Distribution box with secondary double-sided grounding

A distribution box with secondary double-sided grounding ensures safety and reliability by connecting both the box and its components to a low-resistance earth path, typically using two separate grounding connections.

Grounding Principles

Double-sided grounding involves connecting the distribution box to the main grounding system and also providing a secondary grounding path for internal components, such as mounting plates or connected equipment. This approach reduces the risk of electric shock, protects equipment from transient overvoltages, and stabilizes system voltage levels .

Typical Implementation

1. Primary Ground Connection: Attach a ground wire from a threaded stud at the bottom of the distribution box to the mounting plate. This ensures the box itself is safely grounded .
2. Secondary Ground Connection: Connect a second ground wire from the mounting plate to the central factory or substation grounding point. This creates a parallel path for fault currents, improving safety and reducing ground resistance .
3. Wire Sizing: In the US, a 10 AWG (5.26 mm²) wire is typically used, while in other markets a 6 mm² wire is standard. For connections between mounting plates and equipment, larger wires (e.g., 16 mm² or 6 AWG) may be required depending on cable length and number of connected devices .
4. Resistance Requirements: The total ground resistance between all system parts should be less than 0.1 Ohm to ensure effective fault current dissipation .

Benefits of Double-Sided Grounding

• Personnel Safety: Provides a low-resistance path for fault currents, reducing the risk of electric shock .
• Equipment Protection: Protects transformers, switchgear, and connected devices from lightning strikes, transient overvoltages, and ground faults .
• System Stability: Maintains voltage stability across the distribution system, ensuring proper operation of protective devices like circuit breakers and relays .
• Fault Detection: Facilitates rapid identification and isolation of faults, improving reliability and minimizing downtime .

Considerations in Secondary Distribution Systems

In secondary selective or spot network systems, each transformer may supply the entire load, and grounding must account for multiple parallel paths to prevent reverse power flow and ensure reliable operation . Proper grounding also supports high-impedance or solidly grounded configurations, depending on system design, to manage fault currents effectively .

Summary

A distribution box with secondary double-sided grounding uses two separate grounding connections—one to the box or mounting plate and another to the central grounding system—to ensure safety, equipment protection, and system reliability. Correct wire sizing, low-resistance connections, and adherence to grounding standards are essential for effective implementation .