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Protective Current Transformer

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Protective Current Transformer are used to convert current/ voltage from one value to another lower standard value. A current transformer serves two basic functions:

  • To provide a standard current on the secondary which is typically 5 Amps or 1 Amp and
  • Provides dielectric isolation of primary from the secondary.
  • Current Transformers are generally used for measurement and or protective applications. In measurement Current Transformers one is concerned with the accuracy of the secondary current over a wide operating range.
  • Typically the errors are guaranteed to be within specified limits between 5% to 120% of the rated current. The nature of errors are current [Ratio] error [in%] and phase displacement.


  • A burden
  • Accuracy class and
  • Accuracy limit factor. In other words, one specifies the errors of the Protective Current Transformers at the rated full load current as well as at a current given by the accuracy limit factor times the full load current.
  • There exists a class of current transformers known as PS Current Transformers whose performance can not be specified either as a measuring current transformer or protective current transformer. PS class current transformer.

PS class current transformers are characterized by

  • Nominal ratio,
  • Knee point voltage
  • Exciting current at KPV ora stated percentage and [iv] Secondary resistance at 75 C.

Constructional Features Of Current Transformers:

  • Current transformers can be of the ring type, bar primary type, or the wound primary type. We, at NAVAGO use Electrolytic Grade Copper for the primary and secondary winding. In ring and bar primary type current transformer, the primary ampere turns [AT] is fixed and is equal to, the primary current. The difference between a bar primary and a ring type current transformer is that in the bar primary version, the bar is fixed integrally with the CT. for large outputs or for increased accuracy, at low price it may not be possible to manufacture ring or bar primary current transformers.
  • Hence one uses wound primary current transformers in which the primary consists of a number of turns.

Special Current Transformers

  • Summation Current Transformers – Current Transformer intended to summate the currents in a number of feeders in association with the feeder current transformer is called a summation transformer. The performance/testing of these current transformers are governed by IS:6949-1992.
  • The CB CT or a zero sequence CT is used as a very sensitive method of detecting earth fault currents. It is the basic scheme for a power circuit carrying small earth currents. Protection operation at primary current of 5 to 10 amps is feasible. Information required for manufacture of a CB CT.
  • Primary operating current.
  • Type of relay and setting of relay.
  • Finished dimensions, especially inner diameter.
  • Lead resistance from Relay to CT.
  • System voltage and insulation level.

Note: If the above information is given, NAVAGO ensures that the CT manufactured develops the requisite KPV, etc, using the optimum turns ratio and core size resulting in minimum core size. The relays which are generally used for earth fault protection.


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