Question

The ratio error of a CT is due to ________.

• A. power factor
• B. supply voltage fluctuations
• C. exciting current
• D. negative component of current in the secondary

Hint:

Ratio error in CT? Think exciting current — it "steals" some current from the ideal transformation!

Answer 1

The Correct Option is C

Step 1: Understanding Current Transformers (CTs)
A CT steps down high current levels to measurable values. It has a primary winding (connected in series with the line) and a secondary winding connected to the meter.
Step 2: Definition of Ratio Error
The ratio error is the deviation between the actual transformation ratio and the ideal transformation ratio. It arises when the CT does not produce an exact scaled replica of the primary current.
Step 3: Role of Exciting Current
- The exciting (or magnetizing) current is required to magnetize the CT core.
- It does not flow into the secondary circuit but contributes to the total primary current.
- As a result, the secondary current is less than expected, leading to ratio error.
Step 4: Eliminate Other Options
- (1) Power factor → affects phase angle error, not ratio error.
- (2) Voltage fluctuation → irrelevant to CT operation.
- (4) Negative component → not a standard cause of ratio error.
Conclusion:
Option (3) is correct — the exciting current is the key reason for the ratio error in CTs.