In low-voltage switchgear, busbars are arranged in a structured sequence—main, sub, neutral, and earthing—within isolated compartments to ensure safe power distribution, thermal stability, and compliance with IEC 61439.

Busbar Arrangement and Sequence

In a typical LV switchgear panel, the busbar system is the primary current-carrying path that distributes power from the incomer to outgoing feeders or functional units. The standard sequence generally follows:

1. Main Busbar – Carries the full load from the incomer and distributes it to sub-busbars or directly to feeders .
2. Sub-Busbar – Optional intermediate busbars that branch from the main bus to supply specific sections or compartments .
3. Neutral Busbar – Provides a return path for unbalanced currents in three-phase systems and is usually positioned parallel to the main busbars .
4. Earthing Busbar – Ensures safety by connecting all metallic parts to ground, typically located at the rear or bottom of the panel . Busbars can be arranged horizontally across compartments or vertically, depending on panel design, space constraints, and maintenance requirements . Each busbar is typically made of high-conductivity copper and may be single or double-layered to handle rated currents and short-circuit stresses .

Compartmentalization and Safety

IEC 61439 defines Forms of Internal Separation (Form 1 to Form 4b) to segregate busbars, functional units, and terminals. Higher forms, like Form 4b, require separate compartments for each functional unit, enhancing arc fault containment and personnel protection . The busbar compartment is isolated from neighboring compartments to prevent fault propagation and facilitate maintenance .

Thermal and Electrical Considerations

Busbar sizing and sequence must account for:

• Rated current and diversity factor – Determines the maximum load the busbar can carry without overheating .
• Thermal limits – IEC 61439 sets a maximum busbar temperature of 140°C under rated load conditions .
• Short-circuit withstand – Busbars must resist mechanical forces during fault conditions, requiring proper thickness, layering, and support .
• Heat dissipation – Horizontal or vertical busbar layouts are designed to avoid heat accumulation and maintain insulation integrity .

Practical Design Tips

• Standardize busbar widths and thicknesses across ratings to simplify manufacturing and improve thermal efficiency .
• Ensure clear phase identification and maintain adequate spacing between busbars for safety and maintenance .
• Use high-quality joints and insulation to reduce impedance, improve mechanical strength, and prevent corrosion . By following this sequence and design approach, LV switchgear achieves safe, reliable, and efficient power distribution while complying with IEC 61439 standards .