Occurrence

A numerical value that ranks the risk associated with a failure mode by multiplying three independently scored factors: Severity (impact of the failure), Occurrence (likelihood of the failure happening), and Detection (likelihood of detecting the failure before it reaches the end user or causes harm). RPN ranges from 1 to 1,000, where higher values indicate greater risk priority.

A rating of the worst potential consequence of a failure mode, scored on a scale of 1 (negligible) to 10 (catastrophic/hazardous without warning). Severity considers the impact on safety, environment, operations, and financial loss. Severity ratings should only change if the design or process is fundamentally altered to eliminate or mitigate the failure effect.

A rating of the ability of current controls to detect the failure mode or its cause before the failure reaches the end user or causes harm, scored on a scale of 1 (almost certain detection) to 10 (no detection capability). Detection evaluates the effectiveness of existing inspection, monitoring, and testing methods. Lower detection scores indicate better detection capability.

The average number of failures per unit of time for a population of items, typically expressed as failures per hour, per year, or per million hours. Failure rate (lambda) is the reciprocal of MTBF for constant-failure-rate systems. It is a statistical average measure, distinct from the instantaneous hazard rate, and is most accurate when the failure rate is approximately constant (beta near 1).

The underlying reason or mechanism that initiates the process leading to a failure mode. A failure cause is the root-level factor — design deficiency, material defect, manufacturing flaw, operational condition, degradation mechanism, or human error — that creates the conditions for the failure mode to manifest. Each failure mode may have multiple independent causes.

A quantitative or semi-quantitative measure of the likelihood that an equipment item will fail within a specified time period. PoF is determined by the generic failure frequency of the equipment type, modified by damage factors that account for specific active degradation mechanisms, and adjusted by a management system factor reflecting the quality of the facility's integrity management program.