The rated value of a 35kV busbar depends on its material, cross-section, cooling conditions, and standards, typically allowing continuous currents from several hundred to a few thousand amperes with thermal limits up to 105°C for bare copper.

Continuous Current Rating

For a 35kV medium-voltage busbar, the continuous current rating is determined by the busbar material (usually copper or aluminum), cross-sectional area, and cooling conditions. For copper busbars, IEC 61439-1 recommends a current density of 1.5–2.5 A/mm² for enclosed busbars depending on ventilation and enclosure type. For example, a 2000A copper busbar at 2.0 A/mm² requires a cross-sectional area of 1000 mm², which can be achieved with multiple bars in parallel to allow margin for heating . The ambient temperature is typically assumed to be 35°C, with a maximum busbar temperature rise of 70K, resulting in a maximum busbar temperature of 105°C for bare copper . Thermal limits must also consider the lowest permissible temperature of components in contact with the busbar, such as fuse bases or outgoing cables .

Short-Circuit Withstand

Busbars must withstand short-circuit currents both thermally and mechanically. Thermal withstand ensures the busbar temperature does not exceed the short-time limit (250°C for copper per IEC 61439-1) during a fault. Electrodynamic withstand ensures that the electromagnetic forces between parallel busbars do not exceed mechanical strength, calculated based on peak short-circuit current and busbar spacing .

Medium-Voltage Switchgear Integration

For 35kV busbars integrated into metal-enclosed switchgear, such as Siemens 8DA/B or similar systems, the busbars are factory-assembled, type-tested, and designed for single or double busbar configurations. These systems are rated for long-term operation with service life exceeding 40 years under normal conditions, and the busbars are hermetically sealed and safe-to-touch .

Practical Considerations

• Material: Copper is preferred for higher conductivity and thermal performance.
• Cross-section: Must be sized to carry the rated current without exceeding temperature limits.
• Enclosure: IP-rated enclosures improve cooling and allow slightly higher current ratings.
• Thermal reserve: Limiting busbar temperature to 85°C is advisable for safety and longevity . In summary, a 35kV busbar's rated value is not a single number but a combination of continuous current rating, thermal limits, and short-circuit withstand, typically designed according to IEC 61439-1 and DIN 43 671 standards, with continuous currents ranging from several hundred to several thousand amperes depending on busbar size, material, and cooling conditions .