FAT vs SAT: what is the difference between a factory acceptance test and a site acceptance test?
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The two tests sit at opposite ends of the delivery chain and commit the buyer to different things when signed. International practice treats them as distinct, defined acceptance events, and IEC/ISA 62381 covers both. Yet purchase contracts routinely collapse them into a single vague “acceptance test”, and that is where disputes start. This guide covers what each test verifies, what each signature triggers, and why the gap between them is where acceptance problems live. For the factory test itself in depth, see the companion guide What is a factory acceptance test for industrial equipment?
What a FAT covers, and where it happens
The FAT takes place on the supplier’s factory floor, once manufacturing and internal QC are complete and before anything is packed for transport. It runs against an approved FAT procedure with defined test steps, acceptance criteria and sign-off rules, agreed before the test date. If the factory has already run the tests before the witness arrives, or the unit is already packed, the witnessing evidence is weakened and some checks are blocked entirely. Sinospect’s factory acceptance test checklist sets out those steps: readiness, identity, functional witnessing, non-conformities and the release decision, section by section.
A properly scoped FAT verifies three layers:
Build. Dimensional checks against approved drawings, including interface points: flange ratings, facings and drilling patterns checked against the approved interface drawings, not against whichever revision the workshop happened to build from. Materials verified against certificates, nameplates, tag numbers, completeness against the bill of materials, surface treatment and preservation readiness.
Function. Dry runs and, where practical, load runs; control sequences, cycle times, alarms, emergency stops and safety interlocks proven against the project control philosophy, not just the supplier’s standard demonstration; electrical panel inspection against schematics, including control voltages. On one packaged skid, the main motor data can match the purchase order while an auxiliary control panel has been built for the wrong site control voltage, invisible during the factory run because the supplier tested through a temporary transformer. At site it stops energisation, forces panel rework and delays commissioning even though the equipment appeared to run in the factory.
Documentation. Manuals, calibration certificates, material certificates, as-built wiring drawings, lifting instructions and spare parts lists reviewed against the contractual documentation list. An equipment package can be technically present and still not be ready for site: missing as-built drawings or calibration records do not stop a factory demonstration, but they stop installation, energisation or handover.
A FAT is also the moment to confirm something no factory run answers: whether the package can be installed, operated and maintained in the position shown on the project layout. A filter housing or junction box that will face a wall or an adjacent package at site is a real finding, caught by checking the general arrangement drawing, and it becomes very expensive to discover after the equipment is grouted in.
Conditions at FAT are the factory’s: stable utilities, substitute media, simulated signals, often partial load. That is the FAT’s limitation, and also its value: any fault found is corrected by the people who built the machine, on their own premises, at their own cost, before the shipment payment moves. It is the last point in the procurement where the buyer holds full leverage: the equipment is still on the supplier’s floor, and a punch list still blocks the container.
What a SAT covers, and where it happens
The SAT takes place at the buyer’s site, after the equipment has been delivered, installed, connected and commissioned. Where the FAT asked whether it was built to specification, the SAT asks whether it performs to specification here.
A serious SAT checklist covers:
- Delivered condition: transport damage, serial-number match against the FAT report, preservation state on arrival.
- Installation: foundation, anchoring, levelling, alignment, reassembly of items removed for transport, guarding reinstated.
- Utilities as found: actual voltage, frequency and power quality at the terminals; compressed air, cooling water, drainage; not the datasheet values, the values available on the day.
- Function as installed: the sequences, interlocks and alarms proven at FAT, re-run with site utilities and field wiring.
- Performance: runs with the real process medium at contractual capacity, sustained over an agreed duration.
- Integration: signal exchange with upstream and downstream equipment and the plant control system, safety circuits proven across vendor boundaries.
- Handover: operator and maintenance training delivered, manuals on site, commissioning spares received.
Nothing here is controlled: site power may sag, ambient heat, humidity and dust differ from the factory floor, and the product behaves differently from the test medium. That is the point: the SAT is the only test run under the conditions the equipment will live in.
Its signature is the heaviest in the chain. SAT acceptance typically releases final payment, starts the warranty period and transfers care, custody and control to the buyer. Suppliers push for it early; buyers should not sign against an open punch list: every item left open converts from a supplier obligation into a buyer negotiation.
