Reliability Engineering - Reliability Engineering Vs Safety Engineering

Reliability Engineering Vs Safety Engineering

Reliability engineering differs from safety engineering with respect to the kind of hazards that are considered. Reliability engineering is in the end only concerned with cost. It relates to all Reliability hazards that could transform into incidents with a particular level of loss of revenue for the company or the customer. These can be cost due to loss of production due to system unavailability, unexpected high or low demands for spares, repair costs, man hours, (multiple) re-designs, interruptions on normal production (e.g. due to high repair times or due to unexpected demands for non-stocked spares) and many other indirect costs.

Safety engineering, on the other hand, is more specific and regulated. It relates to only very specific and system Safety Hazards that could potentially lead to severe accidents. The related functional reliability requirements are sometimes extremely high. It deals with unwanted dangerous events (for life and environment) in the same sense as reliability engineering, but does normally not directly look at cost and is not concerned with repair actions after failure / accidents (on system level). Another difference is the level of impact of failures on society and the control of governments. Safety engineering is often strictly controlled by governments (e.g. Nuclear, Aerospace, Defense, Rail and Oil industries).

Furthermore, safety engineering and reliability engineering may even have contradicting requirements. This relates to system level architecture choices. For example, in train signal control systems it is common practice to use a fail-safe system design concept. In this concept the so-called "wrong side failures" need to be fully controlled to an extreme low failure rate. These failures are related to possible severe effects, like frontal collisions (2* GREEN lights). Systems are designed in a way that the far majority of failures will simply result in a temporary or total loss of signals or open contacts of relays and generate RED lights for all trains. This is the safe state. All trains are stopped immediately. This fail-safe logic might unfortunately lower the reliability of the system. The reason for this is the higher risk of false tripping as any full or temporary, intermittent failure is quickly latched in a shut-down (safe)state. Different solutions are available for this issue. See chapter Fault Tolerance below.