The Phenomenon of Equipotential in the Distribution Box
In the distribution box, the **equipotential phenomenon** refers to connecting all the metal components inside
the distribution box (such as the enclosure, PE wire, grounding busbar, metal brackets, etc.) into a **whole with
equal potential** through conductors, so as to eliminate the potential difference between different metal components
and ensure personal safety and the stable operation of equipment. The following are its specific manifestations,
principles, and related key points:
### I. The Essence of the Equipotential Phenomenon
1. **Zero Potential Difference**
Under normal circumstances, the metal enclosure, PE wire (protective grounding wire), grounding busbar, metal
mounting brackets, etc. inside the distribution box are connected through **equipotential connection conductors**
(such as copper busbars, braided wires), making their **potentials equal** (theoretically, the voltage difference is 0V).
2. **Safety Principle**
When a leakage occurs inside the distribution box (such as a phase wire coming into contact with the enclosure),
the potential of the enclosure will rise. However, since all metal components are at the same potential, when a person
touches different metal components simultaneously, there will be no **straddle voltage** formed, thus avoiding electric
shock accidents.
YouTube：https://youtube.com/shorts/ynr62lf9Zpo
### II. Specific Manifestations of Equipotential in the Distribution Box
#### 1. Normal State
- **Measurement Phenomenon**:
Use the DC voltage range of a multimeter to measure the voltage between the enclosure of the distribution box, the
PE wire, and the grounding busbar. The **voltage should be close to 0V** (usually less than 1V).
▶ Example: Voltage between enclosure → PE wire = 0V, Voltage between grounding busbar → metal bracket = 0V.
- **Connection Methods**:
- The metal enclosure of the distribution box is directly connected to the PE busbar through **grounding bolts**;
- The internal metal brackets and mounting rails are conducted with the box body through **screws + spring washers**;
- The PE busbar and the grounding busbar are short-circuited through **copper busbars or thick wires**
(as shown in the figure).
#### 2. Fault State
- **Leakage of Phase Wire to the Enclosure**:
If the insulation of the phase wire is damaged and comes into contact with the enclosure, the potential of the enclosure
= phase voltage (such as 220V). However, since the enclosure and the PE wire are at the same potential, the **leakage
current returns to the neutral point of the power supply through the PE wire**, triggering the residual current device
(RCD) to trip.
▶ At this time, the measured voltage between the enclosure and the neutral wire (N) ≈ 220V, but the voltage between
the enclosure and the PE wire is still 0V.
### III. Key Components of Equipotential Connection
| Component Name | Function | Example Picture |
| PE Busbar | Concentrally connect the PE wires of all equipment | [PE Busbar](Example Picture 1) |
| Grounding Busbar | Connect to the grounding electrode of the earth | [Grounding Busbar](Example Picture 2) |
| Equipotential Bonding Wire | Copper busbar, braided copper wire, or yellow-green double-colored wire | [Bonding Wire](Example Picture 3) |
| Metal Enclosure | The distribution box itself, which needs to be directly conducted with the PE busbar |
### IV. Differences between Equipotential and Grounding
| Item | Equipotential Connection | Grounding |
| Purpose | Eliminate the potential difference between internal metal components | Establish a low-impedance connection with the earth |
| Connection Object | Between metal components inside the box | Between components inside the box and the earth |
| Voltage Performance | Voltage between components ≈ 0V | Voltage between components and the earth ≈ 0V |
### V. Common Problems and Troubleshooting
1. **Manifestations of Equipotential Failure**
- The measured voltage between the enclosure and the PE wire > 1V;
- When there is a leakage, the RCD does not trip, and there is a risk of electric shock when a person touches the enclosure.
2. **Troubleshooting Methods**
- **Visual Inspection**: Check whether the connection screws are loose and whether the copper busbar is oxidized;
- **Conductivity Test**: Use the resistance range of a multimeter to measure the resistance from the enclosure to the
PE busbar, which should be < 0.1Ω;
- **Simulated Leakage Test**: Disconnect the phase wire from the load, artificially short-circuit the phase wire and the
enclosure, and observe whether the RCD acts.
### VI. Specifications and Standards
- **National Standard Requirements**:
The GB 50169-2016 "Code for Construction and Acceptance of Grounding Devices" stipulates that the enclosure of the
distribution box must be **reliably connected with equal potential to the PE wire**, and the grounding resistance ≤ 4Ω.
- **Household Scenarios**:
The equipotential terminal box in the bathroom needs to be connected to the PE busbar of the distribution box to achieve
"whole-house equipotential".
### Conclusion
The equipotential phenomenon in the distribution box is the **core design of electrical safety**. By "short-circuiting"
the potentials of all metal components, it avoids electric shock and equipment damage. In daily maintenance, the
reliability of the connection should be focused on to ensure that the equipotential system is always effective.
