CT are instruments that measure the large current on the primary side into small current on the secondary side according to the principle of electromagnetic induction. CT consists of a closed core and winding. The number of windings of its primary side is small, it may be linked up the line  which to be measured, so it often have all the current flows through the line. The secondary winding number is more, it may be concatenated in measuring instrument and protection circuit. When CT is working, its secondary side circuit is always closed, so the impedance of series coil in measuring instrument and protection circuit  is very small, the working state of  CT is close to a short circuit.

There's no resistance on the primary side of micro CT. The ordinary micro CT is a transformer with a parallel resistance on the secondary side， and it output current. The micro current converter in the micro CT is the resistance in parallel at the secondary side and output voltage. Micro CT are divided into multi-turn widing type (plug in the circuit board) and through heart type.