Safety and SIS · Glossary

What is Proof Test Interval?

Symbol: TImonths or yearsSafety and SIS

Proof test interval is the time between periodic tests that reveal dangerous undetected failures in a safety function.

Proof Test Interval Explained

A proof test exercises the SIF to find hidden faults. The interval used in the PFDavg calculation must match how often the plant actually tests.

Shorter intervals lower PFDavg but cost production and testing effort; turnaround schedules often set the practical interval.

TI_max ≈ 2 × PFDavg_target ÷ λ_DU (1oo1)

PFDavg_target	required PFDavg
λ_DU	dangerous undetected rate

Example: Target 5 × 10⁻³, λDU = 1 × 10⁻⁶ /h: TI ≈ 10,000 h, about 14 months.

What is Proof Test Interval?

Proof test interval is the time between periodic tests that reveal dangerous undetected failures in a safety function.

Can the test interval be extended?

Only if the PFDavg is recalculated and still meets the target, ideally supported by test records.

What must a proof test record show?

As found results, failures found, repairs and the as left condition.

What is SIL (safety integrity level)

A practical guide on instrumentationblog.in with more detail on proof test interval and how it is applied on real plants.

All safety and sis terms · Full glossary A to Z