Single-core cables are laid in cable trays

Single-core cables in cable trays are typically laid in flat or trefoil formations, with careful phase arrangement to ensure balanced currents, proper heat dissipation, and compliance with standards.

Cable Construction and Considerations

Single-core cables consist of a central conductor, usually copper or aluminum, surrounded by insulation and sometimes a metallic sheath or armor for mechanical protection and environmental resistance . When multiple single-core cables are used per phase, imbalances in current distribution can occur if the physical arrangement is not optimized, even if the total phase current is balanced . This can lead to overheating and reduced cable lifespan.

Common Formations

Trefoil Formation:

• Cables are arranged in a triangular pattern, keeping each phase equidistant.
• Reduces circulating currents and magnetic field imbalances.
• Commonly used for low and medium voltage applications up to 132 kV.
• Space-efficient but may reduce heat dissipation due to cable contact . Flat Formation:
• Cables are laid side by side in a single row.
• Easier to install and allows better heat dissipation for the middle phase.
• Can cause voltage imbalance due to magnetic field interactions, often mitigated by phase transposition . Triplex Formation:
• Pre-wound trefoil arrangement supplied by the manufacturer.
• Simplifies installation and maintains phase rotation consistency .

Phase Arrangement and Current Balancing

Correct phase sequence and cable arrangement are critical to ensure balanced current flow and prevent overheating . For single-core cables, the arrangement should follow standards such as IEC 60364-5-52 and Electrical High Current Facilities Regulation. In high-power systems, improper arrangement can cause imbalanced currents, leading to insulation degradation and reduced cable lifespan .

Additional Considerations

• Segregation: Cables of different voltage levels or functions should be separated to avoid electromagnetic interference .
• Screen Grounding: The grounding and bonding of cable screens affect current distribution and losses; options include one-side, two-side, or cross-bonding schemes .
• Installation Space: Trefoil is preferred where space is limited, while flat formation may require more tray width but improves cooling .
• Maintenance and Identification: Cables should be clearly marked along the tray for easy identification and maintenance . In summary, choosing the optimal formation and phase arrangement for single-core cables in trays depends on voltage level, current capacity, space constraints, and thermal considerations. Trefoil is generally preferred for compact installations, while flat formation is used when heat dissipation is a priority. Proper adherence to standards ensures balanced currents, safe operation, and extended cable life.