The CTER-91 provides a variable voltage output and digital instrumentation for measuring the excitation voltage and excitation current that results as the voltage applied to the CT under test is increased.
As the CT under test begins to saturate, a large increase in current will be detected for a small increase in voltage.
The ratio test is performed by comparing a voltage applied to the secondary winding to the resulting voltage produced on the primary winding.
For example, if 1 volt per turn is applied to the secondary winding, the voltage present on the primary winding would be 1 volt.
More specifically, if 120 volts were applied to the secondary of a 600/5 current transformer (120:1 ratio), 1 volt would be present on the primary winding.
Polarity of the current transformer under test is determined by special circuitry that divides the voltage applied to the secondary winding by either 10 or 100, depending on switch position, then adds the result to the primary voltage.
An increase in magnitude indicates correct polarity, and a decrease shows incorrect polarity.