Primary and secondary cables share the same cable tray

Primary and secondary cable trays must be installed on dedicated containment systems with a minimum 300 mm horizontal separation or equivalent fire-rated barriers to ensure circuit independence and compliance with safety standards.

Separation and Safety Principles

Primary and secondary cable trays are used to segregate critical circuits such as life-safety, emergency power, or network distribution systems. The main goal is to prevent a failure in one system from affecting the other, maintaining operational reliability and compliance with electrical and fire safety standards.

U

K standards such as BS 7671, BS 8519, and BS 5839 emphasize that circuits of safety services must be independent and either separated by distance or protected by barriers. For rooftop or parallel installations, a 300 mm horizontal air gap between primary and secondary trays is commonly adopted. Where maintaining this gap is impractical, a 120-minute fire-rated baffle (e.g., Durasteel®) can be used to achieve equivalent separation.

Each tray should be lidded steel to provide mechanical protection and fire resistance for the cables. Structural supports like frames or brackets provide load-bearing capacity but do not require fire rating, as fire resistance applies to the cable containment itself.

Tray Types and Installation Considerations

Cable trays can be ladder, perforated, solid-bottom, or wire mesh depending on the application. Key considerations include: Ladder trays: Ideal for heavy-duty power cables, providing ventilation and easy access for maintenance. Perforated trays: Suitable for moderate cable loads, allowing airflow to reduce heat buildup.

Solid-bottom trays: Used for sensitive signal or fiber optic cables, offering full protection. Wire mesh trays: Lightweight and flexible, often used for instrumentation or control cables. Each tray should be mounted securely on walls, ceilings, or suspended supports, with proper spacing and alignment to maintain the required separation between primary and secondary systems.

High-

Voltage and Network ApplicationsIn network or transformer vaults, primary and secondary connections involve: Bus to bus ties connecting parallel busesInter-vault ties linking network protectors and paralleling busesPhase grouping with cables of the same size installed in the same duct or tray These connections must follow dedicated containment practices to prevent interference and ensure safe operation. Proper tray selection, lidding, and separation are critical for both electrical safety and fire compliance.

Summary

Maintain ≥300 mm horizontal separation or use fire-rated barriers between primary and secondary trays. Use dedicated lidded steel trays for each system. Select tray type based on cable type, load, and environmental conditions.

Ensure secure mounting and structural support without compromising fire safety.

Follow B

S 7671, BS 8519, and BS 5839 for life-safety and network installations. This approach ensures circuit independence, fire safety, and compliance while providing flexibility for maintenance and future expansion.