SafetyCritical Systems

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Safety-Critical Systems

• T 79.232
• Ilkka Herttua

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Safety Context Diagram
HUMAN
PROCESS
- Operating Rules
SYSTEM
- Hardware - Software
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Critical Applications

• Computer based systems used in avionics, chemical
  process and nuclear power plants.
• A failure in the system endangers human lives
  directly or through environment pollution. Large
  scale economic influence.

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Safety Definition

• Safety
• Safety is a property of a system that it will
  not endanger human life or the environment.
• Safety-Critical System
• A system that is intended to achieve, on its
  own, the necessary level of safety integrity for
  the implementation of the required safety
  functions.

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Safety Definition

• Safety integrity
• The likelihood of a safety-related system
  achieving its required safety features under all
  the stated conditions within a stated operational
  environment and within a stated period of time.
• SIL levels 0 to 4. SIL 4 is the highest safety
  integrity level.
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Developing safety-related systems

• To achieve safety
• - safety requirements
• - quality management
• - design / system architecture
• - defined design/manufacture processes
• - certification and approval processes
• - known behaviour of the system in all
  conditions
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V - Lifecycle model
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Safety Requirements

• Requirements are the stakeholders (customer)
  demands what they want the system to do. Not
  defining how !!!
• Safety requirements are defining what the system
  must do and must not do in order to ensure
  safety. Positive and negative functionality.
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Specification

• Supplier instructions how to build the system.
  Derived from the required functionality
  requirements.
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Verification and validation

• Verification is the process of determining that a
  system or module meets its specification.
• Validation is the process of determining that a
  system is appropriate for its purpose.
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Fault, error and failure

• A fault is defect within the system.
• Random faults hardware components,
  systematic faults software/hardware design and
  manufacture processes.
• An error is a deviation from the required
  operation of the system or subsystem.
• A system failure occurs when the system fails to
  perform its required function. (Significant,
  major and minor)
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Fault management

• Fault management techniques
• Fault avoidance in entire system design phase
• Fault removal - before system enters service
• Fault detection during service to minimising
  effects
• Fault tolerance operate correctly in the
  presence of faults
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RAM

• Reliability is the probability of a component or
  system functioning correctly over a given period
  of time under a given set of operating
  conditions. (MTBF mean time between failure.)
• The availability of a system is the probability
  that the system will be functioning correctly at
  any given time.
• Maintainability Maintenance is the action taken
  to retain a system in or return a system to its
  designed operating condition. (MTTR mean time to
  repair.)
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Hazard Analysis

• A Hazard is situation in which there is actual or
  potential danger to people or to environment.
• Analytical techniques
• - Failure modes and effects analysis (FMEA)
• - Failure modes, effects and criticality
  analysis (FMECA)
• - Hazard and operability studies (HAZOP)
• - Event tree analysis (ETA)
• - Fault tree analysis (FTA)

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Home assignments

• 1.12 (primary, functional and indirect safety)
• 2.4 (unavailability)
• 3.23 (fault tree)
• Email before 10. February to herttua_at_eurolock.org