Current Transformers (CT) are used to reduce primary current of hundreds or thousands of amps to a nominal value of five or one amp. Most are of toroidal design consisting of a ring type core with many turns around the core comprising the secondary. The primary passes through the center of the ring.

Testing a CT involves verifying the turns ratio, polarity, and excitation. It is also useful to measure the phase shift between primary and secondary and the winding resistance.

The ideal method of testing CT’s would be to apply nominal current on the primary and measure the current on the secondary. It is almost impossible to produce the nominal current for the testing purposes and in the most cases it is not practical and safe.

The conventional method of testing CT’s is to apply voltage to the secondary and measure the induced primary voltage. The comparison of the voltages gives the CT turns ratio. Polarity or phase shift is measured at the same time.

The conventional method of testing CT excitation is to apply increasing voltage to the secondary winding and record applied voltage and resulting current.

Many CT’s have a secondary winding with many taps, typically up to 5. Ratio of every tap is essential but excitation is usually measured on one tap only, usually the full winding.