The maximum short-circuit current of a 35kV busbar depends on its material, cross-sectional area, and spacing, and is determined by thermal and electrodynamic withstand limits per IEC 61439-1 and IEC 60865-1.

Thermal and Electrodynamic Considerations

The short-circuit withstand of a busbar has two main components:

• Thermal withstand ensures the busbar does not exceed its maximum temperature during a fault. For copper, the short-time temperature limit is typically 250°C. The required cross-sectional area $A$ can be estimated using the adiabatic formula: $A\ge \frac{I·\sqrt{t}}{k}$, where $I$ is the RMS short-circuit current, $t$ is the fault duration in seconds, and $k$ is a material constant (143 for copper, 13 for aluminium) .
• Electrodynamic withstand ensures the busbar and supports can withstand the peak electromagnetic forces during a short circuit. The force per unit length between parallel busbars is: $F=0.2·\frac{i_p^2}{d}$ (N/m), where $i_p$ is the peak short-circuit current and $d$ is the center-to-center spacing between phases . The busbar support spacing must limit deflection and stress below yield limits.

Standards and Typical Ratings

• IEC 61439-1 and IEC 60865-1 provide guidelines for short-circuit current ratings and busbar sizing.
• ANSI C37.23 specifies design requirements for busbars and bus ducts rated up to 35kV, including short-circuit withstand and mechanical strength .
• UL 508A and UL 891 tables provide practical SCCR values for busbar arrangements, e.g., edge-to-edge spacing with proper insulators can achieve SCCR up to 50 kA for certain 35kV switchgear .

Practical Estimation

For a 35kV busbar, the maximum short-circuit current is not a fixed number but depends on:

• Busbar material: Copper allows higher currents than aluminium for the same cross-section.
• Cross-sectional area: Larger busbars carry higher short-circuit currents.
• Support spacing and arrangement: Closer spacing and robust supports increase electrodynamic withstand.
• Fault duration: Typically 1 second for short-time ratings. As an example, a properly designed copper busbar in a 35kV switchgear with adequate cross-section and spacing can typically withstand tens of kiloamperes RMS for 1 second, often in the range of 40–50 kA, depending on the exact design and standards compliance .

Conclusion

The maximum short-circuit current of a 35kV busbar is determined by its thermal and electrodynamic limits, busbar material, cross-section, and spacing. Using IEC 61439-1, IEC 60865-1, and ANSI C37.23 standards, designers can calculate or select busbars capable of withstanding short-circuit currents up to 50 kA RMS for typical high-voltage switchgear applications. Proper design ensures both electrical safety and mechanical integrity during fault conditions.