| Term | Meaning |
|---|---|
| BPCS | Basic process control system: the control system that runs the plant normally. |
| β, βD | Common cause factors: fraction of undetected (β) and detected (βD) dangerous failures that affect all redundant channels at once. |
| C&E | Cause and effect matrix: causes as rows, actions as columns. |
| DC | Diagnostic coverage: λDD / (λDD + λDU). |
| Demand | A process condition that requires the SIF to act. |
| Enabling condition | Probability that the plant is in the state where the cause can lead to the consequence. |
| FMEDA | Failure modes, effects and diagnostic analysis: the analysis behind a device's failure rates by category. |
| HAZOP | Hazard and operability study using guide words on a design intent. |
| HFT | Hardware fault tolerance: number of faults a subsystem can have and still perform its function. |
| HIPPS | High integrity pressure protection system. |
| IEF | Initiating event frequency, per year. |
| IPL | Independent protection layer. |
| λSD, λSU, λDD, λDU | Failure rates per hour: safe detected, safe undetected, dangerous detected, dangerous undetected. |
| LOPA | Layer of protection analysis. |
| MEF | Mitigated event frequency: IEF reduced by enabling conditions, modifiers and IPLs, before the SIF. |
| MooN | M out of N voting. |
| MRT | Mean repair time after a failure found at a proof test. |
| MTTFS | Mean time to fail spuriously (to a spurious trip). |
| MTTR | Mean time to restoration after a detected failure. |
| Mission time | Period over which PFDavg is averaged; often the service life or renewal interval. |
| PFDavg | Average probability of failure on demand (low demand mode). |
| PFH | Average frequency of dangerous failure per hour (high demand or continuous mode). |
| Process safety time | Time from the start of a failure to the hazard if no protection acts. |
| Proof test | Periodic test to reveal dangerous undetected failures. |
| PST, PSTC | Partial stroke test and its coverage of λDU. |
| PTC | Proof test coverage: fraction of λDU revealed by the proof test. |
| Route 1H / 2H | IEC 61508 routes for architectural constraints (SFF based, or field data based). |
| RRF | Risk reduction factor, 1 / PFDavg. |
| SC | Systematic capability (1 to 4). |
| SFF | Safe failure fraction. |
| SIF | Safety instrumented function. |
| SIL | Safety integrity level (1 to 4). |
| SIS | Safety instrumented system: sensors, logic solvers and final elements implementing SIFs. |
| SRS | Safety requirements specification. |
| T1 | Proof test interval. |
| TMEL | Target mitigated event likelihood: the tolerable frequency for the consequence. |
| Type A / Type B | IEC 61508 element types: simple with well-defined failure modes (A), or complex (B). |
Written for First Out AI as engineering guidance. It supports engineering work and does not replace it; check every value against your standards, certificates and company rules with a competent functional safety engineer.