Siemens SIMATIC S7 F Manual page 171

Safety
Risk Parameters
The risk parameters have the following meaning in accordance with DIN V 19250:
Parameters
Extent of injury or
damage
S1
S2
S3
S4
Frequency and exposure time
A1
A2
Possibility of avoiding hazard
G1
G2
Probability of the unwanted occurrence
W1
W2
W3
Safety Integrity Level in Accordance with IEC 61508
For each Safety Integrity Level (SIL), IEC 61508 defines the probability of failure of
a safety function allocated to a safety-related system as a target measure.
Safety integrity
level
4
3
2
1
The actuators and sensors generally contribute most to these failure probabilities.
Each safety function always comprises the entire chain, from the collection and
processing of information to the intended action.
The equipment involved, such as the S7 F/FH programmable controller, sensors
and actuators, must in its entirety fulfill the AK and SIL determined as a result of
risk assessment.
If control functions and associated protection functions are implemented together in
the same S7 F/FH, this is said to be high-demand or continuous mode.
Fail-Safe Systems
A5E00085588-03
Meaning
Minor injuries; minor harmful effects on the environment
Serious irreversible injuries of one or more persons or fatality of
a person;
Temporary, seriously harmful effects on the environment
Several fatalities;
Lasting, seriously harmful effects on the environment
Catastrophic repercussions, large number of fatalities
Rare to more often
Frequent to continuous
Possible in certain circumstances
Rarely possible
Very low
Low
Relatively high
Low Demand Mode of Operation
(Average probability of failure to
perform its design function on
demand)
≥ 10
-5 -4
to < 10
≥ 10
-4 -3
to < 10
≥ 10
-3 -2
to < 10
≥ 10
-2 -1
to < 10
High Demand or Continuous
Mode of Operation
(Probability of a hazardous
failure per hour)
≥ 10
-9 -8
to < 10
≥ 10
-8 -7
to < 10
≥ 10
-7 -6
to < 10
≥ 10
-6 -5
to < 10
7-5