A C.T “Current Transformer” is a type of instrument transformer designed to step down the current in the secondary for protection and measurement of proportional primary current.
These transformers with low range ampere meters are used to measure the current in the high voltage circuits. They are also used to step down the current at a specific ratio to insulate the instrument from the high voltage lines.
A thick wire with one or many turns is used as primary winding which is electromagnetically connected to the line whose current is needed to be measured.
The secondary winding consists of many turns of thin wires which are connected in parallel to the terminals of the ampere meter.
In terms of voltage, it is a step-up transformer which steps-down the level of current. If the turns ratio between primary and secondary is 100:05; then it will step up the level of voltage in terms of 20:01 or it will step down the level of current in terms of 01:20.
Therefore, if we know the current ratio and the rating of AC ampere meter, we may determine the line current. This is the same concept used in the clamp meters.
