The answer first: inspect before you release the balance — 12 checks, 5 routine tests
A pre-shipment inspection (PSI) exists for one reason: to catch a defect while the manufacturer still holds your money and the transformer is still on its own factory floor — not six weeks later, bolted to a pad 6,000 km from where you can fix anything cheaply. A competent PSI verifies 12 physical and documentary checks and witnesses 5 routine tests required by IEC 60076-1, and it should be completed and closed before you release the shipping balance — typically 30–40% of contract value. The five routine tests every unit must pass are: winding resistance, voltage ratio and vector group, short-circuit impedance and load loss, no-load loss and no-load current, and the dielectric (applied-voltage) withstand test. This guide gives you the 12-point checklist, the standard behind each test, and the exact documents to demand before the unit is crated. If you are still choosing the factory, start one step earlier with our 12-Point Factory Audit Checklist — the audit is how you pick the factory, the PSI is how you hold it to what it promised.
What a PSI is — and where it fits in the order timeline
PSI is a final, pre-crating inspection performed after manufacturing and testing are complete but before packing and shipping. It is distinct from two other inspections that happen earlier:
- Factory audit (pre-order): evaluates the factory's capability, certifications, and processes — the decision to buy at all.
- During-production inspection (DPI): checks winding, core stacking, and assembly mid-manufacture, when corrections are cheapest.
- Pre-shipment inspection (PSI): verifies the finished, tested unit against the purchase order — the decision to release the balance.
The PSI is normally done by your own engineer, an appointed third party (SGS, Bureau Veritas, TÜV, Intertek), or remotely via live video. For a distribution transformer, the on-site portion takes 4–8 hours including test witnessing, and typically costs a few hundred to a few thousand USD — cheap insurance against a US$15,000–80,000 unit (as of 2026) arriving wrong.
The 5 routine tests — what to witness (IEC 60076-1)
IEC 60076-1 (GB 1094.1 in China) requires a defined set of routine tests on every single transformer before it may be shipped. These are not optional and are the backbone of any PSI:
| Routine test | What it verifies | Typical pass criterion |
|---|---|---|
| 1. Winding resistance | Conductor continuity, correct cross-section, no bad joints | Within manufacturer tolerance (typically better than ±0.5%) |
| 2. Voltage ratio & vector group | Turns ratio and phase displacement match the nameplate | Ratio within ±0.5% (IEC 60076-1) |
| 3. Short-circuit impedance & load loss | Uk% and copper losses match the data sheet | Impedance ±10%, load loss +15% |
| 4. No-load loss & no-load current | Core losses match the efficiency grade you paid for | No-load loss +15% (IEC 60076-1) |
| 5. Applied-voltage (dielectric) test | Insulation withstands the separate-source AC voltage | No breakdown / no flashover at test voltage |
Point 4 is the one that most directly protects your money: if you ordered an S13 or S20 low-loss unit, the measured no-load loss must sit inside the IEC 60076-1 tolerance, not just “close.” Cross-check the measured values against the efficiency grade in our S11 vs S13 vs S20 loss-grade comparison — a unit that fails the no-load loss test is a unit that will quietly burn extra electricity for 30 years.
Routine tests vs type tests — don't demand the wrong thing
A recurring buyer mistake is demanding a type test (lightning impulse, temperature rise, short-circuit withstand) on your serial unit. Type tests are performed once per design, not once per unit, and are covered by the manufacturer's type-test report for that design family. Requiring them on every unit does not add protection — it adds cost and lead time:
- Lightning impulse test (IEC 60076-3): a type test proving the insulation design; the report, not a re-test, is what you accept on a serial unit.
- Temperature-rise test (IEC 60076-2): a type test; winding hot-spot and oil rise are design properties.
- Short-circuit withstand test (IEC 60076-5): a type test, except for larger units where the customer may require a routine version. See our short-circuit current guide for why Uk% and the mechanical withstand margin matter.
The correct demand is: routine tests on your unit, witnessed live; type-test reports for the design family, verified to match your rating.
The 12-point PSI checklist
| # | Check | What to look for |
|---|---|---|
| 1 | Nameplate | Rating, voltages, vector group, impedance, year and serial number match the PO and drawings |
| 2 | Routine test report | All 5 tests present, values inside IEC 60076-1 tolerance, signed and dated |
| 3 | Type-test reports | Lightning impulse, temperature rise and short-circuit — for the correct design family |
| 4 | Dimension & footprint | Matches the approved GA drawing; will it fit your pad or room? |
| 5 | Tank & finish | No dents, leaks or corrosion; paint thickness and color per spec |
| 6 | Bushings & fittings | Correct type and creepage distance; no cracks; gaskets seated |
| 7 | Tap changer & accessories | Operates through full range; gauges, thermometer and relief device installed |
| 8 | Oil / resin | Oil type and dielectric strength certified (or dry-type resin condition) |
| 9 | Accessories & spares | Bushings, gaskets and spare fuses per the packing list — nothing missing |
| 10 | Markings & language | Warnings, terminal labels and instructions in the language you specified |
| 11 | Packing & sealing | Oil-tight, desiccant in the conservator, suitable for ocean freight |
| 12 | Documentation pack | Test reports, certs (CE/UL/CSA as applicable), drawings, manual, warranty |
The documents to demand before crating
No PSI is complete until you hold the paper trail. At minimum, insist on:
- Routine test report for your exact serial number, signed by the factory's QC.
- Type-test reports for the design family, with the test laboratory named.
- Material certificates for core steel, copper and insulation (traceability to the batch).
- Certificates: CE (EN 50588 / IEC), UL/CSA for North America, or CCC for China — whichever your market requires.
- Warranty certificate stating the coverage period and claims process.
- Packing list, invoice and certificate of origin for customs.
If the factory will not put a value in writing, treat the omission as a red flag — a professional manufacturer supplies the full pack as a matter of course. For the North American and EU certification requirements specifically, our standards comparison guide maps which cert applies to which market.
Who should inspect — and what QDTB provides
For high-value or first orders, use an independent third party; for repeat orders, your own engineer on a factory visit is usually enough. Whichever route you take, agree the inspection protocol before production starts so there is no argument about what “pass” means. QDTB supports both: every unit ships with a complete signed routine test report, type-test reports for the design family, and material certs, and we welcome third-party inspection at our factory at any agreed stage. Contact our team to schedule an inspection window, or see the ranges to specify against — oil-immersed and dry-type.