Proof test coverage is the fraction of λDU that the test reveals. It depends on the method, not on the device alone. The table is qualitative guidance for thinking through the failure modes; use the device FMEDA or certificate values where they exist.
| Device | Test method | Reveals | Does not reveal | Coverage tendency |
|---|---|---|---|---|
| Pressure transmitter | Signal injection at the electronics only | Output and logic chain faults | Sensor, impulse line, tapping blockage | Low to medium |
| Pressure transmitter | Calibrated pressure at the manifold up to the trip point | Sensor, electronics, output | Blocked process tapping upstream of the manifold | High |
| Level transmitter | Simulated echo or internal test function | Electronics, part of the measurement | Build-up, actual level response | Medium |
| Level transmitter | Raising the real level to the trip point | Almost all modes | Very few | Highest |
| Temperature | Simulated RTD input | Transmitter, logic | Sensor drift, thermowell | Low to medium |
| Temperature | Sensor in a calibration bath at the trip point | Sensor and transmitter | Thermowell response | High |
| Switch | Shorting the contact | Wiring and logic | The sensing element | Low |
| Switch | Actuation by the process element | Almost all modes | Few | High |
| Shutdown valve | Full stroke, visual end position | Stuck valve, actuator, solenoid | Slow stroke, seat leakage | Medium |
| Shutdown valve | Full stroke with timing | Plus slow stroke | Seat leakage | Medium to high |
| Shutdown valve | Full stroke, timing and leak test | Plus seat leakage | Behaviour at full process conditions | High |
| Motor trip | Trip command, motor stops | Contactor, trip path | Welded second contactor if not checked separately | High if each contactor is checked |
| Logic solver | Function test of voting, delays, program version | Application logic | Hardware faults covered by diagnostics | High |
How to use it
- Write the test method in the procedure exactly as it will be done.
- List the device's dangerous undetected failure modes (from the FMEDA or a simple FMEA).
- Mark which ones the method reveals; the share of λDU covered is the PTC.
- Enter it in First Out AI and record the basis in the source field or the SRS.
Written for First Out AI as engineering guidance. It supports engineering work and does not replace it; check every value against your standards, certificates and company rules with a competent functional safety engineer.