Compliance Guide Grounding and Bonding for Metal Cable Trays

In electrical engineering practice, the grounding and bonding of metal cable trays remains a crucial safety consideration. Whether these measures are necessary—and how to properly implement them—directly impacts electrical safety and system reliability. This analysis examines the grounding and bonding requirements for metal cable trays across various applications, providing clear guidance for electrical engineers and technicians to ensure compliance with relevant standards.

Consider this scenario: An unexpected electrical fault sends current surging through a metal cable tray. Without proper grounding, this tray becomes a hidden hazard, potentially causing electric shock. But are grounding or bonding always mandatory for metal cable trays? The answer depends on multiple factors including the tray's purpose, the type of cables it carries, and its installation environment.

To properly assess requirements, we must first clarify these essential terms:

• Grounding (Earthing): Connecting electrical equipment's metal enclosure or conductive parts to earth, creating a low-impedance path for fault current to flow, enabling protective devices to disconnect power and prevent electric shock.
• Bonding: Interconnecting metal parts that might develop potential differences or introduce external potentials, equalizing their voltages to reduce shock risk.
• Exposed-Conductive-Part: Touchable conductive parts of equipment that normally aren't energized but may become live during faults (e.g., metal enclosures).
• Extraneous-Conductive-Part: Conductive elements not part of the electrical installation but capable of introducing potential (e.g., metal pipes, building structures).

According to IEC standards, whether metal cable trays need grounding depends on if they constitute exposed-conductive parts:

If designed to function as protective conductors (PE) carrying fault current, trays must be reliably grounded. They must meet PE requirements including adequate cross-section for expected fault current and proper electrical continuity, as specified in IEC 60364-5-54.

For trays merely supporting cables, requirements vary by cable type:

• Metal-Sheathed Cables: With cables like MICC (mineral insulated), fault current primarily flows through the cable's metal sheath. The tray typically isn't considered exposed-conductive and usually doesn't require grounding—though the sheath must be properly grounded.
• Non-Metallic Cables: For PVC or XLPE insulated cables providing protection equivalent to Class II equipment, fault current won't flow through the tray even during insulation failure. Thus, grounding is generally unnecessary per IEC 60364-4-41.

Bonding depends on whether trays might introduce external potentials:

Most internal building trays, having no earth contact, don't require bonding.

Trays extending from outdoors or contacting earth (e.g., buried sections) may introduce external potentials. These require bonding to the main earthing terminal (MET) to prevent hazardous voltage differences.

Additional factors may influence requirements:

• Corrosive Environments: Require corrosion-resistant materials/coatings and regular inspection of grounding connections.
• EMC Requirements: Sensitive facilities (hospitals, labs) may need trays grounded for electromagnetic shielding.
• Lightning Protection: Trays may need integration with lightning protection systems per IEC 62305.

Proper execution requires:

• Selecting appropriately sized grounding/bonding conductors (typically copper/aluminum with corrosion protection)
• Ensuring low-resistance connections (via bolts, welding) with clean surfaces and conductive paste
• Conducting periodic inspections for corrosion, loose connections, or resistance changes

Key reference documents include:

• IEC 60364 series (low-voltage electrical installations)
• IEC 61557 series (protective measure testing equipment)
• IEC 62305 series (lightning protection)
• National standards like GB 50054 (China's low-voltage design code)

Grounding and bonding requirements for metal cable trays aren't universal but require case-by-case evaluation considering function, cable types, environment, and applicable standards. This careful approach ensures electrical safety for personnel and equipment.