Occasionally a transformer winding becomes completely shorted. In most cases, this activates the overcurrent-protective device (circuit breaker or fuse) and de-energizes the circuit.
But in some cases, the transformer may continue trying to operate with excessive overheating due to the very large circulating current. This heat will often melt the insulation inside the transformer, which is easily detected.
Also, there will be no voltage output across the shorted winding and the secondary circuit supplied by that winding will be dead.
The short may be in the external secondary circuit or it may be in the transformer’s winding.
To determine its location, disconnect the secondary circuit from the winding and take a reading with a voltmeter. If the voltage is normal with the external circuit disconnected, then the problem is in the external circuit.
However, if the voltage reading is still zero across the secondary leads, the transformer is shorted and must be replaced.
Insulation breakdown is quite common in older transformers especially those that have been overloaded. At some point, insulation breaks or deteriorates and bare conductors become exposed. The exposed wire often comes into contact with the transformer housing and grounds the winding.
If a winding develops a ground, and a point in the external circuit connected to this winding is also grounded, part of the winding will be shorted out. The symptoms are overheating, usually detected by feel or smell, and a low voltage reading as indicated on a voltmeter scale. In most cases, transformers with this condition must be replaced.
A megohmmeter is used to test for this condition. Disconnect the leads from both the primary and secondary windings. Tests can then be performed on either winding by connecting the megger negative test lead to an associated ground and the positive test lead to the winding to be measured.
Insulation resistance should then be measured between the windings themselves, by connecting one test lead to the primary and the second test lead to the secondary.