; death to one person Death to several people Very many people killed
Frequency of, and exposure time in, the hazardous zone (F)
F1 F2
Rare to more often exposure in the hazardous zone Frequent to permanent exposure in the hazardous zone
Possibility of avoiding the hazardous event (P)
P1 P2
Possible under certain conditions Almost impossible
Probability of the unwanted occurrence (W)
W1
A very slight probability that the unwanted occurrences will come to and only a few unwanted occurrences are likely A slight probability that the unwanted occurrences will come to and few unwanted occurrences are likely A relatively high probability that the unwanted occurrences will come to and frequent unwanted occurrences are likely
W2 W3
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2
SIL Definition MEM and GAME
Minimum Endogenous Mortality A new technical system must not add an unjustifiable amount of risk to the risk budget of a person. The starting point is the endogenous mortality of a 15-20 year old human being. Considering only endogenous fatality causes, this is a rate of 2 10-4 / year.
GAME principle A new technical system shall be globally be at least as good as the old one,
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Tolerable Rates of Dangerous Failures and SILs
Safety Integrity Level
Rate of Dangerous Failures
SIL 1
10-6/h … < 10-5/h
SIL 2
10-7/h … < 10-6/h
SIL 3
10-8/h … < 10-7/h
SIL 4
10-9/h … < 10-8/h
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3
Example – Track Worker Warning System Risk graph Parameters: C3 (since several people can be killed when the track worker crew is not warned.), F2 (since the persons are frequently in the dangerous zone), W2 or W3. Result: SIL 3 or SIL 4 MEM The risk 10-9/h.
of
a
single
technical
system
shall
not
exceed
Assuming that only in 20% of the failure of the track worker warning system will lead to accidents. Result: rate of dangerous failures is 5 10-9/h, yielding SIL 4.
7
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Example – Track Worker Warning System GAME a)
watch-out (optimistic version)
b)
simple, single watch-out (pessimistic version)
c)occupational health.
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4
GAME a)
watch-out (optimistic version)
A single track worker is dedicated. Two signals given. Probability for human failures of 5 10-4. Assume 2 trains per hour. Result: 2*(5*10-4)2/h = 5 10-7/h giving SIL2. b)
simple, single watch-out (pessimistic version)
Single track worker One failure of watch out leads to accident. Two trains per hour. Result: 2*5*10-4/h = 10-3/h giving SIL 0.
9
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GAME c)
Occupational health
Inputs: - Kuhlmann gives a target for construction workers of 2 10-7/h. R -Railway Safety reports an average rate of 3 fatalities per 20000 workers per year, giving a rate of 3/(20000 * 1500h) = 10-7/h, assuming 1500 working hours per year. Result: rate of 2 10-7/h or 10-7/h giving SIL 2.
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5
Results Principle
SIL
IEC 61508
SIL 3 / SIL 4
MEM
SIL 4
GAME: watch versions)
out
(optimistic SIL 2
GAME: watch out (pessimistic SIL 0 version) GAME: occupational health
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SIL 2
11
Conclusions
• Results of the different approaches applied to the example mainly coincide.
• More stringent requirements also lead to a higher SIL. • The highest SIL comes from the requirement that the level of safety for a track worker should be the same as in normal life, i.e. application of the MEM principle.
• The lowest SIL is obtained as a result of the requirement that the system should be as good as a controller of site safety under very pessimistic assumptions.
• Other approaches give intermediate results. • The result obtained from the risk graph fits into the range of other results. www. tuv .c om
12
6
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1
Introduction
• Methods for derivation of Safety Integrity Levels • Minimum Endogeneous Mortality (MEM) • As Low As Reasonably Practicable (ALARP), • Globalement Aussi Équivalent (GAME), • IEC 61508 risk graph Study of different principles and comparison
www. tuv .c om
2
1
Safety Integrity Level Definition – IEC 61508 • Risk Graph fromIEC 61508 W3
C1 P1
F1
Starting point for risk reduction C2 estimation
P1 P2
F1
C3
F2
C4
C = Consequence risk parameter
W1
a
-
-
b
P2
F2
W2
a c
b
a
d
c
b
e
d
c
f
e
d
g
f
e
h
g
f
a, b, c, d, e, f, g, h represent the necessary minimum risk reduction. The link between the necessary minimum risk reduction and the safety integrity level is shown in the table.
