Your FAT Passed Every Normal Cycle? Add the Missing Negative Tests
A successful normal cycle proves only the path that was exercised. A useful factory acceptance test also checks how the application handles invalid, missing, delayed, or contradictory information. Design those tests before commissioning pressure encourages shortcuts.
Original software test framework; it is not a safety acceptance procedure.
What is happening?
Negative tests examine specified failure behavior and recovery. Simulation can provide controlled inputs without creating hazards on machinery. Physical acceptance, especially for safety functions, requires its own qualified procedure and cannot be replaced by a generic software checklist.
Check these five things
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Map each requirement to an observable expected result.
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List invalid inputs and interruptions relevant to that requirement.
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Choose simulation or another approved test environment.
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Record recovery expectations as well as initial fault response.
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Keep test identity, configuration version, actual result, and evidence together.
Worked example
A teaching recipe transfer passes when every field arrives. Additional tests interrupt it midway, repeat the request, and supply an out-of-range field. The test passes only if the active recipe remains a complete accepted set and the status explains rejection or recovery.
Quick diagnostic reference
| Test class | Example |
|---|---|
| Missing | Required field absent |
| Delayed | Response arrives late |
| Duplicate | Same request repeated |
| Invalid | Value outside defined range |
The mistake to avoid
Do not invent live fault injections by disconnecting devices or defeating safeguards. Select test methods through the approved commissioning and risk-control process.
How to verify the fix
Trace every acceptance requirement to a recorded test result. Retest affected cases after changes and explicitly identify conditions that remain untested.
Field-work boundary: These are educational diagnostic guides. Use the exact equipment manuals and approved site procedures. Electrical testing and machinery changes belong to qualified, authorized personnel; control hazardous energy and validate affected functions before release.
Technical reference
Siemens: Engineering commissioning and acceptance documentation. The examples and diagnostic tables above are original teaching material; they do not replace the product manual.