A factory acceptance test with a real script removes most commissioning risk before shipment. But some things can only be proven on site — and pretending otherwise inflates both tests.
A factory acceptance test is the last moment a control system can be corrected cheaply. On our floor, a fault is a wiring change and a retest the same afternoon. On your site it is travel, downtime and an audience. So the purpose of a FAT is simple: leave nothing discoverable in the workshop to be discovered in the field.
A real FAT runs from a script, not a demonstration. Every I/O point exercised and confirmed against the EPLAN documentation, every interlock deliberately violated to prove it trips, every alarm forced and its message checked, every operating sequence stepped through on the HMI. The customer signs each line. "We powered it up and it looked fine" is not a FAT; it is a photo opportunity.
Simulation extends how far the workshop test can reach. With the machine simulated against the PLC, we run sequences, fault reactions and recovery scenarios that would be slow or risky to stage on real equipment. A well-simulated FAT means the logic arrives on site already exercised, and commissioning becomes verification rather than debugging.
Be equally clear about what a FAT cannot prove. It cannot prove the field wiring, the sensor mounted two millimetres off, the network behaviour on the plant backbone, or the mechanical reality of a machine under load. These belong to the site acceptance test, and no workshop rehearsal replaces them. A FAT that claims to make the SAT a formality is overselling.
The paperwork is half the value. A signed FAT protocol with the punch list and its agreed deadlines is what turns "we saw some issues" into a managed closure. We ship the cabinet with the protocol, and the SAT begins by retesting only the punch items plus the site-dependent scope — not by repeating three days of workshop testing at field rates.
For buyers, one practical demand: require the FAT script with the quotation, not after the build. The script tells you what the supplier intends to prove, and its thinness or thoroughness tells you what kind of commissioning you are about to fund.
— GANI Engineering engineering team