What does a transformer failure look like?
The isolation transformer has one winding in the series loop and one feeding a single light — which winding fails decides whether one light goes out or the whole circuit does.
A series isolation transformer has its primary winding in the series loop and a separate secondary winding feeding one fixture. That split decides the symptom. An open primary breaks the loop, so every light on the circuit goes dark and the CCR raises an open-circuit alarm. An open secondary puts out only its own fitting while the loop stays complete.
Insulation breakdown to earth or between the windings shows as an earth-fault alarm or a falling insulation reading, and shorted turns as a hot transformer or a dim fitting. Series circuits run at high voltage, so a suspected transformer failure is isolated, locked out and proved dead before it is touched.
- Read the symptom. Whole circuit out with an open-circuit alarm, one light out, or an earth-fault alarm.
- Isolate. CCR off, locked out and tagged; prove the circuit dead.
- Look first. Check the housing, potting, leads and connectors, and the pit for water.
- Test. With the transformer disconnected, check primary and secondary continuity, then insulation to earth and primary to secondary.
- Replace or repair. Fit the correct rating, or repair the connector if that was the fault.
- Re-test & record. Prove the circuit before restoring and record the findings.
Part of the series loop: an open winding breaks the loop and the whole circuit is out.
Feeds one fixture: an open winding, lead or fixture puts out one light and the loop stays complete.
Frequently Asked Questions
Q1What is a transformer failure in airfield lighting?
A fault in a series isolation transformer, the unit that sits in the series loop with its primary winding and feeds one fixture through a separate secondary winding. It can fail by an open winding, insulation breakdown to earth or between the windings, shorted turns, or damaged and wet leads and connectors.
Q2Why does an open primary darken the whole circuit but an open secondary only one light?
The primary winding is part of the series loop, so if it opens the loop is broken and current stops for every fixture, and the CCR raises an open-circuit alarm. The secondary feeds only its own fixture, so an open secondary puts out that one light while the loop stays complete.
Q3How can a failed transformer be told from a failed lamp or fixture?
By the pattern and then by testing. A whole circuit out points at the loop, including a transformer primary. One light out with the loop running points at the secondary side: the secondary winding, its connector, the lead or the fixture. With the circuit isolated, continuity and insulation tests on the transformer, disconnected from the circuit, settle which part has failed.
Q4What causes a transformer to fail?
Moisture in the pit or at a connector, ageing or damaged insulation and potting, mechanical damage to the housing or leads, and overheating from overload or poor connections. Several of these often go together, which is why wet pits and damaged connectors are checked first.
Q5Why is a transformer tested disconnected from the circuit?
So the reading belongs to the transformer alone and the test cannot be affected by, or affect, the rest of the series circuit. The circuit is isolated and locked out first, and the manufacturer's test procedure and limits are followed.
Q6Is a failed isolation transformer dangerous?
It can be. Series circuits run at high voltage, an open secondary can raise the secondary voltage and heat the transformer, and broken-down insulation can put voltage where it should not be. The circuit is isolated, locked out and proved dead before anyone works on the transformer or its leads.
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