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FAT, SAT and Commissioning: A Practical Guide
A Factory Acceptance Test (FAT) proves a system meets its specification at the builder's facility, before shipment, while problems are cheap to fix. A Site Acceptance Test (SAT) re-verifies the essentials after installation, in production conditions. Commissioning is the engineering work between and around them. Together they replace 'it should work' with demonstrated evidence.
Published · Last reviewed by Geek Engineering senior engineering review.
What a FAT is — and is not
The FAT is a scripted demonstration, at the integrator's or builder's facility, that the system does what the functional specification says — executed against a written checklist, with results recorded and deviations logged. It is not a casual demo, and it is not the moment to discover what the specification should have said.
- Run with your real products or representative samples wherever possible
- Every specification requirement mapped to at least one test step
- Failure modes exercised deliberately: faults, E-stops, power loss, recovery
- Punch list recorded with severity and owner for every deviation
What belongs in a SAT
After installation, the SAT verifies what shipping and site integration could have changed, plus what could only be tested in place: real utilities, real network, real upstream and downstream equipment, real operators. A SAT is normally shorter than the FAT — it re-verifies essentials and site-specific items rather than repeating everything.
Commissioning: the work between the tests
Commissioning covers installation checks, loop and I/O verification, tuning, dry runs, and the gradual introduction of product. Plan it as engineering with prerequisites and exit criteria per phase — not as an undifferentiated block of days that evaporates under startup pressure.
- Pre-power checks: wiring verification, grounding, safety circuits
- I/O and loop checkout against the drawings
- Sequence testing without product, then with product at reduced rate
- Ramp to rate with data recorded, then the formal SAT
Who does what
Buyers who send no one to the FAT, or send someone without authority to accept or reject, forfeit the leverage the FAT exists to provide.
- Builder/integrator: writes test procedures against the spec, runs the tests, fixes deviations
- Buyer: reviews procedures in advance, witnesses the tests, supplies product and operators, signs or refuses sign-off
- Both: agree the punch-list severity scale and what blocks shipment or acceptance versus what gets a dated commitment
Common failure patterns
- FAT run with golden samples only — production variety arrives later, unannounced
- Acceptance criteria negotiated during the test instead of before it
- Punch-list items accepted verbally and never closed
- Commissioning days consumed by work that should have finished before shipment
- SAT skipped entirely because 'it passed FAT' — then site conditions differ
FAT and SAT compared
| Factory acceptance test (FAT) | Site acceptance test (SAT) | |
|---|---|---|
| Where | At the builder's or integrator's facility | On your floor, in its final position |
| Who attends | Your engineering and operations representatives, plus the build team | Site operations, maintenance and the commissioning engineer |
| What it proves | The system meets its specification in a controlled environment | It works connected to real utilities, upstream and downstream equipment, and real product |
| What it does not prove | Anything about site power, networks, integration or real product variation | That the original specification was the right one |
| Typical outcome | A punch list resolved before shipment | Signed acceptance and the start of the support period |
Key takeaways
- FAT before shipment is where problems are cheapest; never waive it to save schedule.
- Test failure modes deliberately — fault recovery is where systems earn trust.
- Punch lists need severity, owners, and dates, or they become folklore.
- SAT re-verifies in real conditions; passing FAT does not make it optional.
Related questions
Can the FAT be witnessed remotely?
Partially, and it is better than nothing: live video works for sequence demonstrations and HMI walkthroughs. But physical product behavior, build quality, and the feel of a machine under fault conditions are hard to judge through a camera. For significant systems, in-person witnessing repays its travel cost.
What happens if the system fails the FAT?
That is the FAT doing its job. Deviations get logged, classified, and fixed at the builder's facility, and critical items trigger a re-test before shipment. A failed FAT at the factory is an inconvenience; the same failures discovered on your floor during startup are a crisis.
Who writes the FAT procedure?
The builder or integrator writes it, because it must trace to the functional specification they are contracted against — but the buyer reviews and approves it in advance. If the procedure skips requirements or tests only sunny-day paths, the review is where you catch that.
How long should commissioning take?
It scales with system complexity, site readiness, and how much was genuinely finished at FAT. What matters more than a universal number is that commissioning days are itemized in the proposal, prerequisites (utilities, product, operators) are met on schedule, and rate ramp-up is data-driven rather than declared.
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