The grounding resistance of a cable tray system should ideally be 4 ohms or less to ensure safety and effective fault current conduction.

Importance of Grounding

Cable tray grounding provides a safe path for stray electrical currents to flow into the earth, protecting personnel and equipment from electric shock, fire, or equipment damage . Metallic cable trays can serve as the Equipment Grounding Conductor (EGC), but they must be properly bonded and electrically continuous to meet safety standards .

NEC Requirements

According to the National Electrical Code (NEC):

• All metallic cable trays must be grounded as required in Article 250.96, regardless of whether they are used as an EGC .
• Cable trays may be used as the EGC in qualifying facilities, but the installation must ensure electrical continuity and sufficient capacity to safely conduct fault currents .
• NEC Section 250-51 specifies that the grounding path must be permanent, electrically continuous, and have low impedance to limit voltage to ground and facilitate operation of protective devices .

Resistance Guidelines

• Target resistance to ground: Ideally 4 ohms or less for effective grounding .
• High resistance can impair the system's performance, potentially delaying fault clearing and increasing the risk of electrical hazards .
• For aluminum trays, a bare copper conductor should be avoided due to potential electrolytic corrosion; insulated conductors with proper bonding connections are recommended .

Installation Practices

• Bonding: Use grounding clamps to bond EGCs to each or alternate tray sections to maintain continuity and mechanical stability under fault conditions .
• Parallel EGCs: Running a single conductor cable as a common EGC in parallel with the tray can enhance safety and reduce impedance .
• Material considerations: Ensure connectors are compatible with tray material (e.g., tin or zinc-plated for aluminum) to prevent corrosion .
• Inspection: Regularly check for loose connections, corrosion, or damage that could increase resistance to ground.

Summary

A properly grounded cable tray system ensures low-resistance, continuous paths for fault currents, meeting NEC standards and protecting both personnel and equipment. Maintaining a resistance of 4 ohms or less, using proper bonding techniques, and selecting compatible materials are key to achieving a safe and reliable grounding system .