According to APR 4761A, safety can be defined as the condition in which the risk of harm to persons or damage to property is reduced to, and maintained at or below, an acceptable level through a continuing process of hazard identification and risk management.
This definition highlights the importance of identifying hazards and assessing risks in order to mitigate them and maintain an acceptable level of safety. It emphasizes the need for a continuous process of risk management to ensure that safety is maintained over time, as new hazards and risks may arise. Additionally, it recognizes that safety is ultimately about reducing the risk of harm to people and property.
IEEE Std-1228 (1994) defines software safety as "freedom from software hazard," where software hazard is defined as "a software condition that is a prerequisite to an accident," and an accident is defined as "an unplanned event or series of events that results in death, injury, illness, environmental damage, or damage to or loss of equipment or property". Here we assume that the term "property" also includes intellectual property.
The Elevator Paradox: Safety vs. Reliability
• Reliability is the probability that a system performs its intended function continuously over time.
• Safety is the degree to which a system avoids causing unintentional harm, damage, or hazard.
The Trade-Off in Action
Consider an inoperative elevator. Because it cannot move, shut its doors on anyone, or drop between floors, it is 100% safe. However, because it cannot transport passengers to their desired floor, its reliability is 0%.
To make an active elevator safer, designers add interlocks, sensors, and safety controls—such as door-closure sensors. If a safety sensor fails, it deliberately halts the elevator. Safety increases, but overall operational availability and reliability decrease.
System Safety Engineering balances this fundamental trade-off, ensuring systems fail safely without destroying operational availability.
System Safety is a rational pursuit of acceptable hazard risk in which the system is treated as an integral part of a System-of-Systems , taking into account the interactions among system's constituent parts. System Safety is an integral part of the interdisciplinary approach of systems engineering and its pursuit of systems that meet stakeholder expectations.
The methods of System Safety are diverse and are driven by many factors, including:
System Safety Assessment relies on analytical results, in part due to the high cost of testing limiting the ability to rely on test-fail-fix strategies for designing a safe and reliable system.
Increasing system complexity, which makes it necessary to leverage both traditional and modern hazard evaluation mechanisms in order to identify and analyze comprehensively the full set of credible mishap scenarios that have the potential to lead to adverse consequences, considering all hazard causes and propagation pathways through the system.
The development of systems that operate at the edge of engineering capability, requiring a high degree of discipline in system realization and system operation management and oversight.
The use of unproven technology, requiring engineering conservatism to protect against unknown mishap risks while at the same time requiring allowances for novel solutions.
See also:
Safety Assessment
Safety Management
ALD Safety Services for Certification
SAE 4754A