What should be connected to the thicker small busbar at the top of the cabinet

The thicker small busbar at the top of a cabinet is generally intended to connect to the main incoming power supply or to a primary distribution bus, ensuring it can carry the full system current safely.

Purpose of the Top Busbar

In low-voltage switchgear and controlgear assemblies, the top busbar often serves as the main power input or primary distribution point. Its increased thickness allows it to handle higher currents without excessive heating, providing a backbone for distributing power to downstream circuits or smaller busbars within the cabinet (IEC 61439 standards) .

Connection Considerations

1. Main Power Source: The top busbar is typically connected to the incoming supply from the utility or transformer. This ensures that all downstream circuits receive power through a robust, low-resistance path .
2. Current Rating: The busbar must be sized to carry the continuous full-load current of the system without exceeding permissible temperature rise limits. Thicker busbars are used to accommodate higher currents safely .
3. Short-Circuit Withstand: The busbar should be capable of withstanding short-circuit currents for a short duration (usually 1–3 seconds) without mechanical deformation or insulation failure .
4. Insulation and Plating: Proper insulation and plating (tin, silver, or nickel) at the connection points reduce voltage drop and prevent corrosion. The top busbar may also be mounted with hardware to ensure mechanical stability .
5. Distribution to Downstream Circuits: From the top busbar, power is typically distributed to vertical or horizontal busbars, circuit breakers, or other protective devices within the cabinet. This ensures balanced load distribution and compliance with safety standards .

Safety and Standards

• Follow IEC 61439 guidelines for low-voltage switchgear, which specify temperature rise limits, current-carrying capacity, and mechanical support requirements .
• Ensure proper spacing and clearance to prevent flashovers and maintain insulation integrity .
• Verify that all connections are tightened and plated surfaces are clean to minimize resistance and heating . Key Takeaway: The thicker small busbar at the top of a cabinet should be connected to the main incoming power supply or primary distribution bus, designed to safely carry the full system current and distribute power to downstream circuits while complying with IEC 61439 standards and mechanical support requirements.