The secondary side of the current transformer in the power distribution cabinet needs to be grounded

In the power distribution cabinet, the **secondary side of the current transformer (CT) must be reliably 

grounded**, which is an important measure to ensure personal safety, stable operation of equipment, 

and measurement accuracy. The following are the reasons, methods, and related key points for its grounding:

### I. Core Reasons for Secondary Side Grounding

#### 1. Prevent High Voltage from Intruding into the Secondary Circuit

- **Risk of Insulation Breakdown**:

There is **capacitive coupling** between the primary side (high voltage side) and the secondary side 

(low voltage side) of the current transformer. When the insulation of the primary side is damaged, high

 voltage may be coupled to the secondary side through capacitance, resulting in a **dangerous high

 voltage** in the secondary circuit (for example, in a 10kV system, thousands of volts may intrude into 

the secondary side).

**Function of Grounding**: Fix the potential of the secondary side to the earth potential (0V) to prevent 

the high voltage from endangering the safety of operators and equipment.

#### 2. Eliminate Electromagnetic Interference (EMI)

- **Impact of Environmental Interference**:

The large current busbars in the power distribution cabinet will generate an alternating magnetic field. If 

the secondary circuit is not grounded, an **interference voltage** may be induced in the secondary winding,

 affecting the accuracy of measuring instruments (such as ammeters, power meters) or protective devices

 (such as relays).

**Function of Grounding**: Provide a low-impedance shielding path to introduce the interference current 

into the earth and ensure the stability of the secondary signal.

YouTube：https://youtube.com/shorts/dgcmKZEAnvc 

III. Differences from the Grounding of the Voltage Transformer (PT)

3. Ensure the Reliable Operation of the Protective Device

- **Grounding Fault Scenario**:

When a single-phase grounding fault occurs in the system, the zero-sequence current needs to be transmitted 

to the protective device through the CT secondary circuit. If the secondary side is not grounded, it may lead to 

the **failure of zero-sequence current measurement** and the protective device may refuse to operate.

### II. Grounding Methods and Requirements

#### 1. Principle of Single-point Grounding

- **Only One Grounding Point is Allowed**:

The secondary circuit should be grounded at a single point near the outlet end of the secondary winding of 

the transformer or at the grounding busbar of the power distribution cabinet to avoid forming a **grounding 

loop**.

▶ Example of an incorrect practice: Grounding at both the CT terminal and the instrument terminal at the same

 time may generate a circulating current due to the potential difference between the two places, interfering

 with the measurement.

#### 2. Selection of Grounding Location

- **Common Wiring Methods**:

  - Connect the **S2 terminal** of the CT secondary winding directly to the **PE grounding busbar** of the 

power distribution cabinet (as shown in Figure 1);

  - If the CT is installed inside the bushing of the busbar, the two ends of the **secondary cable shielding layer*

* can be grounded (it needs to be coordinated with the single-point grounding design).

#### 3. Specifications of Grounding Wires

- **Cross-sectional Area Requirements**:

The grounding wire on the secondary side should be a **yellow-green double-colored copper wire**, and the 

cross-sectional area should be ≥ **4mm²** (as specified in the national standard GB 50169).

▶ If it is used in the protection circuit, it needs to meet the thermal stability requirements (for example, it will

 not melt under a system fault current of 100A).

### III. Differences from the Grounding of the Voltage Transformer (PT)

### IV. Common Problems and Troubleshooting

#### 1. Hazards of Grounding Failure

- **Risk of Electric Shock**: When a high voltage appears in the secondary circuit, people may get an electric 

shock when touching the CT terminals or instruments;

- **Maloperation/Refusal to Operate of the Protection**: Interference signals may cause the relay to malfunction,

 or the zero-sequence current cannot be detected.

#### 2. Troubleshooting Methods

1. **Conductivity Test**:

Use the resistance range of a multimeter to measure the resistance between the S2 terminal of the CT secondary

 winding and the grounding busbar, which should be **≤0.1Ω** (good contact).

2. **Voltage Measurement**:

Disconnect the secondary load and measure the voltage between the S2 terminal and the earth. It should 

normally be **≈0V**; if there is a voltage, it may be due to loose grounding or insulation damage.

3. **Insulation Test**:

Use a 500V megohmmeter to measure the insulation resistance between the primary side and the secondary

 side, which should be **≥100MΩ** (to prevent high voltage caused by capacitive coupling).

### V. Relevant Standards

- **GB 50062-2008 "Code for Design of Relaying Protection and Automatic Devices for Electric Power

 Installations"**:

Article 15.0.12 stipulates that the CT secondary circuit must have a grounding point and be reliably connected

 to the protective grounding system.

- **IEC 60044-1 "Current Transformers"**:

It requires that the current-carrying capacity of the grounding wire of the secondary winding should be at

 least 120% of the rated secondary current.

### Conclusion

The grounding of the secondary side of the current transformer is a **mandatory safety requirement**. Its 

core function is to **fix the potential of the secondary circuit to the earth potential** to prevent the intrusion

 of high voltage and electromagnetic interference. In actual operation, the principle of **single-point grounding

** should be strictly followed, and the reliability of the grounding should be checked regularly to ensure the 

safe operation of the power system.