Typical problem areas with CPTs include open circuits, partial shorts, complete shorts, and grounded windings. Your first check should be at the primary of the CPT to verify that primary power is available. Sometimes a fuse on the primary blows, cutting off power to the CPT.
Partial shorts. Sometimes a partial short will occur in the CPT's secondary, causing a voltage drop across it. Overheating of the CPT is the usual symptom because large circulating currents are flowing through the shorted windings.
Troubleshooting here involves a sequence of steps. First, you should verify that line power is available by taking a voltage reading at the CPT's primary. Once this is verified, you should take a reading at the secondary. If the readings are somewhat lower than normal, you can suspect a partial short. This is the best method of determining a partial short condition.
Another method is to use a very sensitive ohmmeter with the CPT's leads disconnected and the system deenergized. Here, a lower than normal resistance reading indicates a partial short. However, the difference in resistance from normal will be very slight; thus the need for a very sensitive ohmmeter.
Complete shorts. Sometimes a CPT's winding becomes completely shorted, activating a CB or fuse to protect the circuit by deenergizing it. However, there may be instances where the CPT continues to operate, resulting in excessive overheating (due to very large circulating current) and melting of CPT winding insulation. The most apparent symptom is a strong odor. Another is no voltage output across the shorted winding.
Be careful here as the short may be in the external secondary circuit and not the CPT's winding. The best way to find the location is to disconnect the external secondary circuit from the CPT and take a voltage reading at the CPT's secondary. If the voltage is normal, the problem is in the external secondary circuit. If the voltage is zero across the secondary leads, the CPT is shorted and should be replaced.
Grounded windings. In older transformers, especially those that have been overloaded, insulation breakdown is a very common problem. Here, the insulation physically breaks or deteriorates to the point where the winding's bare wire becomes exposed. When the exposed wire comes in contact with a grounded surface (such as the CPT housing), the CPT becomes shorted to ground.
Should the above condition develop, and a point in the external secondary circuit also becomes grounded, part of the CPT winding will be shorted out. Here, the symptoms are CPT overheating, which can be detected by touch or smell and a low voltage reading at the CPT's secondary. The only alternative here is to replace the damaged CPT.
The best method for detecting this condition is to use a megohmmeter. First, disconnect the leads from the primary and secondary windings. Second, connect the megohmmeter's negative test lead to an associated ground and its positive test lead to the winding being tested, and record the reading. Finally, take an insulation resistance reading between the windings themselves by connecting one test lead to the primary and the other to the secondary. Record these readings.