Understanding Factory Acceptance Testing, Site Acceptance Testing, and the role of structured verification in technology project delivery.
When a System Passes Testing but Still Fails on Site
A technology solution may perform correctly in a controlled test environment and encounter unexpected problems after installation.
The software functions as specified, test devices communicate successfully, and the expected workflows produce the correct results.
Yet once deployed, differences in network configuration, device firmware, system permissions, or third-party interfaces can affect its behavior.
This does not necessarily mean the earlier testing was ineffective. It may mean the system is now operating under conditions that were not available during the original tests.
Factory Acceptance Testing (FAT) and Site Acceptance Testing (SAT) address different stages of technical verification.
Understanding these differences helps project teams define realistic acceptance criteria, identify problems at appropriate stages, and manage the transition from engineering development to operational deployment.
What Is Factory Acceptance Testing (FAT)?
Factory Acceptance Testing is a structured verification process conducted before a system or solution is released for delivery or deployment, typically in a vendor-controlled or designated test environment.
Its purpose is to demonstrate that the equipment, software, configuration, or integrated solution satisfies the requirements included within the agreed FAT scope.
For technology and systems integration projects, FAT may verify:
- Software functionality and configuration.
- Supported communication interfaces.
- Event processing and application workflows.
- System responses under defined test conditions.
- Handling of selected errors and abnormal scenarios.
- Compliance with agreed functional acceptance criteria.
Where physical equipment is unavailable, simulators or representative devices may be used, provided their limitations are understood and documented.
A successful FAT establishes evidence of functionality within the tested environment. It does not automatically verify every condition the system will encounter after installation.
What Is Site Acceptance Testing (SAT)?
Site Acceptance Testing verifies the installed system against agreed site acceptance requirements.
Unlike FAT, SAT takes place within the actual deployment environment, where the system interacts with installed equipment, site infrastructure, and operational configurations.
Depending on the project, SAT may include:
- Verification of installed hardware and software configurations.
- Communication with actual field equipment.
- Validation of network connectivity and access permissions.
- Functional testing under site-specific conditions.
- Verification of required alarms, commands, and system responses.
- Confirmation that identified defects have been resolved.
SAT helps establish whether the installed solution meets its applicable acceptance requirements.
For larger multi-vendor projects, separate Site Integration Testing (SIT) may also be required to verify combined system behavior after individual subsystems have completed their acceptance testing.
FAT vs SAT: Key Differences
| Area | Factory Acceptance Testing | Site Acceptance Testing |
|---|---|---|
| Environment | Controlled or representative test environment | Actual installation environment |
| Main objective | Verify agreed functionality before deployment | Verify the installed system against site requirements |
| Equipment | Test equipment, simulators, or representative devices | Installed devices and site infrastructure |
| Dependencies | Defined and controlled where possible | Actual network, interfaces, permissions, and configurations |
| Typical findings | Software defects, logic errors, missing functionality | Installation issues, configuration differences, site-specific faults |
| Acceptance evidence | FAT results and recorded deviations | SAT results and recorded deviations |
The precise boundaries depend on the project specification, contract, and agreed testing strategy.
Practical Example: A Centralized Monitoring Application
Consider a centralized monitoring application designed to receive alarms from several independent subsystems.
During FAT, the engineering team uses representative interfaces to generate predefined alarms and verify that the application processes them correctly.
The test confirms that alarm classifications, event mappings, and response workflows behave according to the approved test procedures.
After deployment, the application connects to the actual site systems.
During SAT, the project team discovers that one subsystem uses a different event configuration from the test environment. Another system has additional access restrictions, while a third generates event sequences that were not represented during FAT.
The application may have passed every agreed factory test, yet additional configuration or engineering work is required before the installed solution satisfies the site’s operational requirements.
This illustrates why successful FAT completion does not eliminate the need for site validation.
Three Practices That Strengthen FAT and SAT
1. Establish Acceptance Criteria Before Testing
Testing should be linked to documented requirements.
Each test should identify the relevant function, necessary prerequisites, actions to perform, expected results, and evidence required for acceptance.
Without clear criteria, test execution can become subjective, particularly when multiple vendors or contractors participate.
2. Record Deviations and Retest Results
A failed test should produce an identifiable record of the issue, its impact, assigned responsibility, and corrective action.
After resolution, the affected functionality should be retested, with related scenarios reconsidered where necessary.
This creates traceability between identified problems and their eventual closure.
3. Control Changes Between FAT and SAT
Software versions, configurations, interface definitions, and other relevant changes should be documented after FAT.
Otherwise, the version deployed at site may differ from the version previously tested, making acceptance evidence difficult to interpret.
Where changes affect validated functionality, appropriate regression testing should be considered.
FAT, SAT, and Commissioning Are Related but Different
Acceptance testing supports project delivery, but it should not be confused with the entire commissioning process.
Commissioning may include installation verification, system configuration, interface checks, operational preparation, training, documentation, and handover activities.
FAT and SAT provide formal verification at defined stages of that wider process.
The appropriate testing sequence should be established according to the project scope, technical risks, and contractual requirements.
Better Testing Begins with Better Preparation
FAT and SAT become more effective when technical requirements, interface responsibilities, test environments, and acceptance criteria are established before execution begins.
The objective is not to demonstrate that a system can pass a checklist under ideal conditions.
It is to generate meaningful evidence that the solution satisfies its defined requirements and is ready to progress toward operational use.
At Core Solutions & Technologies (CST), our Projects & Services division supports OEMs, system integrators, and technology partners through integration testing, FAT, SAT, troubleshooting, commissioning assistance, and technical project delivery.
Our engineering approach emphasizes clear requirements, structured validation, and practical resolution of technical issues.
Preparing for FAT, SAT, or technology project commissioning?
Explore CST’s Technology Project Delivery and Specialist Engineering Services capabilities, or contact our team to discuss your testing and commissioning requirements.
About CST Pulse
CST Pulse is the engineering and technology insight platform of Core Solutions & Technologies, covering practical ICT & ELV and Engineering Projects topics for OEMs, system integrators, technology partners and end customers across Middle East & Africa.
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