PFH

Probability of Failure per Hour (PFH) is a quantitative metric representing the average probability that a safety function will fail dangerously in any given hour of operation. It is the primary numerical target for SIL classification in high demand or continuous mode systems under IEC 61508.

SIL targets: SIL 1 ≥ 10⁻⁶ to 10⁻⁵, SIL 2 ≥ 10⁻⁷ to 10⁻⁶, SIL 3 ≥ 10⁻⁸ to 10⁻⁷, SIL 4 ≥ 10⁻⁹ to 10⁻⁸ (dangerous failures per hour).