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Safety can be defined by referring to two existing safety standards:
- IEC 61508 (International Electrotechnical Commission (IEC), a functional safety standard for the general electronics market developed by the IEC.
- IEC 61508 Definition:
Safety is the freedom from unacceptable risk of physical injury or of damage to the health of people, either directly, or indirectly as a result of damage to property or to the environment. Functional Safety is part of the overall safety that depends on a system or equipment operating correctly in response to its inputs.
- ISO 26262 (IEC), a functional safety standard for automobiles from ISO. It has rapidly gaining acceptance as the guideline for the automotive engineer since its release.*
- ISO 26262 Definition:
Absence of unacceptable risk due to hazards caused by mal-functional behavior of electrical and/or electronic systems and the interactions of these systems.
- All systems will have some inherent, quantifiable failure rate. It is not possible to develop a system with zero failure rate.
- For each application, there is some tolerable failure rate which does not lead to unacceptable risk.
- Acceptable failure rates vary per application, based on the potential for direct or indirect physical injury in the event of system malfunction.
- The hazards and risks of applications can be analyzed and assigned categories based on the level of acceptable risk. These categories are known as Safety Integrity Levels, or SILs.
Operational issue in a system which may lead to a failure
Result of a fault which leads to an inability to execute safety critical functionality
Fault Tolerance
Ability to continue safe operation after a fault
System where a fault which may lead to failures is detected and the system is put into a safe state such that faults may not propagate to other systems
System where a fault which may lead to failures is detected and the system can continue operation without loss of safety function
Ability to execute operations in system without failure (generally independent of consideration for a safety function)
Amount of time in which a safety function is available divided by total system operation time. Systems with high reliability and fail functional systems tend to have higher availability than fail safe systems
Ability to detect, resist, or prevent tampering with product functionality
Availability + Reliability + Safety + Security + Maintainability
4 specific steps in the evolution of functional safety:
- Fail-safe: the system goes into safe mode when a failure occurs.
- Fail-silent: the system recognizes that it is receiving the wrong information due to a fault, so the ongoing operation moves to degraded mode.
- Fail-operational: sometimes also referred to as fault-tolerant, a failure in one component does not stop the whole system from working correctly, the system reconfigures itself to compensate for the fault.
- High-dependability: this is advanced failure prediction.
Failures in a functional safety system can be broadly classified into two categories: Systematic and Random failures
− Result from a failure in design or manufacturing − Often a result of failure to follow best practices − Occurrence of systematic failures can be reduced through continual and rigorous process improvement and robust analysis of any new technology
− Result from random defects or soft errors inherent to process or usage condition − Rate of random faults cannot generally be reduced; focus must be on the detection and handling of random faults to prevent application failure
| Standard | Targeted End Equipment Applications |
|---|---|
| IEC 61508 | Electrical, Electronic, Programmable Electronic Systems |
| ISO 26262 | Road Vehicles (except Mopeds) up to 3500Kg* |
| EN 50129 | Railway Signaling |
| ISO 22201 | Elevator / Escalator |
| IEC 61511 | Process Industry (Chemical, Oil Refining etc.) |
| IEC 61800 | Adjustable speed AC motor drive |
| IEC 62061 | Industry Machinery (electronics) |
| ISO 13849 | Industry Machinery |
| IEC 60730 | Automatic Controls for Household use |