Safety-Critical Systems 2 T Risk analysis and design for safety Ilkka Herttua

V - Lifecycle model System Acceptance System Integration & Test Module Integration & Test Requirements Analysis Requirements Model Test Scenarios Software Implementation & Unit Test Software Design Requirements Document Systems Analysis & Design Functional / Architechural - Model Specification Document Knowledge Base * * Configuration controlled Knowledge that is increasing in Understanding until Completion of the System: Requirements Documentation Requirements Traceability Model Data/Parameters Test Definition/Vectors

Overall safety lifecycle

Risk Analysis Risk is a combination of the severity (class) and frequency (probability) of the hazardous event. Risk Analysis is a process of evaluating the probability of hazardous events. The Value of life?? Value of life is estimated between 0.75M –2M GBP. USA numbers higher.

Risk Analysis Classes: - Catastrophic – multiple deaths >10 - Critical – a death or severe injuries - Marginal – a severe injury - Insignificant – a minor injury Frequency Categories: Frequent 0,1 events/year Probable0,01 Occasional0,001 Remote0,0001 Improbable0,00001 Incredible0,000001

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)

Fault Tree Analysis 1 The diagram shows a heater controller for a tank of toxic liquid. The computer controls the heater using a power switch on the basis of information obtained from a temperature sensor. The sensor is connected to the computer via an electronic interface that supplies a binary signal indicating when the liquid is up to its required temperature. The top event of the fault tree is the liquid being heated above its required temperature.

Fault event not fully traced to its source Basic event, input Fault event resulting from other events OR connection

Risk acceptability National/international decision – level of an acceptable loss (ethical, political and economical) Risk Analysis Evaluation: ALARP – as low as reasonable practical (UK, USA) “Societal risk has to be examined when there is a possibility of a catastrophe involving a large number of casualties” GAMAB – Globalement Au Moins Aussi Bon = not greater than before (France) “All new systems must offer a level of risk globally at least as good as the one offered by any equivalent existing system” MEM – minimum endogenous mortality “Hazard due to a new system would not significantly augment the figure of the minimum endogenous mortality for an individual”

Risk acceptability Tolerable hazard rate (THR) – A hazard rate which guarantees that the resulting risk does not exceed a target individual risk SIL 4 = < THR < per hour and per function SIL 3 = < THR < SIL 2 = < THR < SIL 1 =10 -6 < THR < Potential Loss of Life (PLL) expected number of casualties per year

Current situation / critical systems Based on the data on recent failures of critical systems, the following can be concluded: a)Failures become more and more distributed and often nation-wide (e.g. commercial systems like credit card denial of authorisation) b)The source of failure is more rarely in hardware (physical faults), and more frequently in system design or end-user operation / interaction (software). c)The harm caused by failures is mostly economical, but sometimes health and safety concerns are also involved. d)Failures can impact many different aspects of dependability (dependability = ability to deliver service that can justifiably be trusted).

Examples of computer failures in critical systems

Driving force: federation Safety-related systems have traditionally been based on the idea of federation. This means, a failure of any equipment should be confined, and should not cause the collapse of the entire system. When computers were introduced to safety-critical systems, the principle of federation was in most cases kept in force. Applying federation means that Boeing 757 / 767 flight management control system has 80 distinct microprocessors (300, if redundancy is taken into account). Although having this number of microprocessors is no longer too expensive, there are other problems caused by the principle of federation.

Designing for Safety Faults groups: - requirement/specification errors - random component failures - systematic faults in design (software) Approaches to tackle problems - right system architecture (fault-tolerant) - reliability engineering (component, system) - quality management (designing and producing processes)

Designing for Safety Hierarchical design - simple modules, encapsulated functionality - separated safety kernel – safety critical functions Maintainability - preventative versa corrective maintenance - scheduled maintenance routines for whole lifecycle - easy to find faults and repair – short MTTR mean time to repair Human error - Proper HMI

Hardware Faults Intermittent faults - Fault occurs and recurrs over time (loose connector) Transient faults - Fault occurs and may not recurr (lightning) - Electromagnetic interference Permanent faults - Fault persists / physical processor failure (design fault – over current)

Fault tolerance hardware - Achieved mainly by redundancy Redundancy - Adds cost, weight, power consumption, complexity Other means: - Improved maintenance, single system with better materials (higher MTBF) Fault Tolerance

Redundancy types Active Redundancy: - Redundant units are always operating. Dynamic Redundancy (standby): - Failure has to be detected - Changeover to other modul

Hardware redundancy techniques Active techniques: - Parallel (k of N) - Voting (majority/simple) Standby : - Operating - hot stand by - Non-operating – cold stand by

Reliability prediction Electronic Component - Based on propability and statictical - MIL-Handbook 217 – experimental data on actual device behaviour - Manufacture information and allocated circuit types -Bath tube curve; burn in – useful life – wear out

Safety-Critical Hardware Fault Detection: - Routines to check that hardware works - Signal comparisons - Information redundancy –parity check etc.. - Watchdog timers - Bus monitoring – check that processor alive - Power monitoring

Safety-Critical Hardware Possible hardware: COTS Microprocessors - No safety firmware, least assurance - Redundancy makes better, but common failures possible - Fabrication failures, microcode and documentation errors - Use components which have history and statistics.

Safety-Critical Hardware Special Microprocessors - Collins Avionics/Rockwell AAMP2 - Used in Boeing (30+ pieces) - High cost – bench testing, documentation, formal verification - Other models: SparcV7, TSC695E, ERC32 (ESA radiation-tolerant), 68HC908GP32 (airbag)

Safety-Critical Hardware Programmable Logic Controllers PLC Contains power supply, interface and one or more processors. Designed for high MTBFs Firmware Programm stored in EEPROMS Programmed with ladder or function block diagrams

Safety management Safety culture/policy of the organisation - Task for management ( Targets ) Safety planning - Task for safety manager ( How to ) Safety reporting - All personal - Safety log / validation reports

Home assignments 4.18 (tolerable risk) 5.10 (incompleteness within specification) before 2. March to