Question

In a 1000/5 A C.T., the number of secondary turns would be ________

(a) 200

(b) 199 or 198

(c) 5

(d) 1000

The question was asked at a job interview.

This interesting question is from Design Features of Current Transformers in chapter Extension of Instrument Ranges of Electrical Measurements

Answer 1

Correct choice is (b) 199 or 198

For explanation I would say: The optimum number of secondary turns in a C.T. is the value that makes the transformation ratio (R) and nominal ratio (Kn) equal. It is usually one or two turns less than the actual value. Thus in a 1000/5 A, though the number of turns would be 200 ideally, it is considered as 198 or 199 in order to keep R and Kn equal.