A low-voltage outgoing switch busbar is a rigid conductor within switchgear that safely distributes current from the main bus to outgoing feeders, designed according to IEC 61439 or UL standards for thermal, mechanical, and electrical performance.

Function and Role

Low-voltage outgoing busbars serve as the current path from the main bus to individual outgoing circuits in a switchboard or switchgear assembly. They ensure reliable power distribution while maintaining mechanical stability, thermal management, and electrical safety. Outgoing busbars are typically connected to circuit breakers, fuses, or other protective devices, forming the interface between the main power source and downstream loads .

Design Considerations

• Material: Copper or aluminum is commonly used. Copper offers higher conductivity and smaller cross-section, while aluminum is lighter and cost-effective .
• Profile: Flat or rectangular bars maximize surface area for heat dissipation and simplify bolted connections .
• Insulation: Busbars may be bare or insulated with heat-shrink sleeves, epoxy coatings, or finger-safe covers to prevent accidental contact and improve safety .
• Supports: Insulators and busbar supports maintain proper creepage and clearance distances and withstand thermal and electrodynamic stresses .

Standards and Compliance

• IEC 61439-1/-2: Defines construction, verification, and thermal limits for low-voltage switchgear busbars, including maximum working temperature (typically 140°C) and diversity factors for load calculation .
• UL 891: Applicable for North American switchboards, ensuring safety and performance of internal busbars .
• UL 1558 / IEEE C37.20.1: Relevant for low-voltage power circuit breaker switchgear in North America .

Thermal and Electrical Performance

• Current Rating: Determined by busbar cross-section, material, and system diversity factor. For example, a 22-circuit system with a total requirement of 2700 A and a diversity factor of 0.6 would require a main busbar rated for 1620 A .
• Heat Management: Busbar geometry, spacing, and layering are designed to dissipate heat efficiently and prevent hotspots, ensuring long-term reliability .

Practical Design Tips

• Standardize busbar widths and adjust thickness or layering for higher currents to maintain modularity and ease of installation .
• Ensure horizontal busbars for main distribution and vertical risers for outgoing feeders to optimize space and accessibility .
• Use modular systems with quick assembly contacts to simplify installation and maintenance . Low-voltage outgoing switch busbars are therefore critical components in switchgear, balancing electrical performance, thermal stability, mechanical strength, and compliance with international standards to ensure safe and reliable power distribution.