FAT vs SAT at a glance
| Factory acceptance test (FAT) | Site acceptance test (SAT) | |
|---|---|---|
| Location | Supplier’s factory | Buyer’s project site |
| Timing | After manufacture, before packing and shipment | After delivery, installation and commissioning |
| Conditions | Controlled: factory utilities, substitute media, simulated signals | Real: site power, real process media, live interfaces, site ambient |
| Who attends | Buyer’s engineer or appointed witness; supplier QC and project engineer | Buyer’s engineering and operations team; supplier’s commissioning engineer; other vendors where interfaces are tested |
| What is tested | Build against drawings and specification; function under factory conditions; documentation pack | Installed condition, utilities, integration and performance under site conditions; handover readiness |
| Evidence produced | Witnessed FAT report, punch list, test and calibration records | SAT report, performance run data, commissioning records, training and handover sign-offs |
| What signing commits you to | Release for shipment; usually a payment milestone; acceptance of build quality as inspected | Final acceptance; final payment; warranty start; transfer of care, custody and control |
| Typical failure categories | Dimensional and interface errors, wrong or unverified materials, missing items, control logic faults, documentation gaps | Transport damage, installation errors, utility mismatches, integration faults, performance shortfall under real load |
| Who pays for failures found here | The supplier: rework on its own premises at its own cost; a well-drafted contract also puts re-test witness costs on the supplier | Contested: depends on root cause; even where the supplier is liable, site costs (cranage, idle crews, schedule delay) largely fall on the buyer and are hard to recover |
Why “passed FAT” does not mean “will pass SAT”
Between the factory floor and site sign-off, five things change. Each can turn a clean FAT report into a failed SAT.
Transport and handling. Weeks at sea, multiple lifts, storage at both ends, often handled by parties who were never in the factory. Vibration loosens terminals, moisture finds unsealed enclosures, a bad lift bends a frame. And a passed FAT does not protect the buyer if the equipment is not preserved for transport: open nozzles, unsealed glands and unprotected machined surfaces can turn a factory-accepted package into an arrival-damage dispute. A clean FAT report should be followed by packing checks, arrival inspection and preservation records, not treated as the final condition report.
Installation variables. At FAT the machine stood on the supplier’s prepared floor, assembled by the people who built it. At site, foundations, grouting, levelling and reassembly are executed by a local contractor working from drawings, and the package is connected to long pipe runs, field cables and adjacent equipment the factory never reproduced. Misalignment, pipe stress and vibration often appear only after installation.
Utilities. The FAT ran on stable factory power, dry instrument air and clean water. The site may offer generator power with voltage dips, a different supply standard, humid air and hard water. FAT proves performance under recorded factory conditions; SAT proves whether the same package works with the real utilities available at site, which is why the FAT procedure should record exactly what temporary utilities were used.
Integration. At FAT, external signals are simulated with jumpers, switches or test software. A package may pass a local start-stop demonstration while still missing a permissive, alarm or trip required by the project control philosophy, an issue that belongs in the FAT punch list, not in the site commissioning room. At SAT the simulations disappear: the package must exchange live handshakes with other vendors’ equipment and the plant control system. Multi-vendor lines fail at the seams, not inside the machines. The closer the FAT simulation is to the real site interface, the fewer surprises the commissioning team faces at SAT.
Ambient. Temperature, humidity, dust and altitude, none of them present in the factory test. For equipment shipping to high-ambient or high-humidity sites, performance can deviate measurably from the bench result.
What bridges the gap is unglamorous and contractual: a marine packing specification verified at pre-shipment inspection; a utilities and interface schedule frozen at design review and signed by both sides, so “site conditions” cannot later serve as a defence; a controlled technical review whenever the project design changes after FAT documents are frozen; supplier commissioning attendance with rates agreed in advance; and commissioning spares shipped with the unit, not ordered after the first failure. With those measures in place, the SAT confirms what the factory record already showed rather than surfacing problems for the first time at site, where they are most expensive to fix.
When the contract should require both, and how to word the acceptance sequence
Both tests are justified whenever equipment is engineered to order, contains automation, depends on site conditions for its performance, or crosses an ocean. For a standard catalogue machine, a FAT combined with a pre-shipment inspection may be enough; for anything engineered, the acceptance sequence should be explicit in the purchase contract rather than assembled from assumptions after the order.
A clean sequence reads: FAT → release for shipment → pre-shipment inspection → delivery → installation → commissioning → SAT → final acceptance certificate → warranty start. Each arrow should map to defined evidence and, where money moves, a payment milestone.
The classic dispute is the supplier arguing that the FAT signature meant final acceptance, usually after the package fails SAT on an interface problem. The defence is built into the contract before the order is signed:
Wording points that decide outcomes later:
- Attach the FAT and SAT protocols as contract annexes, agreed before manufacture starts. A test protocol negotiated at the factory gate is the supplier’s protocol.
