Current Transformer (C.T.)

A current transformer (C.T.) is used to measure high alternating current in a power system. The primary of the transformer has a few turns of thick wire where as the secondary has many turns of very fine wire as shown fig below. It is clear from the figure that a current transformer is simply a well designed step up transformer. Since voltage is stepped up, the current is stepped down which can be measured with low range ac ammeter.

The primary of the ct is connected in series with the line whose current is to be measured as shown in fig. The secondary of the transformer is connected across low range (0-5) Amp ac ammeter.

NpIp=NsIs

Or Ip/Is = Ns/Np

Where 

Ip = Line current (Primary)

Is = Ac ammeter current (Secondary)

Ip/Is is called CT ratio

Therefore Ip/Is = C.T. ratio

Or Ip = Is×C.T. ratio

Line current (Ip) = Ac ammeter reading × C.T. ratio

For Example

If the reading of ac ammeter is 1A and the C.T. ratio 100:1

Then

Line current (Ip) = 1×100

                             = 100A