The polarity of a CT is determined by the direction in which the coils are wound around the transformer core (clockwise or counterclockwise) and by how the leads are brought out of the CT case. All current transformers are subtractive polarity and should have the following designations to visually identify the direction of current flow:

H1 - primary current, line facing direction

H2 – primary current, load facing direction

X1 - secondary current

The polarity marks on a CT designate the relative instantaneous directions of the currents. The polarity test proves that the predicted direction of secondary CT current (leaving) is correct for a given direction of primary current (entering).

Taking care to observe proper polarity is important when installing and connecting current transformer to power metering andprotective relays. At the same instant of time, that the primary current is entering the primary terminal the corresponding secondary current should be leaving the similarly marked secondary terminal.

A CT under test is assumed to have correct polarity if instantaneous current direction for primary and secondary current is opposite to each other. CT Polarity is critical when CT's are being used together in single-phase or three-phase applications.

Most modern day CT test equipment is capable of performing the ratio test automatically using a simplified test lead setup and will display polarity as correct or incorrect. CT polarity is verified manually by utilizing a 9V battery and analog voltmeter with the following test procedure:

CT Polarity Test Procedure

Disconnect all power prior to testing and connect the analog voltmeter to the secondary terminal of the CT to be tested. The positive terminal of the meter is connected to terminal X1 of the CT while the negative terminal is connected to X2.

Run a piece of wire through the high side of the CT window and shortly make contact with the positive end of the 9-volt battery to the H1 side (sometimes marked with a dot) and the negative end to the H2 side. It is important to avoid continuous contact, which will short circuit the battery.

If polarity is correct, the momentary contact causes a small deflection in the analog meter in the positive direction. If the deflection is negative, the polarity of the current transformer is reversed. The terminals X1 and X2 need to be reversed and the test can be carried out.

Note: Polarity is not important when connecting to ammeters and voltmeters. Polarity is important only when connecting to wattmeters, watt-hour meters, varmeters, and induction-type relays. To maintain polarity, the H1 side of the CT must be toward the source of power; then the X1 secondary terminal is the polarity connection.