Current transformer ratio and phase error by test ring method

This method of testing may be indicated briefly as follows: A special current transformer or test ring is connected with its primary and secondary respectively in series with the primary and secondary of the current transformer under test. The number of turns in the secondary coil of the special current transformer is varied until the primary and secondary ampere turns of the special current transformer or test ring are equal to each other and show a balance. Then the ratio of turns in the special current transformer is the current ratio of the transformer under test. Of course, the primary and secondary ampere turns can never actually balance, because they are not exactly in step or opposition, but a point can be found where the vector difference between them is at right angles to one of them and this point is taken as the balance point. This actual minimum value of vector difference gives a measure of the phase difference between primary and reversed secondary amperes.