What Are The Grounding Protection Features Of A Distribution Box?

Grounding protection for distribution boxes is a systematic safety project, not a single protective measure. It comprises multiple lines of defense, including different types of protection, technical specifications, grounding systems, construction standards, and inspection and maintenance. Its main purpose is to ensure personal safety, protect equipment stability, and prevent fires.

1. Protective Grounding (PE Wire Connection)

All normally non-energized metal parts of the distribution box, such as the metal casing, door panels, mounting brackets, and metal conduits, must be reliably connected to the grounding terminal block using a PE wire (yellow-green double-color wire), and then led to the main grounding electrode.

Function: Prevents the metal casing of the distribution box from becoming energized due to insulation failure, thus preventing electric shock accidents. When the casing becomes energized, a short circuit is formed in the grounding loop, prompting the circuit breaker or fuse to quickly disconnect the power supply.

2. System Grounding

The transformer neutral point is directly grounded, providing a zero-potential reference for the system. In the main distribution box, the neutral wire is connected to the PE wire and grounded; in sub-distribution boxes, the two must be separated.

Function: Ensures stable system operation and forms a fault current loop in the event of a single-phase ground fault.

3. Repeated Grounding
In a TN system, in addition to the grounding of the transformer at the power supply end, a secondary grounding is performed at a key point on the PE or PEN line.

Purpose: This reduces the PE line's potential to ground, lowering the risk of electric shock during neutral or ground faults and enhancing the sensitivity of protective devices.

4. Equipotential Bonding

Install an equipotential bonding terminal block near the distribution box to connect all components, including the PE busbar, metal pipes, and reinforcing steel. This is especially crucial in damp locations such as bathrooms and swimming pools.

Function: Eliminates potential differences between different metal components around the distribution box, preventing electric shock accidents caused by lightning strikes or fault voltage introduction.

5. Lightning Protection Grounding

The grounding terminal of the surge protector installed in the distribution box must be connected to the PE busbar or an independent lightning protection grounding device using the shortest and thickest possible wire. The grounding resistance is typically required to be ≤ 4Ω (≤ 10Ω for independent lightning protection).

Function: Quickly discharges the large current generated by lightning to the ground through down conductors and grounding devices to protect the distribution box and its internal equipment from lightning damage.

6. Ground Fault Protection (Residual Current Circuit Breaker)

Install a residual current operated circuit breaker (RCCB). When the leakage current reaches a threshold (e.g., 30mA), it automatically cuts off the power supply, serving as an important supplement to the grounding system.

Function: It can automatically cut off the power supply when a ground fault occurs. It is not a grounding method, but a protective measure that works in conjunction with grounding.