For Indoor/Outdoor Current Transformers for use on H.S.C up to 36 kV: Two cores are usually considered as adequate. One core is used for metering purpose and the other is used for general protection application. A third core for differential protection is sometimes specified.

Current Transformers for use on H.S.V. of 72.5 kV and above: It is common to specify 5 cores for applications as follows.
  • Core 1 - Metering
  • Core 2 - Over current and Earth fault protection.
  • Core 3 - Transformer differential protection.
  • Core 4 - Bus - differential protection.
  • Core 5 - Distance protection.Al
All applications of Current transformers for this range, may or may not require all 5 corers. (Current Transformers for use on H.S.V. of 72.5 kV are usually specified with only three cores.) It will be readily appreciated that reduction in the number of cores shall affect the price of Current Transformers. Thus, it is recommended that only required number of cores be specified. (The present approach to have spare cores may be discontinued as far as is practicable.)