Necessary minimum risk reduction
Safety integrity level
-
No safety requirements
a
No special safety requirements 1 2 3 4 An E/E/PE SRS is not sufficient
F = Frequency and exposure time risk parameter P = Possibility of avoiding hazard risk parameter W = Probability of the unwanted occurrence a, b, c ... h = Estimates of the required risk reduction for the SRSs
b, c d e, f g h
3
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Safety Integrity Level Definition – IEC 61508 Risk parameter Consequence (C)
Classification C1 C2 C3 C4
Minor injury Serious permanent injury to one or more persons; death to one person Death to several people Very many people killed
Frequency of, and exposure time in, the hazardous zone (F)
F1 F2
Rare to more often exposure in the hazardous zone Frequent to permanent exposure in the hazardous zone
Possibility of avoiding the hazardous event (P)
P1 P2
Possible under certain conditions Almost impossible
Probability of the unwanted occurrence (W)
W1
A very slight probability that the unwanted occurrences will come to and only a few unwanted occurrences are likely A slight probability that the unwanted occurrences will come to and few unwanted occurrences are likely A relatively high probability that the unwanted occurrences will come to and frequent unwanted occurrences are likely
W2 W3
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4
2
SIL Definition MEM and GAME
Minimum Endogenous Mortality A new technical system must not add an unjustifiable amount of risk to the risk budget of a person. The starting point is the endogenous mortality of a 15-20 year old human being. Considering only endogenous fatality causes, this is a rate of 2 10-4 / year.
GAME principle A new technical system shall be globally be at least as good as the old one,
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5
Tolerable Rates of Dangerous Failures and SILs
Safety Integrity Level
Rate of Dangerous Failures
SIL 1
10-6/h … < 10-5/h
SIL 2
10-7/h … < 10-6/h
SIL 3
10-8/h … < 10-7/h
SIL 4
10-9/h … < 10-8/h
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3
Example – Track Worker Warning System Risk graph Parameters: C3 (since several people can be killed when the track worker crew is not warned.), F2 (since the persons are frequently in the dangerous zone), W2 or W3. Result: SIL 3 or SIL 4 MEM The risk 10-9/h.
of
a
single
technical
system
shall
not
exceed
Assuming that only in 20% of the failure of the track worker warning system will lead to accidents. Result: rate of dangerous failures is 5 10-9/h, yielding SIL 4.
7
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Example – Track Worker Warning System GAME a)
watch-out (optimistic version)
b)
simple, single watch-out (pessimistic version)
c)occupational health.
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8
4
GAME a)
watch-out (optimistic version)
A single track worker is dedicated. Two signals given. Probability for human failures of 5 10-4. Assume 2 trains per hour. Result: 2*(5*10-4)2/h = 5 10-7/h giving SIL2. b)
simple, single watch-out (pessimistic version)
Single track worker One failure of watch out leads to accident. Two trains per hour. Result: 2*5*10-4/h = 10-3/h giving SIL 0.
9
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GAME c)
Occupational health
Inputs: - Kuhlmann gives a target for construction workers of 2 10-7/h. R -Railway Safety reports an average rate of 3 fatalities per 20000 workers per year, giving a rate of 3/(20000 * 1500h) = 10-7/h, assuming 1500 working hours per year. Result: rate of 2 10-7/h or 10-7/h giving SIL 2.
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10
5
Results Principle
SIL
IEC 61508
SIL 3 / SIL 4
MEM
SIL 4
GAME: watch versions)
out
(optimistic SIL 2
GAME: watch out (pessimistic SIL 0 version) GAME: occupational health
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SIL 2
11
Conclusions
• Results of the different approaches applied to the example mainly coincide.
• More stringent requirements also lead to a higher SIL. • The highest SIL comes from the requirement that the level of safety for a track worker should be the same as in normal life, i.e. application of the MEM principle.
• The lowest SIL is obtained as a result of the requirement that the system should be as good as a controller of site safety under very pessimistic assumptions.
• Other approaches give intermediate results. • The result obtained from the risk graph fits into the range of other results. www. tuv .c om
12
6
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