Build Safety-Critical Designs with UML-based Fault Tree Analysis - The basics
Reliability and Safety
Reliability and safety are mostly independent concerns. Reliability refers to the probability that a system or component will meet its functional and quality of service (e.g. timeliness) requirements within a specified timeframe. While this sounds similar to our previous definition of safety, but the two concepts are importantly different.
A safe system is one which does not lead to accidents. It may fail all the time and still be safe. A reliable system may fail infrequently but when it does fail it does so with catastrophic consequences " such a system is not safe.
A handgun, for example, is a very reliable piece of equipment, but can easily lead to accidents even in the absence of a system fault. On the other hand, my old Plymouth station wagon refuses turn on at all, therefore it is very safe even though it is unreliable.
In general, reliability is a separate concern from safety, and it is important to maintain the distinction. For the most part, in systems that have a fail-safe state, reliability is an opposing concern to safety.
Reliability is improved when the system continues to provide services even it creates a hazardous situation. If the system is creating a hazardous situation, and there is a fail-safe state, then entering the fail-safe state improves system safety but decreases system reliability.
Consider a medical treatment laser. If a memory cell in the controller seems faulty, the safest thing the system can do is to shut down with the laser is de-energized (its fail-safe state), even if it is relatively unlikely that the detected fault could lead to a hazard. This decreases the system reliability. In such systems, a pessimistic policy is likely to be safer than an optimistic policy.
Many systems don't have a fail-safe state. If you're flying at 600 knots and 35,000 feet, it is not safe to shut off the jet engine if it is suspected of having a fault.
Similarly, in a drive-by-wire car, the last thing I want to see is an "Abort, Retry, Ignore" message appear on my dashboard when I'm driving down the freeway at 85 (ah, excuse me, 55) mph. In such systems, increasing reliability (such as by adding redundant delivery channels) also improves the system safety.
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| Figure 2: Safety vs. Reliability |
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