Many circuits on aircraft use step down transformers for control voltages. Typical user systems are lights and 28VAC position synchros.

Troubleshooting a short that is causing a breaker trip is a fairly simple task. Isolating the user systems by disconnecting the LV connection is the first step. If the control breaker remains in, the short is "downstream" from the transformer. If the breaker is still popping, the HV line should be disconnected to isolate the transformer itself. If the breaker stays in, the transformer is shorted.

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APPROVED AIRCRAFT MANUFACTURER MAINTENANCE MANUAL PROCEDURES SHOULD ALWAYS BE FOLLOWED.