How to determine if the polarity of a current transformer is correct?
Dec 15, 2025| 1. DC Method
Connect the positive terminal of a 1.5V dry cell battery to the primary side P1 and the negative terminal to P2; connect the secondary side S1 to the positive terminal of a pointer-type multimeter and S2 to the negative terminal.
If the pointer swings positive when the circuit is closed and reverses when it is opened, the polarity is correct (P1 and S1 are the same polarity terminals).
2. AC Method
Apply low-voltage AC power to the primary side and connect a voltmeter to the secondary side. If U1 + U2 ≈ the vector sum of the voltages of the two windings, the polarity is correct.
Use a phase meter to measure; a phase difference close to 0° indicates the same polarity, and 180° indicates opposite polarity.
Instrument Method
Use a current transformer calibrator to directly test the polarity, following the instrument's instructions.
Precautions
The polarity markings on the nameplate must be clearly understood (e.g., P1/S1 are the same polarity terminals).
Ensuring the secondary circuit is not open during testing to avoid high-voltage hazards.
Incorrect polarity can cause the meter to reverse or the protection system to malfunction.
