Diagnostic Tests VS Proof Tests

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Thread Starter

iabdelmohsen

IEC 61511-2003 defines Proof Test as follows: "test performed to reveal undetected faults in a safety instrumented system so that, if necessary, the system can be restored to its designed functionality" (IEC 61511-2003 part 1 clause 3.2.58).

Diagnostic tests are normally referred to as online tests or automated tests and are performed either continuously or very frequently. Diagnostic tests detect dangerous failures and can change them to "safe" failures by bringing the process to a safe state or alarm operations/maintenance personnel to take some action.

On the other hand proof tests verify that the device will respond as expected to an unsafe condition and has not experienced a dangerous undetected failure. Diagnostic tests are usually performed online through diagnostic hardware and software features often built into components (e.g., memory test, CPU test, internal watch dog, and HART diagnostics). Some forms of diagnostics are external to the components such as automated partial stroke test for final
elements.

Proof tests are usually performed at pre-defined test intervals per the Safety Requirement Specification (SRS). Ideally real operating process conditions should be present or simulated for the proof tests and can be divided into parts versus complete end to end testing depending upon safety conditions.

The advantage of diagnostic tests is that they can detect failures online. Whereas a proof test is performed at a predefined time interval, then there is chance that the process could be running with a failed safety function for a long time before it is revealed.
 
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