Current Transformers

The accuracy of CT is a function of the magnetic performance of the steel core. Toridally wound cores with high permeability and low loss are used to optimize performance and physical size of the transformers. High grade insulation is used to insulate between the windings and the core and between winding layers. Maximum mechanical and electrical performance is achieved by distributing all windings evenly around the periphery of core.

  Accuracy Class:

In the case of metering CT s, accuracy class is typically, 0.2, 0.5, 1 or 3. This means that the errors have to be within the limits specified in the standards for that particular accuracy class. The metering CT has to be accurate from 5% to 120% of the rated primary current, at 25% and 100% of the rated burden at the specified power factor. In the case of protection CT s, the CT s should pass both the ratio and phase errors at the specified accuracy class, usually 5P or 10P, as well as composite error at the accuracy limit factor of the CT.

  Instruments Security Factor

To protect the instruments and meters from being damaged by high currents during fault conditions, a metering core must be saturated typically between 5 and 20 times the rated currents. The rated Instrument Security Factor (FS) indicates the over current as a multiple of the rated current at which the metering core will saturate. It is thus limiting the secondary current to FS times the rated current. ISF for GR meter CTS are deigned to less than 5. The safety of the metering equipment is greatest when GR CTS are used.


Every CT is tested in accordance with IS-2705 for ratio accuracy and phase angle errors with microprocessor based automatic instrument transformer test set with facilities for an automatic print out of test results.

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