FAT vs. SAT: What’s the Difference and Why Both Matter in Industrial Automation
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Introduction
In the world of industrial automation, process control, and system integration, successful project delivery hinges on rigorous testing. Two crucial milestones in any automation or instrumentation project are the FAT (Factory Acceptance Test) and the SAT (Site Acceptance Test).
Despite their similar names, FAT and SAT serve distinct but complementary roles, ensuring systems meet technical specifications and function reliably in real-world operating environments.
In this blog, grounded in 30+ years of industry expertise, we’ll explore the differences between FAT and SAT, their importance, what’s typically included in each, and best practices to ensure a successful project handover.
What Is FAT (Factory Acceptance Test)?
🏭 Definition:
The Factory Acceptance Test (FAT) is a pre-delivery test conducted at the manufacturer’s or system integrator’s facility to verify that a system or equipment meets all contractual specifications and functions correctly before it is shipped to the site.
🎯 Main Purpose:
- Identify design or functionality issues early (before shipping)
- Reduce commissioning time and costs
- Ensure the system complies with functional requirements
✅ What’s Included in FAT?
| FAT Components | Details |
|---|---|
| Functional testing | PLC logic, HMI screens, control loops, alarms |
| I/O simulation | Simulated input/output verification |
| Communication tests | Network protocols (Modbus, Profinet, OPC UA, etc.) |
| Hardware verification | Panel inspection, power supply checks, wiring verification |
| Documentation review | P&IDs, loop diagrams, wiring schedules, software backups |
| Customer witness test | End-user or consultant may observe and sign off |
📍 Location:
- Vendor or integrator facility
What Is SAT (Site Acceptance Test)?
🏗️ Definition:
The Site Acceptance Test (SAT) is performed after the system is installed at the site. It confirms that the system operates correctly within the actual operating environment, often after mechanical and electrical integration with other systems.
🎯 Main Purpose:
- Validate system integration with field devices, utilities, and infrastructure
- Ensure proper installation, configuration, and communication
- Test under real process conditions
✅ What’s Included in SAT?
| SAT Components | Details |
|---|---|
| Power-up and installation verification | Cable termination, grounding, enclosure sealing |
| Live I/O testing | Connection to actual sensors, actuators, MCCs |
| Loop checks | End-to-end loop verification from sensor to control output |
| Network and system integration tests | Communication with DCS, SCADA, or higher-level systems |
| Functional and safety testing | Emergency stop, interlocks, alarms, shutdown sequences |
| Operator and maintenance training | Hands-on usage training for end-users |
📍 Location:
- Project site or plant
FAT vs. SAT – Key Differences at a Glance
| Aspect | FAT (Factory Acceptance Test) | SAT (Site Acceptance Test) |
|---|---|---|
| Purpose | Validate design and functionality before delivery | Validate performance and integration on site |
| Location | Vendor or integrator’s facility | Customer or end-user facility |
| Test Environment | Simulated/simulated I/O | Real-world/live I/O |
| Scope | Software logic, panel layout, function test | Installation checks, loop tests, integration |
| Timing | Before shipment | After installation and commissioning |
| Stakeholders | Engineers, integrator, vendor | End-users, site engineers, commissioning team |
| Cost-saving benefit | Detect issues early, reduce shipping of faulty system | Avoid downtime, ensure smooth startup |
Why Both FAT and SAT Are Important
🔍 Skipping FAT?
You risk shipping an untested or non-functional system, leading to delays, increased costs, or even project failure.
🔍 Skipping SAT?
You miss the chance to verify system behavior under actual site conditions, including signal integrity, power supply stability, grounding, and EMI interference.
🎯 FAT ensures your system is built right. SAT ensures it works right on-site.
Typical Workflow in a Control System Project
Engineering Design → Panel Fabrication → FAT → Shipping → Installation → SAT → Commissioning → Handover
Real-World Example: FAT and SAT in a Water Treatment Plant
🧪 FAT:
- Control panels tested with simulated water level sensors and pumps
- All PLC logic validated using test switches and indicators
- Operator reviewed HMI navigation and alarm conditions
🛠️ SAT:
- Panel installed in the pump house
- Actual level transmitters connected to I/O cards
- Loop tested from sensor → PLC → HMI → actuator
- Commissioned auto-start sequence based on water demand
🧠 Outcome: The system was commissioned ahead of schedule with zero rework, thanks to thorough FAT and SAT.
FAT & SAT Best Practices
📋 For FAT:
- Provide the client with a FAT checklist in advance
- Simulate real-world process conditions as closely as possible
- Backup all control logic and HMI applications
- Document all deviations and agreed-upon changes
📋 For SAT:
- Ensure all wiring and mechanical work is complete before starting
- Involve plant operations personnel for training and familiarization
- Record as-found and as-left conditions in SAT documentation
- Test safety systems and ensure redundancy (if applicable)
Common Mistakes to Avoid
| Mistake | Impact |
|---|---|
| Skipping simulated I/O during FAT | Misses logic errors before field wiring |
| Poor documentation after FAT | Creates confusion during installation |
| Incomplete loop checks during SAT | Leads to startup issues and commissioning delays |
| Unclear responsibility ownership | Causes communication gaps between vendors and contractors |
| No post-test summary report | Loss of test traceability and audit trail |
Who Should Witness FAT and SAT?
Depending on the project size and criticality, the following stakeholders may be involved:
| FAT Witnesses | SAT Witnesses |
|---|---|
| System Integrator | Site Electrical Engineer |
| Client Project Manager | Plant Operations Staff |
| Control System Engineer | Instrumentation Technicians |
| Third-Party Inspector (optional) | Commissioning Manager |
Regulatory and Industry Standards
In regulated industries like pharmaceuticals, oil & gas, and energy, FAT and SAT may be mandated by:
- IEC 61511 – Functional safety for SIS
- ISA-88 / ISA-95 – Batch control and MES integration
- GAMP 5 – Validation guidelines for automation systems
- ISO 9001 – Quality management system
Conclusion
Both FAT and SAT are essential checkpoints in ensuring that your automation or instrumentation project is safe, compliant, and ready for full operation. While FAT verifies that the system is built to specification, SAT validates its functionality within its real operating environment.
✅ Key Takeaways:
- FAT is a pre-installation test that simulates real-world behavior.
- SAT is an on-site validation under live conditions with actual hardware.
- Both reduce project risk, ensure compliance, and improve system reliability.
- Proper planning, documentation, and stakeholder involvement are critical.