The primary of a current transformer typically has only one turn. This is not really a turn or wrap around the core but just a conductor or bus going through the “window.” The primary never has more than a very few turns, while the secondary may have a great many turns, depending upon how much the current must be stepped down. In most cases, the primary of a current transformer is a single wire or bus bar, and the secondary is wound on a laminated magnetic core, placed around the conductor in which the current needs to be measured.
If primary current exists and the secondary circuit of a CT is closed, the winding builds and maintains a counter or back EMF to the primary magnetizing force. Should the secondary
be opened with current in the primary, the counter EMF is removed; and the primary magnetizing force builds up an extremely high .