Used Transformer Pre-Purchase Checklist
Buying a used transformer is buying its history, and its history is in its documents. This checklist covers what to ask for, what to check on site, and which gaps should change the price — or stop the purchase — before money moves. It is a buying aid, not a test report, and any unit you are serious about should still be tested by a qualified engineer.
Documents first: nameplate, reports and history
Ask for the nameplate data, all available test reports with dates and conditions, oil and DGA history, maintenance records, and any record of faults, repairs, rewinding or relocation. Check that report serial numbers match the nameplate, and that the history is continuous rather than a single recent test with nothing before it. A gap in history is not proof of a fault, but it is a gap in evidence, and evidence gaps should be priced in.
On site: bushings, tap changer, cooling and leaks
Inspect bushings for cracks, pollution and oil level; operate the tap changer through its range if permitted and note hunting, stiffness or failed steps; check fans, pumps, radiators and conservator level; and look for oil staining, rust, damaged paint and repaired areas. These checks cost nothing and catch the problems that paperwork hides: a unit that looks clean in a report can still show a leaking bushing or a seized tap changer on site.
Tests to require before you pay
At minimum, require insulation resistance, turns ratio, winding resistance, and oil tests including DGA, all under stated conditions and dated. For larger or critical units, add short-circuit impedance, bushing capacitance and tan delta, and frequency response if available. Agree in writing who pays for testing, what happens if a test fails, and that the unit will not be energised for a trial run without the seller's and your engineer's agreement.
Red flags that should stop or reprice the deal
Stop or reprice on: no test reports at all, a DGA trend that is rising without explanation, a tap changer that does not operate cleanly, a serious oil leak, evidence of internal fault or fire, or a nameplate that does not match the reports. Any one of these does not automatically mean the unit is scrap — but each one means the price must reflect the risk and the testing still needed, or the purchase should wait until testing answers the question.
If you want a second pair of eyes on your own report or nameplate before you decide, see the remote transformer services — documents only, fixed price, fixed turnaround, no site visit. For the wider testing picture, start with the transformer testing and commissioning guide.