Control system projects often include two similar-sounding tests that serve very different purposes. Factory acceptance testing and site acceptance testing both appear on almost every automation project schedule. Yet many plant teams are unsure exactly where one test ends and the other begins. This article will clarify the difference between FAT vs SAT, explaining how each step reduces project risk, validates system performance, and helps teams catch issues before full operation. Understanding FAT and SAT specifically helps decision-makers negotiate better contracts and avoid budget surprises.
Understanding FAT vs SAT
A Factory Acceptance Test, or FAT, is a structured test performed at the equipment manufacturer’s facility. Its purpose is to confirm that the system works exactly as specified before it ever leaves the factory.
A Site Acceptance Test, or SAT, is a similar structured test performed once the equipment is installed at the plant. Its purpose is to confirm the system still works correctly after connection to the real plant environment.
Both tests exist to catch problems before they become expensive. The earlier you find a defect, the cheaper and easier it is to fix. For a plant manager, project risk simply means paying more, waiting longer, or getting a system that underperforms.
FAT and SAT reduce that risk at two different points in the project: before shipment and after installation. Waiving a factory acceptance test on a complex system raises the risk of finding problems after shipment, when fixes cost far more. Together, FAT and SAT create two separate opportunities to catch issues before startup. With these two purposes in mind, the sections below explain what happens during each test and how they differ.
Where FAT and SAT Fit in the Project Timeline
Design and engineering come first, followed by manufacturing and software configuration at the vendor’s facility. FAT happens once that configuration is complete, before the vendor packs the equipment for shipment. After shipment, the equipment is installed, wired, and connected to the plant’s own power and network.
SAT happens immediately after installation is complete, right before commissioning and startup begin. Seeing both tests on a single timeline helps teams plan staffing and avoid schedule surprises.
What Happens During a Factory Acceptance Test
A factory acceptance test takes place at the vendor’s facility, before anything ships to the plant. The goal is to verify the system operates as expected before it ships to site, while problems are still cheap to fix. Plant representatives are often invited to attend and observe testing firsthand. Attending in person lets plant staff raise concerns immediately, rather than discovering them weeks later on site. It also creates a signed record that the vendor met the agreed specifications before shipment. A typical FAT covers the following:
- Control logic and configuration software tested against approved specifications
- Alarms and safety interlocks triggered under simulated fault conditions
- Operator screens, trends, and reports checked for accuracy
- Communication between the controller and other connected systems
Issues found during a FAT go onto a punch list and get corrected before shipment. Common findings at this stage include the following:
- Wiring or configuration errors caught before they reach the plant
- Â Software settings that do not match the approved design
- Missing or incomplete documentation needed for the plant’s records
What Happens During a Site Acceptance Test
A site acceptance test happens after installation, using the real hardware in its permanent location. A factory environment is controlled, but a plant site brings real temperature swings, vibration, and electrical noise. These conditions can reveal problems that never appeared during the factory acceptance test.
Industry guidance describes SAT as a simpler protocol confirming that there was no damage to the equipment during transport and that all components remain intact. SAT also verifies the system performs correctly once connected to real field wiring and real plant conditions. Testing a backup controller means deliberately failing the primary one and confirming the backup takes over smoothly. A typical SAT covers the following:
- Installed hardware matching the approved system drawings
- Power and grounding checks completed without fault
- Redundant systems, such as backup controllers, failing over correctly when tested
- Communication networks and operator workstations performing as specified on site
Common findings at this stage include the following:
- Wiring damaged or altered during shipping and installation
- Grounding or power issues specific to the site’s electrical system
- Integration problems with existing plant systems not present at the factory
- Passing a SAT is usually the final step before clearing the plant for startup.
Key Differences Between FAT vs SAT
Although the two tests share many checklist items, several differences matter for planning purposes.
| Aspect | FAT (Factory Acceptance Test) | SAT (Site Acceptance Test) |
| Location | At the vendor’s facility | At the plant site |
| Timing | Before shipment | After installation |
| Purpose | Confirms the system meets its design | Confirms it works once installed |
| Test Environment | Controlled factory conditions | Real plant conditions, including temperature and vibration |
| Attendance | Often select representatives | Usually broader plant staff |
| What Is Tested | Control logic, software, alarms, and displays | Installed hardware, wiring, grounding, and redundancy |
| Common Findings | Software errors and documentation gaps | Shipping damage and site wiring issues |
| Failure Response | Issues logged on a punch list before shipment | Deviation investigated and corrected before startup |
| Outcome | Clears equipment to ship | Clears the plant to start up |
Why Skipping Either Test Increases Risk
Some project teams try to save time by combining or skipping one of the two tests. Skipping FAT on complex equipment raises the risk of discovering serious defects only after installation. A failed SAT does not usually cancel the earlier FAT, but it still requires investigation and correction. Either gap can delay startup and add unplanned cost to a project.
What Decision Makers Should Ask For
Plant leaders do not need to run these tests themselves, but they should take the following steps before signing a contract.
- Request a formal FAT and SAT protocol before signing the contract.
- Confirm who attends each test and who has authority to sign off.
- Define what happens if either test fails, including who covers the fix.
- Reserve scheduled time for corrections between FAT and SAT.
Every test also needs clear acceptance criteria, meaning agreed rules for a pass or a fail. Asking these questions early protects both the project budget and the project timeline.
Bringing FAT and SAT Together
Factory Acceptance Testing and Site Acceptance Testing are not competing steps but complementary checkpoints in every control system project. FAT confirms the system was built correctly, while SAT confirms it was installed correctly on site. Together, they protect your project from costly surprises, schedule delays, and responsibility disputes.
At Petrotech, we plan and execute both types of testing for control system projects across the process industries. Our team knows exactly what to test, document, and ask for you. Contact us today to plan your next FAT and SAT.