chapter  6
29 Pages

The 1950s, 1960s, and Onward

System Safety
WithDrew Rae, Sidney Dekker

Chapter 6 details the contributions of system safety. It was driven by the commitment that safety should get built into the system from the very beginning. And once a system was in operation, system safety specified the requirements for its effective and safe management. This required system safety to recognize the technical, human, and environmental contributors to the creation and erosion of safety, and to map and resolve (to the extent possible) the conflicts and trade-offs between safety and other factors in the design and operation of the system. To do so, systems engineering for safety involves standardized process steps, with many variations in the detail of technique applied at each step. The steps are (semi-)formal modeling of the system under development; analysis of draft system designs; and analysis of the final design to demonstrate safety and to inform post-design safety efforts. From a design perspective, systems can be unsafe through requirements error (designing the wrong system), or implementation error (designing the system wrong). The aim is to prevent foreseeable events and minimize the consequences of unforeseen ones. The increasing complexity of automation and computerization (particularly when added to legacy systems can make this very difficult. System safety, through its formal language and techniques, has defined safety as freedom from unwanted events, and protection against unwanted outcomes. As systems have become more complex and anticipating all pathways to failure becomes virtually impossible, an emphasis is shifting to assuring the presence of capacity to handle unforeseen events, rather than assuring the absence of failure modes.