In a DCS (Distributed Control System) project, successful commissioning does not depend only on drawings, configuration documents, or manufacturer specifications.
A system must be tested and verified step by step before it can be safely put into operation.
In a complete DCS project lifecycle, FAT, SAT, and SIT are three critical testing stages. Many people simply call them “acceptance tests”, but engineers who have worked on real projects know that these three tests have different purposes.
Simply explained:
- FAT (Factory Acceptance Test): Verify the system before shipment from the manufacturer.
- SAT (Site Acceptance Test): Verify the system after installation at the customer’s site.
- SIT (Site Integration Test): Verify communication and cooperation between DCS and other systems.
These three stages work together to move a DCS system from “designed and built” to “ready for operation.”
1. FAT: Factory Acceptance Test – Finding Problems Before Shipment
FAT (Factory Acceptance Test) is performed at the manufacturer’s factory or system integration facility before the equipment is delivered to the site.
The purpose of FAT is to confirm that the DCS system meets the requirements defined in:
- Contract documents
- Technical specifications
- Engineering design documents
- Approved testing procedures
FAT is not simply checking whether the equipment is assembled correctly. It is a detailed verification of both hardware and software functions.
Typical FAT activities include:
Hardware Verification
- DCS cabinet inspection
- Controller and I/O module verification
- Power supply checking
- Redundancy test
- Communication module inspection
Software and Configuration Verification
- Control logic verification
- HMI graphic checking
- Alarm configuration testing
- Trend function verification
- Sequence control testing
- Communication testing
For example, engineers will verify:
- Are the correct I/O modules installed?
- Are all required signals configured?
- Are alarm limits correct?
- Are interlock logic functions implemented according to the design?
A well-performed FAT can prevent many problems before shipment.
When equipment is still at the manufacturer’s facility, it is much easier to modify hardware, correct configuration errors, or update control logic.
Once the system arrives at the site, any major modification may affect installation schedules and commissioning progress.
Therefore, FAT should not be treated as a formal procedure only. The customer, system integrator, engineering company, and third-party inspectors may participate in FAT activities.
All identified issues should be recorded, corrected, and tracked until closure.
2. SAT: Site Acceptance Test – Verifying the Installed System
SAT (Site Acceptance Test) is performed after the DCS equipment arrives at the customer’s site and installation work has been completed.
At this stage, engineers verify whether the system can operate correctly under actual site conditions.
A common question is:
“If FAT has already been completed, why do we still need SAT?”
The reason is simple:
The factory environment and the real plant environment are different.
During transportation, installation, and wiring, many issues may occur.
SAT typically includes:
Installation Verification
- Cabinet installation inspection
- Cable termination checking
- Power supply verification
- Grounding inspection
- Network connection checking
System Function Testing
- I/O channel verification
- Signal loop checking
- Operator station testing
- Alarm display verification
- User access permission checking
For DCS systems with advanced control functions, SAT may also include verification of:
- Control loops
- Sequence functions
- Interlock functions
- Field signal responses
Especially for safety-related functions, engineers should not only check logic diagrams or software configurations.
The actual field signals and operating conditions must be verified.
After successful SAT completion, the DCS system is ready for:
- Loop checking
- Equipment commissioning
- Functional testing
- Plant startup preparation
3. SIT: Site Integration Test – Checking Communication Between Systems
SIT (Site Integration Test) focuses on the interaction between the DCS and other plant systems.
Unlike FAT and SAT, SIT verifies whether different systems can communicate and work together correctly.
Modern industrial plants usually include multiple control systems, such as:
- SIS (Safety Instrumented System)
- ESD system
- PLC system
- MCC system
- Electrical monitoring system
- Fire & Gas system
- Compressor control system
- Boiler control system
- MES system
- Historian database
During SIT, engineers verify more than simple communication.
They check:
- Data exchange accuracy
- Signal consistency
- Status feedback
- Alarm transmission
- Interlock actions
- System response during abnormal conditions
Many problems only appear during system integration.
For example:
- Different tag names between systems
- Incorrect signal direction
- Wrong status feedback
- Communication failure without alarm indication
- Different interpretations of the same interlock condition between DCS and SIS
These problems may not be discovered when each system is tested separately.
That is why SIT is a critical step before plant startup.
FAT vs SAT vs SIT Comparison
| Test | Location | Main Purpose | Typical Checks |
|---|---|---|---|
| FAT | Manufacturer factory | Verify system before shipment | Hardware, software, logic, HMI, alarms, communication |
| SAT | Customer site | Verify installed system operation | Installation, wiring, power supply, grounding, I/O, basic functions |
| SIT | Customer site | Verify system integration | Interfaces, communication, alarms, interlocks, data exchange |
Common Problems Found During Different Testing Stages
Problems Found During FAT
- Incorrect hardware configuration
- Missing I/O points
- Incorrect control logic
- Incomplete HMI graphics
- Wrong alarm settings
Problems Found During SAT
- Wiring errors
- Power supply problems
- Grounding issues
- Network configuration problems
- Incorrect field signals
Problems Found During SIT
- Communication mismatch
- Incorrect signal mapping
- Interlock sequence problems
- Data inconsistency between systems
DCS Testing Sequence Before Startup
A typical DCS project follows this sequence:
Engineering Design
↓
FAT (Factory Acceptance Test)
↓
Transportation & Installation
↓
SAT (Site Acceptance Test)
↓
SIT (Site Integration Test)
↓
Loop Check
↓
Commissioning
↓
Plant Startup
Related Standards
For process automation projects, FAT, SAT, and SIT activities can refer to:
- IEC 62381 – Automation systems in the process industry: Factory acceptance test (FAT), site acceptance test (SAT), and site integration test (SIT)
- Project-specific testing procedures
- Customer engineering specifications
These standards provide guidance for planning, executing, and documenting automation system acceptance testing.
Conclusion
A DCS project is not complete simply because the cabinets have arrived at the plant and the system can be powered on.
Each testing stage has a different purpose:
- FAT finds problems before shipment.
- SAT verifies installation and basic operation at the site.
- SIT ensures that DCS and other systems work together correctly.
A well-executed FAT, SAT, and SIT process can significantly reduce commissioning risks, avoid unexpected delays, and improve the reliability of the final automation system.
In industrial projects, the fastest way to complete commissioning is not to reduce testing — it is to discover and solve problems as early as possible.