Current Transformer (CT)


Current Transformer (CT):-

Current transformer is used to measurement purpose hence it is known as the instruments transformer. the transformer which are used for measurement , protection and control circuit, are known as instrument transformer. for the protective system is used as sensing device. There are to types of instrument transformer

  • Current transformer(CT)
  • Potential transformer(PT)

Current transformer :- 

The instrument transformer which are used in conjunction with ammeter, over current relay etc. Are known as current transformer.
The current transformer is also known as the current converter. Which is step down the value of current, from high value. VA rating of current transformers is less then to the potential transformers. The current ratio is substantially constant for given range of primary current and phase angle error is within specified limits. the current rating of secondary of CT is very less then the primary side. if the CT ratio is 500/5 , then the primary current is 500 amp which is step down at 5 amp at secondary side.


CT is connected in series with the line, where we have to measure the value of current.
Generally current transformer us similar to the normal step up transformer. Which is having a large no. of turns at secondary winding and less no. of turns at primary winding.

There are three basic types of current transformers: wound, toroidal and bar.

  • Wound Current Transformer: – The transformers primary winding is physically connected in series with the conductor that carries the measured current flowing in the circuit. The magnitude of the secondary current is dependent on the turns ratio of the transformer. Wound Current Transformer
  • Toroidal Current Transformer: – These do not contain a primary winding. Instead, the line that carries the current flowing in the network is threaded through a window or hole in the toroidal transformer. Some current transformers have a “split core” which allows it to be opened, installed, and closed, without disconnecting the circuit to which they are attached.Toroidal Current Transformer
  • Bar-type Current Transformer:– This type of current transformer uses the actual cable or bus-bar of the main circuit as the primary winding, which is equivalent to a single turn. They are fully insulated from the high operating voltage of the system and are usually bolted to the current carrying device.Bar-type Current Transformer

The core is made by nickel-iron alloy or grain oriented sheet steel. Core is wound type. Before applying secondary winding ,the core is insulated by means of end collars and wraps.the secondary winding is wounded on insulated core in form of toroidal winding by hand winding.


  • Can be used for extension range of ammeter and watt-meter.
  • There is low power consumption in the measuring circuit.
  • Used for measurement the high value of current.


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