Requirements for grounding materials of underground electrical distribution boxes

Grounding materials for underground electrical distribution boxes must include copper conductors, copper-weld ground rods, and proper connectors, sized and installed to achieve low-resistance grounding and ensure safety.

Grounding Conductors and Rods

For underground distribution systems, bare copper conductors are typically used. For medium voltage (MV) grounding, a 70 mm² bare copper conductor is recommended, while for low voltage (LV) grounding, a 35 mm² bare copper conductor is standard. Ground rods should be 16 mm in diameter and 2400 mm long, or alternatively two rods of 1200 mm each, made of copper-weld steel to ensure durability and conductivity.

Installation and Configuration

Rod Placement: Ground rods should be installed vertically and spaced appropriately to achieve a target resistance of 25 ohms or less. If the desired resistance is not achieved with three rods, additional rods may be added, up to a maximum of eight, or a 50-foot deep ground rod well may be used. Connection: All metal parts of the distribution box, including enclosures, panels, and conduits, must be connected to the grounding system using bolted connectors, grounding lugs, or bushings.

Each feeder and branch circuit should include a separate insulated green ground conductor connected from the panel ground bus to all connected equipment. Bonding: All metal piping, conduit, and structural steel in the vicinity should be bonded to the grounding system. Expansion or deflection fittings require bonding jumpers to maintain continuity.

Material Considerations

Corrosion Resistance: Materials must withstand soil conditions and mechanical stress. Copper and copper-weld rods are preferred for their resistance to corrosion and long-term reliability. Mechanical Protection: Grounding conductors should be protected from physical damage, typically by routing through conduits or using protective sleeves where exposed.

Testing and Verification

Continuity Tests: All grounding conductors must be tested for electrical continuity. Resistance Testing: The grounding system should be tested with an ohmmeter or megger to ensure the resistance to earth meets the 25-ohm requirement.

Summary

In summary, underground electrical distribution boxes require copper conductors and copper-weld ground rods, properly sized and installed to maintain low-resistance grounding. All metal parts must be bonded, corrosion-resistant materials should be used, and the system must be tested to ensure safety and compliance with electrical standards.