FAT vs SAT: what is the difference between a factory acceptance test and a site acceptance test?

If you have just been handed a purchase order and asked to “arrange the FAT and the SAT”, and neither term means anything yet: they are two inspections of the same equipment at two different moments. The FAT happens at the factory that built it, before the balance is paid and before it ships — you will also see the same event called a factory witness test, and, in EPC and oil-and-gas procurement procedures, source inspection or vendor surveillance at the final witness point. The SAT happens at your own site, after the equipment is installed and running. Everything below is the detail of who does what at each, and what each signature costs you if it is given too early.

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?

For the categories of fault that recur most often at the factory stage, see Sinospect’s anonymised record of China FAT/PSI defect patterns.

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. For high-voltage equipment such as transformers and switchgear, type-specific checks like partial discharge testing during FAT sit in this layer, with their own limits and witnessing rules. 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 site acceptance test 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

Which of the two you actually need help with

Most people read this comparison because a decision is waiting, not because the definitions are interesting. Below is where an order usually sits when someone looks the difference up, the question underneath it, and the shortest route to an answer.

One thing worth separating before any of those: the SAT is run by whoever controls your site, and no supplier-side arrangement changes that. The FAT is the one of the pair that happens in somebody else’s building, on somebody else’s schedule, in a country where you may have nobody — which is why it is the half of the sequence buyers most often need to arrange rather than simply attend.

What does a FAT date mean in a project schedule?

A FAT date is the date the supplier commits to having the equipment assembled, internally tested and ready for witnessed factory acceptance. It anchors everything downstream of it: witness travel, vessel booking, letter-of-credit milestones, site readiness. That makes FAT-date slippage the earliest reliable signal that delivery will be late.

In procurement documents, “FAT date” always refers to the factory acceptance test; it is unrelated to the FAT (File Allocation Table) date format used in computing. A supplier that reschedules the FAT date more than once is signalling a delivery risk, and the shipment milestone should follow the witnessed test, not the original calendar.

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.

FAT → SAT → commissioning: what the factory record has to hand over

Buyers rarely think about the two acceptance tests on their own. They think about a sequence — factory acceptance test, site acceptance test, commissioning — and the three names mark three moments with three different owners. The FAT happens at the manufacturer’s works. The SAT and the commissioning happen at your site, run by whoever carries that scope — your project team, your integrator, or the supplier who sold you the equipment.

What travels between the two halves is a record. So the useful question is not who does what — it is what the factory test has to capture so that the people commissioning the plant are not rediscovering it on site, where correction costs the most and the supplier is no longer in the room.

Three things a factory test can never prove, however carefully it is run: performance under the utilities actually available at site, live integration with other vendors’ equipment, and behaviour in the real ambient. Those belong to the SAT and to commissioning. What the FAT can do is name them explicitly, so they reach the commissioning schedule as tasks rather than as surprises.

Sinospect works on the factory side of that sequence: it witnesses the test in China and assembles the record. Where Sinospect supplies the equipment itself, that responsibility does not stop at the factory gate — seeing the machine through to service on site is part of the supply, not a separate engagement. What Sinospect does not do is sell site commissioning as a standalone service on equipment it did not supply. Commissioning bought on its own belongs to an integrator or a commissioning contractor, and a project shopping for one is looking for a different kind of firm.

FAT and SAT on a regulated line

On a pharmaceutical, biotech or food-grade line, neither test is qualification. Qualification — installation, operational and performance qualification, IQ, OQ and PQ — is the end user’s own regulated exercise, run against protocols owned by the end user’s quality system and defended in the end user’s audits. What the FAT and SAT do is generate evidence that qualification can lean on instead of repeating.

That changes what the factory record has to look like. A FAT report adequate for a general industrial buyer — “tests performed, results acceptable” — cannot be cited inside a qualification protocol. What can be cited is dated and signed, identifies the exact unit by serial number, names the test instrument with its calibration status, and puts the acceptance criterion beside the measured value rather than in a separate document. The equipment’s own documentation set — material certificates, surface-finish records, as-built drawings, software and firmware revisions — has to be complete on the same day, because every one of those is easier to obtain while the equipment is still on the supplier’s floor than after it is installed in a controlled area.

Sinospect does not write, approve or execute qualification protocols; that stays with the end user’s quality organisation. The factory-side job is to make sure the record arriving from China is in a form qualification can consume. For cleanroom and controlled-environment packages, where part of the acceptance is only provable once the system is installed, the boundary between what is verifiable at the works and what is genuinely site-only is set out under cleanroom systems FAT witnessing.

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