Current transformer:
Current transformer is nothing but a step-up transformer, whose primary has less turns typically the line consider as the primary winding and the secondary has more turns. That’s why we should keep CT secondary terminal in the closed condition.
Primary winding < Secondary Winding
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Note: As the voltage increases the current decreases. That’s why we gets such a Current ratio 400/5
Refer this:
Open circuit is not allowed at the output end of current source transformer, because high voltage will be generated during open circuit, resulting in insulation damage or electric breakdown or even accidents. However, short circuit is allowed, and the current during short circuit is the rated output current.