Current transformers

Insulation resistance between primary to secondary and earth may be measured with an insulation resistance tester such as a meggar.Insulation resistance of secondary may be measured by using a 1 kV meggar only. Use of 5 kV meggar for this test may damage the insulation of the secondary.Primary current injection set maybe used for ascertaining the ratio of the C.Ts. It may be noted however, that the recorded ratio may not be exactly same as the ratio indicated on the label of the C.T. This is because of the difficulty in taking simultaneous readings of the ammeters on primary and secondary side.

Polarity test may be carried out to check the polarity of each core of the C.T., by using a 1.5 V dry battery cell and a center zero D.C. ammeter. It may please be noted that this test need not be and should not be carried out on core balance C.Ts as the conduction of this test may leave the core of the C.T. slightly magnetized which may result into improper operation of the C.B.C.T. unless it is fully demagnetized. It may be appreciated that the operating point for C.B.C.Ts. on the magnetization characteristic is much below the ankle point of the curve and the performance is likely to be affected due to slight remnant magnetization of the core. Besides this, the relays used with C.B.C.Ts. are not polarity sensitive.