Engineering Guide: MV Switchgear Currents—Rated, Overload, and Short-Circuit Conditions

In the design and operation of power distribution systems, Medium Voltage (MV) switchgear must withstand rigorous challenges under various current conditions. Accurately verifying the temperature rise under rated continuous current, the transient effects of overload current, and the thermal and dynamic stability during short-circuit faults is fundamental to ensuring long-term grid safety. These conditions not only dictate the specifications for busbar cross-sections but also directly impact the reliability limits of insulation supports under thermal aging and electromagnetic stress.

1. Rated Continuous Current of MV Switchgear

The rated continuous current refers to the maximum current level at which the medium voltage switchgear can operate indefinitely without exceeding temperature limits. This includes the main busbars (horizontal busbars) as well as the currents of the incoming, bus-coupler, and outgoing feeders. Typically, the ratings for these components are consistent; however, in scenarios involving multiple parallel incoming feeders, the main busbar rating must be appropriately increased.

Verification of continuous current is typically conducted via a temperature-rise test. The circuit is subjected to the rated continuous current for several hours until thermal equilibrium is reached—defined as the point where the temperature rise is less than 0.5 K per hour.

The current rating is the primary factor determining busbar specifications. A larger cross-sectional area reduces resistance and heat dissipation, whereas a smaller area leads to increased heat and significant temperature rise. Additionally, temperature rise in AC systems is influenced by busbar arrangement and cross-sectional geometry. Maintaining the enclosure structure and layout while increasing the busbar cross-section is an effective way to lower temperature rise.

2. Overload Current of MV Switchgear

During operation, switchgear must also account for overload conditions, where the current exceeds the rated value for a short duration. The primary reason for limiting temperature rise lies in the insulation support structures at busbar junctions. The thermal resistance of insulating materials is finite; for instance, the epoxy resin insulator undergoes accelerated aging when temperatures exceed 125°C, leading to a degradation in dielectric performance and mechanical strength.

Engineering Calculation Example:

Busbar: Single Copper, 80×10 mm (Vertical Installation)

Heat Transfer Coefficient (Kt): 7 W/m2·K

Result: I = 1598 A

While the temperature rise caused by overloads is generally manageable and usually does not cause immediate negative impacts due to their short duration, frequent or prolonged overloads can jeopardize the safety and service life of insulation components. Therefore, thermal stability of the insulation system must be considered a critical boundary when evaluating overload capacity.

3. Short-Circuit Current of MV Switchgear

Beyond normal operation and potential overloads, the most severe challenge to a power system arises from short-circuit faults. Before a fault is cleared by protective devices such as fuses or vacuum circuit breakers, the switchgear must withstand the massive stresses induced by the fault current. This includes the thermal effects of the Short-Time Withstand Current (RMS value) and the electromagnetic stresses of the Peak Withstand Current.

The RMS value of the short-circuit current primarily generates thermal effects. Compared to rated continuous currents (typically 1.25 kA to 3.15 kA), short-circuit magnitudes are far higher (often 25 kA to 40 kA), though they last for a very brief period (typically 1 to 4 seconds). During this interval, the heating is considered an adiabatic process, as there is insufficient time for heat to dissipate via conduction, convection, or radiation. Based on standard formulas, the minimum required busbar cross-section for short-circuit conditions can be derived.

In low-current applications (e.g., a 300 A rating required to withstand a 31.5 kA short circuit), one must compare the cross-sectional requirements for both continuous and short-circuit currents and select the larger value.

• Continuous Current Requirement: For 300 A, a copper bar cross-section of only 150 mm² might suffice.
• Short-Circuit Requirement: Calculated for 31.5 kA / 4 s: Area =I/ₐ√（t/△t）=31500/13×√(4/215)=330mm²

Consequently, for this application, the 300 A copper bar must use at least a 6×60 mm specification.

4. Short-Circuit Dynamic Stability and Breaking Capacity

Short-circuit currents impact busbar selection not only through thermal effects but also through immense electromagnetic forces that test the mechanical integrity of the switchgear. The core of dynamic stability verification is ensuring that the busbar system and insulation supports do not deform or fail under the mechanical stress caused by the peak current. Furthermore, switching devices must possess reliable breaking and making capacities to safely isolate faults.

4.1 Peak Stress and Dynamic Stability

The maximum peak current generates mechanical stress, known as electromagnetic force destruction. Medium voltage switchgear design must focus on the mechanical strength and cantilever strength of insulation supports. In 50 Hz systems, the peak withstand current is typically 2.5 times the RMS value, while in 60 Hz systems, it is 2.6 times.

• 50 Hz Environment: An RMS of 31.5 kA corresponds to a peak current of 78.75 kA (31.5 x 2.5).
• 60 Hz Environment: An RMS of 31.5 kA corresponds to a peak current of 81.9 kA (31.5 x 2.6).

4.2 Breaking and Making Capacity

The short-circuit breaking capacity of switching equipment is usually defined by its RMS value (e.g., 31.5/40/50 kA). Similarly, the short-circuit making current is frequency-dependent, with peak multipliers of 2.5 for 50 Hz and 2.6 for 60 Hz.

It is important to note that while the earthing switch does not have a short-circuit breaking capacity, they must possess a corresponding making capacity. For example, they must meet requirements such as 80 kA or 82 kA prospective short-circuit making currents in 50 Hz or 60 Hz environments, respectively.

5. Conclusion

The performance of MV switchgear fundamentally depends on its ability to adapt to and withstand diverse current conditions. From temperature-rise control for long-term rated operation to thermal stability verification for transient overloads and short circuits, and finally to electromagnetic integrity and breaking reliability during extreme faults—each current dimension directly impacts system safety and longevity.

In engineering practice, it is essential to strictly adhere to relevant industry standards. Based on ensuring adequate clearance and creepage distances, scientific selection of switchgear must be achieved through precise technical calculations and rigorous type-test verification. Only by ensuring that every current parameter is accurately matched to the site conditions can the stable, long-term, and efficient operation of power systems and related projects be guaranteed.

As an experienced medium voltage switchgear manufacturer, Liyond is dedicated to providing professional flexible switchgear solutions for your needs. If you require further clarification or technical support regarding switchgear selection, parameter verification, or applications under special conditions, please feel free to contact us.