What Does
Safety Requirements Specifications Mean?
Safety requirements specifications (SRS) are specifications that describe every required safety function that must be performed by a safety instrumented system (SIS).
SRSs specify both what safety functions must be performed by a system and how well those functions must be performed.
Safeopedia Explains Safety Requirements Specifications
An SRS is designed for two primary purposes. First, it guides the design and creation of a safety system depending on the requirements of the workplace using the system; second, it provides the employees in that workplace with a clear explanation of the safety capabilities with which they are working. A clear understanding of the specific functionality of a safety system is important to mitigate workplace risk. A poor understanding of safety capabilities is a major cause of accidents involving control systems.
SRSs are associated with two major international standards. The first is the ISO 13849 (International Organization for Standardization), a standard which defines all parts of a functional safety analysis—including SRS—for the purposes of machine safety. The second is the IEC 61511 (International Electrotechnical Commission), a process safety standard which mandates the creation of an SRS for all safety instrumented systems.
SRSs have become a standardized component of safety systems due to the high prevalence of workplace accidents that result from control system specification issues. According to a 2003 study by the UK’s Health and Safety Executive (HSE), a poor understanding of a control system’s safety functions due to deficient specification in the description of the safety system is a factor in 44 percent of accidents that involve control systems, the most common causative factor in the study.
A standards-compliant SRS must list both the general safety functions and the task-specific functions required by all of the safety instruments that comprise the safety system. General safety functions described by the SRS include such information as the environment the safety system will need to function in and the standards that it must meet. Specific functions include the specific safety integrity level (SIL) at which each safety instrument must function and each safety function’s response time to a safety incident.
Because SRSs list exact definitions of the safety functions provided by a safety system, they are often used as a contractual document between the company providing the safety system and the company commissioning it.