How to check if the secondary side grounding of a current transformer is good?

I. Appearance and Wiring Inspection (Preliminary Check)
First, conduct a visual inspection to confirm that the basic wiring meets the requirements:

Confirm that the secondary circuit has a grounding point, usually located on the terminal block of the terminal box or protection panel. The S2 (or K2) terminal should be connected to a yellow-green bicolor grounding wire.

Check that the grounding wire is a copper conductor with a cross-sectional area of ​​not less than 4mm², and that there are no breaks, corrosion, or looseness.

Verify that the entire circuit has only one grounding point, avoiding simultaneous grounding in the terminal box and control room to prevent the formation of ground loops that could affect the accuracy of the protection device's operation.

II. Insulation Resistance Test (Core Testing Method)

This is the standard method for determining whether the grounding system is normal:

Temporarily disconnect the grounding connection of the S2 terminal at the terminal box.

Use a 1000V megohmmeter to measure the insulation resistance of the secondary circuit to ground.

If the resistance is greater than 1MΩ, the circuit insulation is good; if it is close to zero, there may be multiple grounding points or short circuit faults.

After the test, restore the original grounding and confirm, in conjunction with the drawings, that there are no other unexpected grounding points. ✅ Judgment Criteria: Insulation qualified and only one grounding point → Grounding system normal.

III. Terminal Method for Detecting Grounding Point Location (Professional Verification)
Used to determine if the grounding point is genuine and effective, and if its location is correct:

Disconnect the connection terminal at the secondary circuit terminal of the transformer to remove the existing grounding point.

Apply a small current from the polarity terminal (K1), grounding the K2 terminal to form a loop.

Observe the voltage change trend:

If the grounding point is on the K2 side, the loop includes the secondary coil, exhibiting high impedance, requiring a higher voltage to establish current;

If the grounding point is on the K1 side, the coil is not connected, and the loop exhibits low impedance.

The difference in voltage amplitude can determine if the grounding point location is correct, verifying the consistency between polarity and grounding.

IV. I-V/Ampere Characteristic Method for Auxiliary Judgment (Suitable for Acceptance Testing or Fault Troubleshooting)
After disconnecting the secondary circuit and removing the grounding point at the outdoor terminal box, use an volt-ampere characteristic meter to apply voltage to the winding and test the excitation curve. If the inflection point voltage is significantly lower, it may indicate an inter-turn short circuit or grounding abnormality; this can be combined with insulation testing for comprehensive judgment.

V. Monitoring During Operation (Indirect Judgment Methods)

For systems already in operation, the grounding status can be indirectly determined through the following methods:

Check if the protection device has alarm messages such as "CT disconnection" or "grounding abnormality".

In a star-connected system, use a clamp meter to measure the neutral current. An abnormal current may indicate multiple grounding points.

Use a residual current detector to check the shunt current on the grounding wire to determine if there are parasitic loops or abnormal grounding paths.

VI. Common Problems and Handling Suggestions

 Grounding wire detachment or loose connection: Immediately reconnect and tighten to ensure reliable contact.

 Multiple grounding points: Locate and remove redundant grounding points to ensure "one and only one" grounding point.

 Incorrectly grounded S1: Should be grounded to S2 (K2) to avoid incorrect high-voltage release path when open-circuited.

 Incorrectly grounded low-voltage CT: For systems of 380V and below, due to the large insulation margin, the secondary side does not need to be grounded; follow the specifications.