- Categorise punch items before shipment release, because “minor open items” is not a definition. A workable scheme: Category A must be closed before shipment; Category B before energisation; Category C may remain open until SAT without affecting safe installation; Category D covers documentation and cosmetic items with an agreed closure date. Items affecting safety, core function, site interfaces, statutory compliance, preservation or commissioning documentation belong in Category A unless the buyer authorises otherwise in writing.
- Name the test in every payment milestone. “Payment after acceptance test” is an invitation to dispute: a payment due after FAT should be linked to the signed factory acceptance and closure of shipment-blocking items; a payment due after SAT to signed site acceptance after installation and commissioning checks.
- Fix the warranty trigger. Warranty should commence from site acceptance following successful SAT, not from FAT release, unless that is expressly agreed. A purchase order that links warranty to undefined “acceptance” will be read by each side in its own favour.
- Define re-test rules. If the FAT fails, the supplier bears the cost of the re-test and the witness’s re-attendance.
- Resist deemed-acceptance clauses that treat equipment as accepted a fixed number of days after delivery whether or not a SAT has been run. Acceptance should lapse by performance, not by calendar.
- Hold a retention payment until SAT sign-off. It is the only money that still talks after the equipment has sailed.
Frequently asked questions
Does FAT replace pre-shipment inspection?
No. The FAT proves function before packing; the pre-shipment inspection, typically weeks later, verifies what actually leaves the factory: quantities against the packing list, closure of punch items, marine packing, preservation and marking, and that nothing was removed or substituted after the test. Equipment is partially disassembled for transport after the FAT; the pre-shipment inspection is the only check on what the buyer will actually receive.
Who should witness the FAT?
The buyer’s own engineer where feasible; otherwise an appointed representative with the technical background for that equipment class and explicit authority to withhold release for shipment. A witness who can observe but not reject is a formality, not a control. A FAT report signed only by the supplier should be read as internal QC, not as acceptance.
What does “FAT date” mean in a project schedule?
The date the supplier commits to having the equipment assembled, internally tested and ready for witnessed factory acceptance. It anchors everything downstream: witness travel, vessel booking, letter-of-credit milestones, site readiness. That makes FAT-date slippage the earliest reliable signal that delivery will be late.
Is the SAT the same thing as commissioning?
No, although they overlap in time. Commissioning is the work: energising, loop checks, calibration, trial runs, the activity of bringing installed equipment into operation. The SAT is the verdict: a formal witnessed test against the contractual protocol, usually run at the end of commissioning, whose signature carries the commercial consequences set out above.
Can the FAT checklist be reused as the SAT checklist?
The skeleton, yes; the content, no. The FAT protocol verifies build and function under factory conditions. The SAT protocol must add what the factory could not test: real utilities, the real process medium, sustained performance at contractual capacity, and integration with the rest of the plant. A SAT that merely repeats the FAT confirms that the equipment survived transport, and nothing else.
What happens if the SAT fails?
Whatever the contract says, which is why the mechanism must exist before the test. Typical provisions: a punch list for non-critical findings, a re-test for critical ones with cost allocation tied to root cause, and an escalation path if contractual performance cannot be met under site conditions. The root-cause split matters: supplier scope (design, manufacture, programming, documentation, supplied components) stays with the supplier; installation, utility and third-party interface defects sit with whichever party owns that scope under the project responsibility matrix. A SAT failure without a contractual mechanism to resolve it is where projects stall.
How Sinospect handles FAT and SAT for China orders
When Sinospect supplies the equipment from China as the principal, the FAT is its own release gate: the factory is paid after the goods pass it, not before, and Sinospect carries the FAT package through to the site team for the SAT. For buyers who run their own procurement, Sinospect provides the same independent control on the China side of this chain: reviewing FAT and SAT protocols against the technical file before manufacture begins, witnessing the FAT through factory acceptance testing in China with the authority the buyer defines, controlling the punch list, running the pre-shipment inspection after punch closure, and assembling the documentation package the site team will rely on for commissioning and the SAT. The SAT itself is run by the buyer’s project team at site, with the FAT package as its reference baseline, which is precisely why the factory-side record has to be complete. If you have a live procurement file, an order placed, a FAT approaching, or an acceptance sequence still under negotiation, submit it for review and we will set out what is covered and where the exposure sits.
Setting up the FAT–SAT sequence for a China order?
Send the equipment scope and where the order stands. Sinospect responds with how the FAT and SAT should be split in the contract, the documents to agree with the supplier and how factory-side witnessing would work.