Safety integrity level (SIL) verification for SLAC radiation safety system

Safety Integrity Level (SIL) is a key concept in functional safety standards. SIL is a performance measure on how reliable a safety system is in performing a particular safety function. To comply with standards, during the system design stage, SIL verification must be performed to demonstrate that the SIL achieved meets/exceeds the SIL that was assigned during risk assessment. Unlike industrial applications, where safety systems are usually composed of certified devices or devices with long usage history, safety systems in large physics laboratories are less standardized and more complex in terms of system architecture and devices used. In addition, custom designed electronics, with limited reliability information, are often employed. Verifying SIL for these systems requires in-depth knowledge of reliability evaluation. In this paper, it is demonstrated how to determine SIL using SLAC radiation safety systems (Personnel Protection System (PPS) and Beam Containment System (BCS)) as examples. PPS utilizes commercial safety rated devices, while BCS contains customized electronics. Choice of standards, methods of evaluation, reliability data gathering process (both from industry and from hardware development) are also discussed. (author)