Requirements for the grounding wire sheath of distribution boxes

The grounding wire sheath of distribution boxes must be properly sized, securely connected to the box and main grounding system, and maintain a low resistance, typically below 0.1 ohm, to ensure safety and electromagnetic compatibility.

Wire Sizing and Material

• For distribution boxes, the grounding wire should be at least 6 mm² in most markets, while in the US a 10 AWG (≈5.26 mm²) wire is commonly used .
• When connecting the mounting plate to spindles or other internal components, larger cross-sectional areas may be required depending on cable length and number of spindles. For example, a 16 mm² wire is recommended for longer tool cables, and in the US, a 6 AWG (≈13.29 mm²) wire ensures proper grounding .
• Copper conductors are preferred for their high conductivity and corrosion resistance. The grounding wire should match or exceed the cross-sectional area of the metal sheath or armor of the cables it connects .

Connection Methods

• Grounding wires must be attached to threaded studs or grounding bolts on the distribution box housing and connected to the main grounding point or mounting plate .
• Contact surfaces should be clean, free of oxidation, paint, or powder coatings, and may use tooth-lock washers to ensure reliable electrical contact .
• Shielding layers of cables should be grounded at both ends only if necessary; otherwise, grounding at one end is preferred to avoid ground loops, especially for long cable runs .

Resistance Requirements

• The total grounding resistance between all system parts should be less than 0.1 ohm to ensure safety and proper fault current dissipation .
• For cable sheaths in high-voltage systems, grounding resistance should generally not exceed 4 ohms, and separate grounding electrodes should be spaced 3–5 meters apart from other grounding points .

Installation Best Practices

• Grounding points should be parallel backups, not sequential links, to prevent a single point of failure .
• Short grounding wire lengths (ideally under 20 cm for shield terminations) reduce inductive effects at high frequencies .
• Regular testing of grounding resistance is recommended, as environmental factors like soil type, moisture, and corrosion can affect performance .
• When cables pass through zero-sequence current transformers, the grounding wire should be insulated until after the transformer to prevent unintended current paths .

Summary

Proper grounding of distribution boxes involves correct wire sizing, secure connections, low resistance, and attention to shielding and environmental factors. Following these requirements ensures personnel safety, equipment protection, and compliance with electrical standards .