How to determine if a current transformer is installed correctly
May 20, 2026| I. Check the primary side installation direction
Current must flow in from terminal P1 and out from terminal P2. P1 should face the power supply side (such as the transformer or incoming line), and P2 should face the load side.
Reversed connection will cause the energy meter to reverse, and the protection device to malfunction or fail to operate.
The direction can be confirmed by checking the equipment nameplate or the primary cable routing.
II. Verify correct polarity (core step)
Use the following two common methods to determine polarity:
1. DC method (battery instantaneous method)
Operation: Connect P1 to the positive terminal of the battery and P2 to the negative terminal on the primary side; connect S1 to the positive terminal of the pointer-type milliammeter and S2 to the negative terminal on the secondary side.
Observation: At the moment the switch is closed, the pointer deflects in the positive direction → P1 and S1 are the same polarity terminals, and the polarity is correct (subtractive polarity).
The pointer deflects in the reverse direction → the polarity is incorrect.
This method is clear in principle, low in cost, and suitable for rapid on-site testing.
2. AC Method (Phase Meter or Transformer Ratio Tester Method): Directly measure the phase difference between the primary and secondary sides using a transformer ratio polarity tester.
If the phase difference is close to 0° → same polarity (correct); if it is 180° → opposite polarity (incorrect).
This method is highly accurate and easy to operate, suitable for batch testing and relay protection commissioning.
III. Check Secondary Side Wiring and Grounding
S1 is connected to the instrument's input terminal. S2 must be reliably grounded, and only one grounding point is allowed.
Multiple grounding points will create circulating currents, leading to malfunctions in the protection system.
The grounding wire should be connected to the dedicated grounding copper busbar in the control room, not to the cabinet or bolts.
IV. Confirm No Open Circuit in the Secondary Circuit
Open circuits on the secondary side are strictly prohibited during operation; otherwise, thousands of volts of high voltage will be induced, endangering safety.
Check that all wiring terminals are secure and free from looseness or oxidation.
Before replacing an instrument, the S1-S2 terminals must be short-circuited first.
V. Dynamic Verification After Power-On
After commissioning, observe whether the readings of the electricity meter and ammeter are normal.
If zero current, negative values, or three-phase imbalance occur, immediately disconnect the power and check the wiring and polarity.
A clamp meter can be used to compare whether the ratio of the primary and secondary currents matches the turns ratio.
