Earthing or Bonding a Metallic Cable Tray: What the Regs Really Say (and the Bits People Miss) 

Understanding cable‐tray earthing comes early in the 18th-Edition module of the electrician courses at Elec Training Birmingham. The base rule sounds simple, yet the real-world detail still trips experienced installers. This deeper guide walks through the logic, adds practical checks, and flags a few mistakes we keep seeing on site. 

1. First decision: is the tray an exposed conductive part?

A quick recap

An exposed conductive part is metal you can touch that is not normally live but could become live during a fault. If the answer is yes, the tray must be earthed so fault current can flow and operate the protective device ¹. 

1.1 Tray used as the protective conductor 

Regulation 543.2.1 allows the tray itself to serve as a CPC if its cross-section and continuity meet the adiabatic equation. Where that choice is made: 

• Verify every joint with serrated or spring washers. 

• Measure end-to-end resistance (≤ 0.05 Ω is the usual site rule of thumb). 

• Record the result on the test sheet, do not rely on visual inspection alone. 

1.2 Tray carries cables only 

Scenario A: metallic-sheathed cables (MICC, armoured, SWA) 
The sheath gives the fault path. Earthing the tray adds another parallel path that may create circulating earth‐leakage currents, a point designers often ignore. 

Scenario B: PVC or LSF insulated cables 
Regulation 412.2.4.1 treats the cable assembly as equivalent to Class II equipment, so a single insulation fault cannot raise the tray to a dangerous voltage. Leaving it isolated is actually safer. 

Key field tip 
If you must earth a tray for functional reasons (static discharge, RFI), do it at one end only. Bonding both ends can form a loop, increasing magnetic coupling and nuisance RCD trips. It are easy to miss that during design reviews. 

2. Second decision: is the tray an extraneous conductive part?

An extraneous conductive part is metal that can import an outside Earth potential ². Ask: 

1 Does the tray leave the building envelope? 
2 Is it in direct structural contact with soil or structural steel that is already bonded? 

If the answer is yes, bond it to the main earthing terminal with a conductor sized per Table 54.8. If no, extra bonding simply wastes copper. 

Special case – rooftop PV or HVAC plant 
Trays that feed external plant often sit on concrete plinths. Moisture can put them at Earth potential seasonally. Spot-bond across the plant isolation gland, then route the bonding conductor back with the tray so it remains in the same EMI zone. 

3. Corrosion, continuity and inspection

• Bimetallic joints: aluminium tray to copper lugs accelerates galvanic attack. Use zinc passivated “Bi-Metal” washers or fit a stainless transition strap. 

• Painted tray: scrape paint at the clamp point or fit an approved piercing earth clamp, otherwise resistance readings can drift after the snag sheet is signed off. 

• Modular sections: new fast-fit trays with clip-in couplers often rely on spring pressure. Always include a 4 mm² bonding link across any expansion joint. 

During Periodic Inspection the tray’s earth path must be verified. The test is dead (continuity) not live, so schedule it when production can tolerate an isolation. 

4. Quick reference table

5. Why this matters for trainees

Knowing when not to earth or bond is as important as knowing when to do it. Fault paths, touch voltages, and Chapter 54 intent become second nature only through practice, which is why our intensive NVQ pathway puts every learner on site within ten weeks. 

If you plan to upskill, the fast-track electrician courses at Elec Training Birmingham run quarterly. They include live testing workshops using tray test rigs, so you leave confident the next time a site agent asks, “Is that tray bonded?”