What is a FAT (Factory Acceptance Test)?
A FAT (Factory Acceptance Test) is a structured, documented test at the supplier’s or system integrator’s premises in which the customer verifies, before delivery, that an installation, machine or control system meets the agreed specifications, drawings and functional requirements. Faults found during the FAT can be fixed quickly in the workshop, with the right people and tools close at hand. On site, with a running plant and a tight start-up schedule, the same problem costs many times more in time and money. The FAT is followed by the SAT (Site Acceptance Test), which tests the system in its final environment.
🧠 What is tested during a FAT?
A FAT covers everything that is already present at the supplier’s factory: the control cabinets, the software and the interfaces between them. Field equipment such as sensors, valves and motors is usually not yet in place and is therefore simulated. Typical elements:
- Hardware and wiring - visual inspection of cabinets, components, labelling, earthing and power supplies, plus a point-to-point check of the wiring to PLC cards and drives
- I/O test - every input and output channel is forced or simulated and checked all the way to the operator screen, including scaling, engineering units and alarm limits
- Functional test - the control logic is worked through step by step against the Functional Design Specification (FDS): start-up, normal operation, shutdown and fault scenarios
- Operation and visualisation - HMI and SCADA screens, navigation, alarm handling and trends
- Communication - interfaces via OPC UA, Modbus or fieldbuses, and the behaviour on loss of communication
- Performance and redundancy - cycle times, controller and network load, and switchover of redundant controllers, power supplies or servers
A FAT can be carried out physically with real hardware, or (partly) virtually against a simulation model. With virtual commissioning, software acceptance can take place before the machine has even been built.
🔄 What is the difference between FAT, SAT and SIT?
The international standard IEC 62381 describes the test phases for automation systems in the process industry. Edition 3.0 was published on 30 July 2024 and added an optional Factory Integration Test (FIT). It replaces the old forms with checklists that let the owner, buyer and vendor agree the scope of testing and their responsibilities in advance.
| Test phase | Where | What is demonstrated | Field equipment |
|---|---|---|---|
| FAT | Supplier’s factory | System meets its specification | Simulated |
| FIT (optional) | Factory or test facility | Interaction with other vendors’ systems | Largely simulated |
| SAT | Customer’s site | System works after transport and installation | Connected |
| SIT | Customer’s site | Integration with the rest of the plant and higher-level systems | Connected, process conditions |
A successful FAT is therefore not final acceptance. Transport damage, field wiring, earthing and real process conditions are only covered at the SAT and SIT. In contracts, however, the FAT is often a milestone tied to a payment instalment or to release for shipment.
🔧 How does a FAT work, step by step?
- Test plan and test procedures - the supplier writes a FAT protocol that is traceable to the requirements (User Requirements, FDS, I/O list, drawings). The customer approves the protocol before the test, not during it.
- Internal pre-FAT - the supplier first tests everything itself. A FAT in which the customer is the first tester almost always overruns.
- Readiness check - frozen software version, up-to-date drawings, calibrated test equipment, simulation ready and the right people available.
- Execution with the customer - each test case is run, the result recorded and signed off by both parties.
- Recording deviations - every finding goes onto a punch list, usually categorised as blocking shipment, to be resolved before the SAT, or administrative.
- Retest and release - resolved items are retested. Then the FAT report is issued and the system is formally released for shipment. That report is the traceable evidence of what was tested and accepted.
Freeze the configuration. After the FAT, every change to software or hardware counts as a change under change management. Otherwise you test a different version at the SAT from the one the customer accepted. Record the exact project version (for example the TIA Portal project version and firmware versions) in the FAT report as well.
