What Is Safety?

Safety is an engineering and management discipline concerned with identifying, analyzing, and controlling hazards so that the risk of harm to people, property, and the environment is reduced to acceptable levels. It applies to the design of systems and products, the conditions under which they are operated, and the organizational processes that govern their development and maintenance. The field draws from systems engineering, human factors, probability theory, and regulatory science, and it spans domains from aerospace and nuclear power to consumer products and occupational health.

Safety is distinct from reliability in an important respect: reliability concerns whether a system performs its intended function, while safety concerns whether a system causes unintended harm. A highly reliable system can still be unsafe if its normal operation exposes workers or the public to hazardous energy, toxins, or other harm-producing agents.

Functional Safety

Functional safety is the aspect of overall safety that depends on the correct operation of safety-related systems, including both the system under control and the safety functions implemented in hardware and software that prevent or mitigate hazards. The governing standard, IEC 61508 published by the International Electrotechnical Commission, defines a risk-based methodology for designing electrical, electronic, and programmable electronic safety-related systems. The standard introduces the concept of Safety Integrity Level (SIL), a discrete scale from SIL 1 (lowest) to SIL 4 (highest) that quantifies the required risk reduction a safety function must achieve. Sector-specific standards derived from IEC 61508 include IEC 62061 for machinery, ISO 26262 for automotive, and DO-178C for airborne software.

The SIL assignment process begins with a hazard and risk analysis that estimates the severity and frequency of potential harms, then specifies what probability of dangerous failure on demand is acceptable for each safety function. This quantitative requirement drives the rigor of the development process, including the redundancy architecture, the diversity of implementation, and the depth of testing required.

System Safety and Aerospace Safety

System safety is a subdiscipline that applies systematic, analytical techniques to identify and eliminate hazards during the design and development of complex systems, rather than relying on post-design add-on protective measures. Techniques include preliminary hazard analysis (PHA), fault tree analysis (FTA), and failure mode and effects criticality analysis (FMECA). Aerospace safety applies these methods in one of the most demanding regulatory environments, where civilian aircraft must demonstrate extremely low catastrophic failure probabilities, on the order of one in a billion flight hours, before they are certificated for commercial service. The FAA Advisory Circular AC 25.1309-1 defines the airworthiness requirements and safety assessment processes for transport-category aircraft systems.

Occupational Safety and Human Factors

Occupational safety concerns the protection of workers from hazards arising in the course of employment, including mechanical, electrical, chemical, ergonomic, and psychosocial risks. In the United States, the Occupational Safety and Health Administration (OSHA) sets and enforces workplace safety standards; internationally, the ISO 45001 standard provides a framework for occupational health and safety management systems. Human factors (also called ergonomics or human-centered design) is the scientific discipline concerned with understanding the interactions between humans and systems, designing systems so that human error is less likely and its consequences less severe. Human factors analysis recognizes that most accidents involve a combination of latent organizational conditions and immediate human actions rather than a single isolated cause, a principle formalized in James Reason's Swiss Cheese model.

Electrical Safety

Electrical safety addresses the hazards of electric shock, arc flash, fire, and equipment damage arising from the generation, transmission, and use of electrical energy. The NFPA 70E Standard for Electrical Safety in the Workplace specifies the protective equipment, safe work practices, and approach boundaries required when workers must operate near energized conductors. Arc flash hazard analysis, which calculates the incident energy a worker might be exposed to during an electrical fault, is now a standard step in facility electrical safety programs.

Applications

Safety has applications in a wide range of fields, including:

• Aerospace and aviation, including aircraft certification and air traffic management
• Process industries such as chemical plants and oil refineries, under IEC 61511
• Automotive systems, including driver assistance and autonomous driving safety cases
• Nuclear power, where probabilistic risk assessment governs plant licensing
• Medical devices, where IEC 60601 and related standards govern patient safety
• Construction and heavy industry, under occupational safety regulations