Grounding inside cable tray shaft

Cable trays inside a shaft must be properly grounded and bonded to provide a low-impedance path for fault currents, ensuring safety and compliance with NEC standards.

Key Principles of Grounding Cable Trays

1. Use of Cable Tray as Equipment Grounding Conductor (EGC): Metallic cable trays can serve as the EGC if they meet the minimum cross-sectional area requirements specified in NEC Table 392.60(A) and are properly bonded across sections . This allows the tray itself to carry fault currents safely to the grounding system. 2. Continuous Electrical Path: All sections of the cable tray must be electrically continuous. Bonding jumpers, clamps, or splice plates should be used to bridge gaps between tray sections, ensuring a reliable path for fault currents . Even if the tray is mechanically continuous, electrical continuity must be verified. 3. Grounding Conductors in the Tray: If individual cables have their own EGCs, these should be bonded to the tray at each or alternate sections using grounding clamps. This not only provides electrical continuity but also mechanically secures the conductors to prevent displacement under fault conditions . 4. Material Considerations:

• Avoid bare copper conductors in aluminum trays to prevent electrolytic corrosion; insulated conductors with exposed ends at bonding points are preferred .
• Steel trays should not be used as the sole EGC for circuits with ground-fault protection above 600 A, and aluminum trays above 2000 A . 5. Safety and EMI Protection: Proper grounding reduces the risk of electric shock, fire hazards, and electromagnetic interference (EMI) in sensitive environments. In shafts containing both power and signal cables, separation or shielding may be necessary to prevent interference .

Installation Best Practices in Shafts

• Plan Grounding Points: Identify locations where the tray will be bonded to the building grounding system.
• Parallel Conductor Placement: Lay grounding conductors parallel to the tray where possible, connecting cable shields or screens at both ends .
• Bonding to Equipment: Ensure the tray is bonded to enclosures, conduit, or other metallic raceways entering or exiting the shaft.
• Inspection and Testing: Verify continuity and low resistance of the grounding path after installation to ensure compliance and safety.

Regulatory Compliance

All grounding practices must comply with NEC Article 250 and NEC Article 392 for cable trays. This includes proper bonding, conductor sizing, and maintaining a continuous low-impedance path for fault currents . Following these standards ensures both personnel safety and system reliability. By adhering to these principles, grounding inside a cable tray shaft can be safely implemented, providing protection against electrical faults, minimizing EMI, and ensuring compliance with electrical codes.