Current Transformers

The Current Transformer

The Current transformer is used to transform the current from a high current into a lower current allowing the power to be measured and controlled. The CT consists of a core, a primary and a secondary winding. A magnetic flux is created as alternating current flows through the primary winding. This induces alternating current in the secondary winding. This alternating current is proportional to the current in its primary. ‘Step-down transformers’ is another term for a Current transformer. Once the current can be safely monitored and measured using an ammeter once it has been reduced it.

Current transformers can perform circuit control and perform roles for safety protection and current limiting. The ratio of primary current to the secondary current is known as the current transformation ratio of the Current transformer. Unlike a voltage transformer, the primary current of a current transformer does not depend on the secondary load current but instead is controlled by an external load. The secondary current is usually rated at a standard 1 Ampere or 5 Amperes for larger primary current ratings. The current transformation ratio of the Current Transformer is usually high. Often the secondary ratings are of the order 5 A, 1 A, 0.1 A, whereas the primary ratings vary from 10 A to 3000 A or more.

Current Transformers and Ammeters

Current transformers and ammeters are used together as a matched pair in which the design of the current transformer gives a maximum secondary current that corresponds to a full-scale deflection on the ammeter. An inverse turns ratio exists between the two currents in the primary and secondary windings typically in current transformers. Which is why calibrating the CT is generally for a specific type of ammeter. A current transformers secondary winding should not be operated into an open circuits. This is because Current transformers are intended to be used as proportional current devices.

The Three types of CT’s

  • Wound Current Transformer – The transformers primary winding connects in series with the conductor carrying the measured current flowing in the circuit. The secondary currents magnitude depends on the turns ratio of the transformer.
  • Toroidal Current Transformer – These CT’s contain no primary winding. The line that carrying the current flowing in the network threads through a window or hole in the toroidal transformer. CT’s with a “split core” can be opened, installed, and closed, without disconnecting from the circuit.
  • Bar-type Current Transformer – This type of CT is known for using the cable or bus-bar of the main circuit as the primary winding, This is equivalent to one turn.


CT’s perform the functions below:

  • A Current transformer supply the protective relays with currents of magnitude proportional to those of power circuit but sufficiently reduced in magnitude.
  • The measuring devices cannot be directly connected to the high magnitude supplies. Thus current transformers supply such devices with currents of magnitude proportional to those of power.
  • Current transformers isolate measuring instruments from high voltage circuits.

Current transformers are used for Electrical substations, Generating stations, Commercial electric power distribution. Shop our range of CT products. We promise Fast Delivery!

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