Understanding Busbar Bolt Loosening: Causes, Mechanisms, and Reliability Solutions

Bolt loosening in busbar connections is a common reliability concern in high- and low-voltage power distribution systems. It can increase contact resistance, causing localized overheating at the joint and potentially leading to electrical failures or safety risks.

Busbar bolt loosening is not caused by a single factor but by the combined effects of material properties, thermal cycling, preload loss, and vibration stresses.

1. Impact of Busbar Material Properties

The mechanical performance of busbars is highly dependent on alloy composition, heat treatment, and manufacturing processes.

For aluminum busbars, processes such as extrusion insulation, welding, punching, bending, and plating may affect the mechanical strength of the terminal area. For example, during aluminum tab welding, the Mg content in 6101 aluminum alloy can significantly influence the mechanical properties of the welded zone. Reduced compressive strength at the connection area may weaken long-term clamping stability.

2. Thermal Cycling-Induced Preload Loss

Thermal cycling is one of the key factors contributing to busbar joint relaxation. During operation, repeated temperature changes caused by charging and discharging cycles generate thermal expansion and contraction, creating cyclic stresses that gradually reduce bolt preload.

Test results show that under an initial preload of 40 kN, aluminum busbar joints decreased to approximately 25.2 kN after 881 thermal cycles, while copper busbar joints maintained about 32.9 kN after 1,236 cycles. Aluminum joints exhibit a faster stress relaxation rate, with significant preload loss occurring during the early service stage.

In addition, repeated dimensional changes caused by thermal expansion can introduce additional mechanical stress. Improper routing design or insufficient expansion allowance may further accelerate connection relaxation.

3. Bolt Preload Relaxation

All bolted joints experience preload loss during service, known as bolt relaxation. Immediately after tightening, microscopic deformation occurs at thread interfaces and bearing surfaces, resulting in initial preload reduction.

To improve long-term clamping force retention, common assembly methods include:

• Torque cycling: Tightening above the target torque, releasing, and retightening to the specified torque.

• Step tightening: Applying preload in stages with stabilization time between tightening steps.

These methods help reduce initial relaxation and improve connection reliability.

4. Vibration-Induced Loosening

Continuous vibration, especially low-frequency cyclic loading, can cause micro-motion and wear between threaded components, resulting in reduced preload.

Busbars with insufficient mechanical support are more vulnerable to vibration-induced loosening. Recommended solutions include:

• Using locking nuts, serrated washers, or other anti-loosening components;

• Applying anaerobic thread-locking adhesives;

• Adding support points to reduce unsupported busbar spans;

• Optimizing busbar stiffness and damping to minimize resonance effects.

5. Recommended Bolt Connection Design

For improved connection reliability, hex flange bolts are recommended due to their larger bearing surface, which helps reduce local stress concentration and maintain stable electrical performance.

Zinc or zinc-nickel alloy coatings are preferred for corrosion protection. Nylon lock nuts are recommended to prevent damage to plated busbar surfaces during assembly while providing reliable locking performance.

Conclusion

Busbar bolt loosening is a multi-factor reliability challenge involving material characteristics, thermal stress, mechanical preload, and vibration conditions. A comprehensive approach covering material selection, manufacturing processes, assembly control, structural support, and connection design is essential for improving long-term electrical safety and reliability.

As a professional supplier of copper and aluminum busbar solutions for high- and low-voltage power distribution systems, RHI understands that every manufacturing step directly affects connection performance — from bending accuracy, terminal welding, plating protection, precision punching, to insulation solutions.

By providing customized high-conductivity and low-resistance busbar solutions, RHI Electric helps customers reduce risks caused by material and process limitations, enabling safer, more reliable, and longer-lasting electrical systems.