How is a transformer tested?
With the circuit isolated and the transformer disconnected, an insulation tester checks between the primary, the secondary and earth, and a low-resistance meter checks that each winding is intact.
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Transformer testing proves a series isolation transformer is sound before it goes back into the loop. With the circuit isolated, locked off, proven dead and the transformer disconnected, an insulation tester is applied primary to secondary, primary to earth and secondary to earth, and each winding is checked for continuity with a low-resistance meter.
Insulation readings should be high and winding readings low and steady. Compare them with the specification and with the last record, because a falling trend usually points to moisture or damage that will become an earth fault or an open circuit. Discharge the leads after each insulation test and record every result.
Frequently Asked Questions
Q1What is transformer testing in AGL?
Checks that a series isolation transformer is electrically sound: insulation resistance between the primary, the secondary and earth, and continuity of each winding. It is done with the circuit isolated, and results are compared against the specification and earlier readings.
Q2Which insulation tests are made on a transformer?
Primary to secondary, primary to earth and secondary to earth, using an insulation tester at the voltage the specification and manufacturer require. A healthy transformer reads high on all three.
Q3Why test winding continuity?
An open primary breaks the whole series loop, and an open secondary leaves its fitting dark and can raise the secondary voltage. A low-resistance reading on each winding shows it is intact, and a change from earlier readings is an early warning.
Q4What must be done before testing a transformer?
Isolate the circuit at the CCR, lock it off and prove it dead, earth it as the site procedure requires, and disconnect the transformer from the fitting and from the loop. Work only under the permit system, and discharge the leads after each insulation test.
Q5What does a falling insulation reading mean?
Often moisture in the pit, a damaged connector or a damaged lead, though it can also be the transformer itself. Look at the trend, not one number: investigate a steady fall before it becomes an earth fault. See Insulation Testing and Earth Fault.
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