Factory Acceptance Testing and Site Acceptance Testing exist to catch the difference between a system that was designed correctly and a system that was built correctly. Skipping either one shifts that discovery to the day something fails.
FAT verifies the system before it leaves the integrator’s premises: does the configured hardware and software actually do what the specification says, under controlled conditions, with every party able to inspect and challenge the result before it is shipped and installed.
SAT verifies the same functionality once installed on site, under real conditions — real network latency, real environmental conditions, real interfaces with other building systems — and confirms it still performs against the accepted specification.
A rigorous SAT should exercise failure modes deliberately: pull a network cable and confirm failover behaviour, simulate a power interruption and confirm recovery, and trigger alarms during a monitored shift to confirm operators actually see and can act on them — not just that the system technically registered the event.