Tuesday, March 13, 2012


Current transformers are used to step down high current to very low current that can be handled comfortably.


  • High currents can not be measured directly using ammeters due to limitations of current carrying capacity of the measuring instruments.
  • Also, high currents can not be used in protective relays. To allow high current to pass, larger size of conductor is required .
  • This leads to practical difficulties in handling the devices
  • The practical solution to this difficulty is to step down the current to a very low value that can be as low as 1 amp or 5 amp.
  • Unlike a regular transformer, a current transformer steps down only current.
  • Voltage developed in the secondary will be negligibly less
  • (ie, current in the secondary x resistance of the winding)
  • The basic principle of CT functioning is ampere-turn matching
  • AT primary = AT secondary


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