In brief
What to carry into the engineering work
- Definitions are written for engineering use and link back to the wider workflow.
- Project specifications and the applicable standards govern where terminology differs.
- Use the glossary as a navigation point, then open the full guide for assumptions and application.
Terms
- BPCS
- Basic Process Control System used for normal process control; it is not automatically independent protection for a scenario it helps cause.
- CCF
- Common-cause failure: one cause that can defeat multiple nominally redundant channels.
- DC
- Diagnostic coverage: fraction of relevant dangerous failures detected by the diagnostic measures.
- HFT
- Hardware fault tolerance: ability to perform the required function in the presence of specified hardware faults.
- LOPA
- Layer of Protection Analysis: semi-quantitative method that estimates scenario frequency using initiating events and independent protection layers.
- MooN
- Voting notation in which M of N channels must act for the subsystem to perform its function.
- MRT
- Maximum restoration time used as a controlled limit for returning a failed or degraded function to its required state.
- MTTR
- Mean time to repair, a statistical repair-time input used by some reliability models.
- PFDavg
- Average probability of failure on demand for a low-demand safety function.
- PFH
- Average frequency of dangerous failure per hour for high-demand or continuous-mode functions.
- PST
- Partial stroke test used to reveal selected final-element failures without a complete stroke.
- PTC
- Proof-test coverage: fraction of relevant dangerous failures revealed by the defined proof test.
- PTI
- Proof-test interval between complete scheduled proof tests.
- RRF
- Risk reduction factor; for low demand, approximately the reciprocal of PFDavg.
- Safe state
- The defined process condition reached or maintained to control the hazardous scenario.
- SC
- Systematic capability supported by the device or subsystem evidence for control and avoidance of systematic faults.
- SFF
- Safe failure fraction used with HFT and device type in Route 1H architectural assessment.
- SIF
- Safety Instrumented Function that detects a defined condition and performs a specified safety action.
- SIL
- Safety Integrity Level specifying a discrete integrity requirement or supported claim for a safety function.
- SIS
- Safety Instrumented System that implements one or more SIFs.
- SRS
- Safety Requirements Specification defining functional and integrity requirements for the SIS and its SIFs.
- Validation
- Confirmation that the installed SIS and SIFs satisfy the SRS in the intended application.
- Verification
- Confirmation that the output of a lifecycle phase satisfies its specified input requirements.
- β factor
- Fraction used to model dangerous failures attributed to common cause in redundant channels.
- λDU
- Dangerous undetected failure rate.
- λDD
- Dangerous detected failure rate.
Primary references
- IEC — Overview of IEC 61508 and functional safety
- IEC 61511-1 — process-industry SIS framework and requirements
Use the applicable edition and national adoption. Project specifications and controlled procedures may add requirements.