In many international projects, switchgear manufacturers are frequently required to provide the thermal loss power of each individual panel. These values serve as fundamental input data for the design of substation ventilation and air-conditioning systems. Without accurate heat dissipation figures, thermal management becomes speculative, and long-term reliability is compromised.
At its core, heat generation in switchgear is proportional to the square of the current. A 4000 A switchgear panel will therefore dissipate significantly more heat than a 3000 A unit. Yet the relationship is not purely linear. As the rated current increases, the switchgear design typically incorporates larger copper busbar cross-sections, more contact fingers, and optimized current paths. These measures reduce resistance. As a result, heat dissipation cannot be estimated by current alone and must be calculated based on actual operating conditions.
Approximate Power Loss Estimation
A simplified method for estimating heat dissipation considers the operating current and the main circuit resistance of the switchgear. Take a 3000 A bus coupler panel as an example. If the per-phase main circuit resistance is 80 μΩ and the current transformer primary current is 3000 A, the total heat dissipation can be calculated as:
W = I² × R × 3
This yields a result of approximately 2160 W.
It should be noted that this calculation is based on DC resistance. In reality, heating is caused by AC resistance. For copper busbars, AC resistance depends on geometry, lamination, spacing, and skin and proximity effects. Rectangular copper busbars typically exhibit AC resistance values that are 1.4 to 1.8 times their DC resistance. Consequently, actual heat generation is often higher than this rough estimate suggests.
Refined Calculation by Component Segmentation
A more rigorous approach breaks the calculation down to individual components. Current transformers, space heaters, secondary control and protection circuits, busbars, and contacts are each evaluated separately. Summing these values produces a result that more closely reflects real operating conditions.
1. Busbar Heat Dissipation
Busbar resistance can be derived from copper resistivity, length, and cross-sectional area. Consider a typical metal-clad outgoing feeder panel. The branch busbar length is 7.5 m. For a 2000 A rating, double 80 × 10 mm copper bars are used.
Copper DC resistivity at 20 °C: 1.75 × 10⁻⁸ Ω·m
Copper resistivity at ~100 °C: 2.32 × 10⁻⁸ Ω·m
AC resistance correction factor: 1.6
The effective resistivity becomes 3.74 × 10⁻⁸ Ω·m.
The resulting branch busbar resistance is approximately 175 μΩ, leading to a heat dissipation of:
2000² × 0.000175 = 701 W
Applying the same method to the main busbar, with a per-phase length of 1 m and identical dimensions, yields an additional 280 W.
Total busbar heat dissipation: 981 W.
2. Circuit Breaker Losses
Circuit breaker heating is determined by contact resistance and arc chute thermal characteristics. A typical 2000 A withdrawable circuit breaker exhibits a per-phase resistance of around 25 μΩ. The resulting heat dissipation is:
2000² × 0.000025 × 3 = 300 W
3. Current Transformer Losses
Current transformer losses are generally consistent with manufacturer test data:
LZZBJ9, 1250/5: 36 W
LZZBJ9, 2500/5: 65 W
LMZB1, 3000/5 (four cores): 67 W
LMZB1, 4000/5 (three cores): 54 W
These losses, while modest individually, become significant when multiplied across phases and panels.
4. Auxiliary and Secondary Systems
Additional heat sources include space heaters and secondary systems. Space heaters typically contribute around 150 W, while control and protection devices add approximately 50 W.
Comprehensive Example
For a 2000 A switchgear panel, total heat dissipation can be summarized as:
Busbars: 981 W
Circuit breaker: 300 W
Current transformers (three units): ~165 W
Heater and secondary systems: ~200 W
Total heat dissipation: approximately 1646 W
While the difference between rough estimation and detailed calculation for a single panel may appear modest, the cumulative effect across a lineup of twenty or thirty panels can be substantial. This directly influences HVAC sizing and operational efficiency.
Rated Current vs. Actual Operating Current
The values above assume full-load operation at rated current. In practice, many switchgear panels operate well below their nominal ratings. A panel rated at 3150 A may carry an actual operating current of only 2800 A. Under these conditions, heat dissipation drops to around 2080 W, far below the rated-current value.
This reflects a common reality in modern substations. Equipment is generously rated, while actual load currents remain comparatively low. Accordingly, real-world thermal stress is often much less severe than theoretical maxima.
Persistent Overheating Issues
Despite this margin, cases of overheating and even catastrophic failure still occur. This is especially evident in high-current switchgear. Substandard materials, insufficient contact pressure, and cost-driven compromises can render even modest currents problematic. Such failures raise legitimate questions about manufacturing quality and the rigor of type testing, particularly for nominally 3150 A assemblies.
Data Demands in International Projects
Overseas projects routinely require extensive baseline data. Noise emission levels, foundation impact resistance, per-panel heat dissipation, operating and charging power for switching mechanisms, and even full 3D models are often mandatory. These inputs enable accurate spatial simulation and thermal analysis, ensuring that substations are designed for durable, long-term operation rather than optimistic assumptions.
Special Considerations for FC Circuits
For fused circuits (FC panels), fuse heating must be calculated separately using manufacturer datasheets. Although operating currents are relatively small, conductor cross-sections and contact areas are also reduced, resulting in higher circuit resistance. When assessing the total heat dissipation of a complete switchgear lineup, FC panels can account for a surprisingly large share of the thermal load.
Accurate heat dissipation analysis is therefore not a formality. It is a technical necessity. Only through meticulous calculation and realistic assumptions can switchgear systems achieve the thermal stability demanded by modern power infrastructure.