Proof Test — Failure-Finding

What it is

Failure-finding (proof test): a periodic functional test of an item whose failure is hidden — the equipment looks normal and the failure would only reveal itself when the function is demanded. The archetypes: the PSV that will not open, the level switch that will not trip, the interlock that will not act.

When it is the right decision (Framework B)

"Hidden" is a dimension of visibility, not of statistics — it overlays any Framework A category (a PSV can be Wear-out at the seat AND Random in sticking; both hidden). The criterion is single: operations does NOT perceive the failure under normal conditions. If the failure is hidden, the proof test is mandatory — there is no CBM of a function that is never exercised.

Periodicity — the three-ceilings rule

where $D$ is the required availability of the protective function and $\lambda_{DU}$ the dangerous-undetected failure rate. Three sources compete — the shortest always prevails:

1. RCM calculation (formula above);
2. IEC 61511 (if the item belongs to a SIF: the SIL sets the interval);
3. NR-13 (PSVs: never beyond the internal inspection of the protected vessel — item 13.5.4.10, Brazilian regulation).

Pitfalls

The proof test exercises the item — a poorly executed test is an infant-mortality opportunity (recalibration, reassembly); the test must simulate the real demand (simulate level at the switch, bench pop test for the PSV) and record a measured value, not an "OK".

On the platform

The strategy selector identifies hiddenness at the second question and applies the formula with your numbers.