🏭 Where is a FAT used?
| Application | Typical FAT focus |
|---|---|
| Machines and packaging lines | Function, cycle time, format changes, emergency stop |
| Skids and process modules | I/O, control loops, interface to the DCS or Module Type Package |
| DCS and SCADA systems | Graphic displays, alarms, historian, redundancy |
| Safety PLC and SIS | Safety functions, cause-and-effect matrix, bypasses |
| OT network components | Switches, firewall rules, segmentation |
| Migrations in existing plants | Equivalence of old and new, cut-over plan (brownfield) |
In the pharmaceutical industry the FAT is part of validation under GAMP 5. The second edition from 2022 encourages you to leverage well-documented supplier testing instead of repeating it on site, provided the quality of that testing has been assessed. For safety-related systems, clause 13 of IEC 61511 covers the FAT of the logic solver and application software. The standard recommends this FAT rather than mandating it; validation of the complete safety function happens later on site.
🔐 How do you include cybersecurity in the FAT?
The FAT is the last opportunity to have security weaknesses fixed before the system is in your production environment. That is why cybersecurity belongs in the test protocol, not in a separate review afterwards:
- Access - check access control, roles, personal accounts and the removal of default passwords
- Hardening - unused ports, services and USB access disabled according to the hardening plan
- Network - Firewall rules and segmentation as designed, checked with vulnerability scanning in a test set-up
- Logging - Audit and logging functions are enabled and record logins and changes
- Recovery - a full backup and a tested restore of controllers and servers
- Documentation - an asset inventory with firmware and software versions
Ask your supplier to show how it meets IEC 62443-2-4 (requirements for service providers) and which system requirements from IEC 62443-3-3 have been implemented. A preliminary security assessment can also be carried out during the FAT, which is recommended for OT systems. This turns Security by Design into something testable rather than a promise.
⚙️ What role does functional safety play?
Besides security, the FAT tests the safe behaviour of the system. Think of fail-safe behaviour on an emergency stop, power failure or loss of communication: do the outputs go to the safe state, and does the system stay off rather than restart by itself? A FAT does not, however, replace the conformity assessment for CE marking. Machine builders still complete that under the Machinery Directive, which will be replaced by the Machinery Regulation from 20 January 2027.
❓ Frequently asked questions
Is a FAT mandatory?
In most cases a FAT is not a legal obligation but a contractual agreement between customer and supplier. Standards such as IEC 62381 and IEC 61511 describe the FAT as good practice. In regulated sectors such as pharmaceuticals, the FAT is in practice a standard part of validation.
Who carries out the FAT?
The supplier or system integrator carries out the FAT and provides the people, the test set-up and the simulation. The customer attends as a witness and signs off the results, often joined by the operations and maintenance organisation and sometimes an independent inspector.
How long does a FAT take?
The duration of a FAT depends on the scope: a small machine can be tested in one or two days, while a large DCS project may take several weeks. A thorough internal pre-FAT and a test protocol approved in advance are the best ways to keep the FAT short.
What happens if the FAT fails?
When a FAT fails, the findings go onto a punch list and the supplier resolves them. Blocking items are retested before the system may be shipped. Minor items can, by agreement, be carried over to the SAT.
Can a FAT be done remotely or virtually?
A FAT can partly be witnessed remotely via video and screen sharing, which has become more common since 2020. A virtual FAT against a simulation model or digital twin mainly tests the software. Physical checks of cabinets and wiring usually still require someone on site.
What is the difference between a FAT and a SAT?
The FAT tests the system at the supplier’s premises with simulated field equipment, while the SAT tests it at the final location with the real installation. The FAT demonstrates that the system meets its specification; the SAT demonstrates that it works in practice after transport and installation.
📌 In summary
A FAT shows the customer, before delivery, that a machine or control system does what was agreed. Hardware, I/O, logic, operation, communication and cybersecurity are tested against a protocol approved in advance, with simulated field equipment. IEC 62381:2024 provides the structure for FAT, SAT and SIT. A strict punch list, a frozen configuration and a clean handover to the SAT make the difference.
A FAT finds faults in the workshop rather than on site, and is the last opportunity to enforce functionality, safety and security before delivery.