Grounding of the fiber optic junction box

Fiber optic junction boxes with metallic components must be grounded if exposed to contact with power conductors, using a bonding conductor to the building's grounding electrode system, or alternatively, metallic members can be interrupted with an insulating joint. When Grounding is RequiredGrounding is only required for fiber optic cables that contain metallic components (conductive fibers or strength members) and are exposed to potential contact with electric power conductors.

According to NE

C 770. 93, optical fiber cables entering a building or terminating outside must either be: Grounded per NEC 770. 100, orInterrupted by an insulating joint or equivalent device to prevent current flow through metallic members.

If the metallic members are not exposed to power conductors, grounding is generally not necessary. Grounding Conductor RequirementsNEC 770. 100 specifies that the bonding conductor should connect the metallic components of the fiber optic cable to the building's grounding electrode system.

While the minimum conductor size allowed is 14 AWG, manufacturers typically recommend a 6 AWG copper conductor to ensure both grounding and bonding compliance with ANSI/TIA/EIA-J-STD-607. The conductor should be run in as straight a line as practicable and be as short as possible, generally less than 20 feet, to maintain continuity and minimize potential differences.

Connection Points

The bonding conductor can be attached to the Telecommunications Main Grounding Busbar (TMGB), Telecommunications Grounding Busbar (TGB), or other accessible grounding points in the building. If a Telecommunications Bonding Backbone (TBB) is present, it serves to equalize potential differences but should not be the sole path for ground fault current.

Practical Considerations

Indoor junction boxes: Metallic components should be bonded to the building's grounding system if exposed to power conductors. Outdoor terminations: Grounding or insulating joints should be installed as close as possible to the point of cable entrance or termination. Non-conductive fiber cables: No grounding is required unless a metallic member is present for strength or locator purposes.

Insulating joints: These can replace grounding if they effectively interrupt metallic continuity, preventing stray currents from traveling through the fiber cable.

Summary

Identify cable type: Conductive vs. non-conductive. Check exposure: Only cables exposed to power conductors require grounding. Use proper conductor: 6 AWG copper is preferred; minimum 14 AWG allowed.

Bond to grounding system: TMGB, TGB, or other approved electrode. Consider insulating joints: Can eliminate the need for grounding if properly installed. Following these practices ensures compliance with NEC 770.

93 and 770. 100, maintains safety, and protects both personnel and equipment from potential electrical hazards.