Does a Metallic Cable Tray Require Earthing or Bonding?
The necessity of earthing or bonding a metallic cable tray depends on its role in the electrical system. The tray may require earthing, bonding, or neither, depending on whether it qualifies as an exposed-conductive-part or an extraneous-conductive-part.
Exposed-Conductive-Part:
A conductive part of equipment that can be touched, is not normally live, but could become live under fault conditions.
Purpose of earthing: To provide a fault current path for operating the overcurrent protective device.
Extraneous-Conductive-Part:
A conductive part that can introduce a potential (e.g., Earth potential) into the system but is not part of the electrical installation.
Purpose of bonding: To prevent potential differences and reduce shock risks.
Evaluating the Need for Earthing
Is the metallic tray an exposed-conductive-part?
Tray Used as a Protective Conductor
If the tray itself is used to carry fault currents (per Regulation 543.2.1), it is an exposed-conductive-part and must be earthed.
Tray Not Used as a Protective Conductor
Scenario 1: The tray carries metallic-sheathed cables (e.g., MICC cables).
The cable sheath provides the fault current path, so the tray is not an exposed-conductive-part and does not require earthing.
Note: Earthing the tray could distribute touch voltages unnecessarily during a fault.
Scenario 2: The tray carries cables with non-metallic sheaths.
These cables provide equivalent protection to Class II equipment, ensuring no fault current flows through the tray.
The tray is not an exposed-conductive-part and does not require earthing.
Caution: Earthing in this case may increase shock risks.
Evaluating the Need for Bonding
Is the metallic tray an extraneous-conductive-part?
Potential Introduction of Earth Potential
- If the tray introduces Earth potential from outside the installation (e.g., carrying cables into a building and contacting Earth externally), it qualifies as an extraneous-conductive-part.
- In this case, bonding is required to mitigate potential differences.
No Introduction of External Potential
- If the tray does not introduce a new potential, it is not an extraneous-conductive-part and does not require bonding.
Special Considerations
Metallic Conduit or Trunking
- Houses insulated cables without an outer sheath and inherently provides a fault path. Earthing may be required depending on its use.
Cable Basket
- Similar principles apply as for metallic trays, depending on its role in fault current paths or potential introduction.
Earthing is required if the tray is an exposed-conductive-part and used as a protective conductor.
Bonding is required if the tray is an extraneous-conductive-part that introduces an Earth potential.
Neither is needed if the tray is neither an exposed-conductive-part nor an extraneous-conductive-part.
Always refer to local electrical regulations (e.g., IEC, BS 7671) for specific requirements.