PFS (probability of failing safely) is a legacy reliability measure for the safe-failure side of a device or function: the dimensionless fraction of time it sits in a safe-failed, or spuriously tripped, state. It still turns up in older reliability texts, including Goble’s earlier work, but modern functional safety practice has largely moved away from it.
Today the industry quantifies spurious behavior with rate-based measures instead, namely the spurious trip rate (STR) and its inverse, mean time to fail spurious (MTTFsp), because “how often will this nuisance-trip?” is more useful to operations than a probability. We at SIL Safe treat PFS mainly as a term to recognize and decode in an old reference, then translate into STR or MTTFsp for any current work. Note that PFS is a probability while STR is a rate, so the two should not be used interchangeably.
Key Points
- PFS is a legacy, probability-based measure of safe (spurious) failure, the fraction of time a device sits in the safe-failed state.
- Modern practice uses rate-based metrics instead: spurious trip rate (STR) and mean time to fail spurious (MTTFsp).
- PFS is a probability, not a rate, so it should not be equated with STR; expect to see it mainly in older texts.
Example
An older reliability handbook reports a valve’s “PFS” alongside its PFD. For current work you would not carry PFS forward; you would describe the same spurious behavior as an STR (trips per year) or an MTTFsp (years between spurious trips).
See Also: spurious trip rate (STR), mean time to fail spurious (MTTFsp)
Cited Sources
- W. M. Goble, “Control Systems Safety Evaluation and Reliability,” ISA — an earlier reliability text where the PFS measure appears.