FAT / SAT protocols: why two commissionings cost less than one

Clients new to modular sometimes push back on our standard two-stage commissioning. "Why test it twice? Can we skip the factory test and just commission on site?" The short answer is that you can, and you will lose two to six weeks the first time something fails. The long answer is what this article is for.

The proposition

A modular building is commissioned once at the factory (Factory Acceptance Testing, or FAT) and once on site (Site Acceptance Testing, or SAT). The FAT is the first date on the building's calendar where the contractor says "this is done" and the client says "I agree, you are done with this part". The SAT is the same agreement after shipment, assembly and utility connection — the handover.

Two commissionings cost roughly 25% more in testing hours than one. They save, in the median project, two to six weeks of schedule risk and 3–8% of contract value in rework costs. That is because things found at FAT are fixed in the factory, with the tool, the spare part and the welder at hand. The same thing found at SAT is fixed on site, with a van, a courier and an overtime bill.

What a FAT actually catches

The FAT, for us, is a 3-to-5-day exercise involving the contractor's commissioning team, the client's commissioning representative, and ideally a third-party witness for certified applications (hospitals, defence). It happens with every module assembled in the factory yard, utilities temporarily connected, and every system energised.

The FAT catches, in roughly this order of frequency: cable mis-termination (a dimmable circuit wired to a non-dimmable driver, or similar), HVAC airflow out of balance, medical gas leaks, incorrect door swing, wrong fixture finish, missing firestopping, wrong label, cabinet door alignment, and software-configuration errors on BMS and nurse-call panels. Those are the mundane ones. On 5–10% of projects, the FAT catches something structural — a weld that failed UT, a joint tolerance out of spec, a panel with delamination. Those are the ones that would have been very expensive to fix on site.

What an SAT can only catch

The SAT exists because not everything can be tested in the factory. You cannot simulate the actual site utility supply — the real voltage, the real water pressure, the real groundwater table. You cannot simulate interaction with the surrounding building or the surrounding terrain. You cannot simulate the actual network on the IT side. So the SAT is where those site-specific interactions get verified.

In practice, 90% of SAT findings are utility or interface issues: wrong pressure on the incoming water, a phase reversed on the incoming power, a dust infiltration issue because the site is sandier than the factory, a roof drainage problem because the site slopes the wrong way. These things cannot be caught at FAT. That is why the SAT is not redundant — it is the only test that can catch them.

The 142-point protocol

Our standard FAT protocol runs to 142 numbered checks, grouped into 14 sections: structural verification (12 points), envelope (9), electrical (18), HVAC (22), plumbing (14), medical gas (11, where applicable), fire safety (10), security (6), IT/BMS (17), interior finishes (8), casework and equipment (7), accessibility (4), labelling and signage (3), documentation handover (1, but with sub-items for every certificate). Each point has a pass/fail criterion, a measurement method and a tolerance. The result is a signed document that lives with the unit forever and is the legal basis for shipment.

The SAT protocol is shorter — around 60 points — because it only re-runs the site-sensitive items plus a reduced sample of the factory items to confirm the unit survived shipment. It is still signed. Both FAT and SAT documents are delivered to the client at handover as part of the project dossier.

Writing FAT/SAT into your contract

If your contract is silent on FAT and SAT, you are relying on the manufacturer's goodwill. Some manufacturers will do both exhaustively. Some will call a two-hour walk-around a FAT. The only way to lock in the level of rigour you need is to write the protocol into the contract: who attends, who signs, what the pass/fail criteria are, and what happens when a point fails (fix-and-retest, or conditional sign-off with a punch list and a deadline).

Our standard contract language makes FAT a condition of shipment and SAT a condition of final payment. We think that is the right shape for any serious modular project. It aligns the incentives: the manufacturer does not get paid until the unit is verified twice, and the client does not get to move the goalposts after the fact. Two commissionings, one shared interest in getting it right.