1. Safety Assessment (Fault Trees) ITV Model-based Analysis and Design of Embedded Software Techniques and methods for Critical Software Anders P. Ravn Aalborg University August 2011

2. Safety Assessment Find hazards that may lead to incidents or mishaps, analyze their relations, and estimate their consequences. May involve probabilistic reasoning (Reliability Engineering). Is PFH < 10 -6 per hour (h -1 ) ?

3. Techniques Fault Trees Event tree (forward analysis from initiating event) Combinations (cause-consequence diagrams) Many tools Kirsten M. Hansen, Anders P. Ravn and Victoria Stavridou, From Safety Analysis to Formal Specification, IEEE Trans. Softw. Eng.24,pp. 573-584, July 1998

4. Fault Trees - Events Primary Events : Basic event – fault in atomic component Undeveloped Event – fault in composite component (may be analyzed later) External event – expected event from environment Intermediate event: Nodes inside a fault-tree

5. Fault Tree - Gates... condition Inhibit gate

6. Example – ”Woke too late” Woke too late Alarm clock fails Phone fails ”Inner clock” fails

7. Example ”Alarm clock fails” Alarm clock fails electronics fail SW fails Power fails Beeper fails Button fails Button read fails Beeper not set

8. Cut Set A cut set is a set of events that causes a top level event A singleton cut set is a single point of failure

9. Example – ”Wake too late” Wake too late Alarm clock fails Phone fails ”Inner clock” fails

10. Example ”Alarm clock fails” Alarm clock fails electronics fail SW fails Power fails Beeper fails Button fails Button read fails Beeper not set

11. Exercise – estimate the PFD Alarm clock fails electronics fail SW fails Power fails Beeper fails Button fails Button read fails Beeper not set