Busbar Support and Insulator Selection: Short-Circuit Forces
busbar support: Busbar support and insulator selection follows the electrodynamic forces that arise between conductors during a short circuit. The force is proportional to the square of the peak short-circuit current and inversely proportional to phase spacing, so shortening the support pitch reduces the load on the insulator while also changing busbar deflection and resonance behaviour.
Contents
A busbar system is often considered finished once its current-carrying capacity has been calculated and a cross-section chosen, yet the mechanical force acting on the bars during a short circuit grows with the square of the current and appears within a tenth of a second. In a system with the wrong support pitch the bars strike each other, insulators crack and damage spreads along the board in a single fault. This article covers how short-circuit forces arise, the relation between support pitch and bending stress, insulator selection criteria, and the effects of thermal expansion and resonance.
How do short-circuit forces act on busbars?
Two parallel conductors carrying current in the same direction attract each other and repel when the currents are opposed; the magnitude of the force is proportional to the product of the currents and inversely proportional to the distance between conductors. Because a short circuit raises current to tens of times the rated value, the force rises by hundreds of times and reaches its peak in the first half cycle, during the asymmetrical peak current. In a three-phase system the centre phase experiences the highest force, and since the direction of the force alternates through the cycle the busbar system is subjected to an oscillating load in the order of 100 Hz. The calculation therefore has to consider dynamic behaviour and not just a static force value.
Support pitch, bending stress and deflection
A busbar behaves like a loaded beam between supports, and because the force is distributed per unit length the bending moment grows with the square of the support pitch. Shortening the pitch reduces bending stress rapidly, but every additional support brings insulator cost, assembly time and a clearance constraint. Three checks are made in design: the bending stress in the bar must remain below the yield limit of the material, the deflection must not violate insulation distances, and the force reaching the support must not exceed the declared mechanical strength of the insulator. Copper and aluminium have different elastic moduli and yield limits, so the same geometry gives different results.
Which criteria govern insulator selection?
An insulator has three separate duties: mechanical support, electrical insulation and providing the creepage path across its surface. On the mechanical side the declared bending and tensile strength is compared with the calculated short-circuit force with an adequate safety margin. On the electrical side impulse withstand and rated insulation voltage must match the Uimp and Ui values of the assembly. Material choice follows the pollution degree: glass-fibre reinforced polyester and epoxy insulators offer different resistance to surface tracking. In condensing or dusty environments ribbed geometries with extended creepage paths are preferred.
Thermal expansion, resonance and joints
| Criterion | Effect | Design decision |
|---|---|---|
| Peak short-circuit current | Force grows with the square of current | Shorten the support pitch or increase the cross-section |
| Phase spacing | Force rises as spacing falls | Optimise together with insulation distances |
| Support pitch | Bending moment grows with pitch squared | Balance insulator count against stress |
| Insulator mechanical strength | The insulator cracks once exceeded | Select the declared value with a safety margin |
| Thermal expansion | Permanent stress on supports over long runs | Add flexible connections or expansion elements |
| Natural frequency | Vibration grows if close to the 100 Hz component | Choose a pitch away from resonance |
Busbar temperature rises noticeably with load, and on long busbar runs thermal expansion produces elongation in the order of millimetres; if that movement is not released, a permanent stress is imposed on the supports. The solution is to use flexible connections or expansion elements at defined intervals. The second issue is resonance: when the natural frequency of the busbar system lies close to the 100 Hz component of the short-circuit force, vibration is amplified and stresses exceed the calculated values. Choosing a support pitch that keeps the natural frequency away from that component is good practice. Busbar joints are the weakest link both electrically and mechanically, so bolt class, torque value and contact surface preparation are part of the calculation.
Assembly, torque and verification
The most common field mistakes are copying a standard support pitch into a project with a different short-circuit level, removing a support to make room for a later outgoing connection, and tightening bolts by feel instead of with a torque wrench. Verification of short-circuit withstand is achieved either by comparison with a tested reference design or by type test, and changing the busbar cross-section or support pitch invalidates that verification. The post-assembly checklist should include support positions, bolt torques, clearances and freedom of movement at flexible connections. At periodic maintenance, torque checks and thermography catch a loosening joint before it becomes a fault.
Key takeaways: busbar support
- Force grows with the square of current — double the short-circuit level and the force quadruples.
- Bending moment grows with the square of the pitch — shortening the pitch is the single most effective measure.
- Insulators are selected mechanically and electrically — declared bending strength and impulse withstand are checked together.
- Changing the support pitch invalidates verification — short-circuit withstand has to be demonstrated again.
- Standard source — IEC publication catalogue: iec.ch.
Frequently asked questions
Is shortening the support pitch always safe?
In terms of mechanical stress yes, but not without limit. A shorter pitch raises the natural frequency and at some point can enter the resonance region; every support also brings clearance and access constraints. The correct approach is to evaluate stress, deflection, frequency and cost together.
Can copper and aluminium busbars use the same support pitch?
Usually not. Because the elastic modulus and yield limit of aluminium differ from copper, the same geometry gives different deflection and stress; aluminium also needs a larger cross-section for the same current, which changes the moment of inertia. A separate calculation is required for each material.
Which current value do I use for the short-circuit force?
Not the r.m.s. value of the rated short-time withstand current but the corresponding peak withstand current, because the greatest mechanical stress occurs during the asymmetrical first peak. The peak value is derived from the r.m.s. value by a factor that depends on the system power factor.
Related articles
- Busbar Current Carrying Capacity and Derating Calculation
- Short Circuit Withstand in Switchboards: Icw, Ipk and Conditional Rating
- Busbar Systems: Copper or Aluminium? Selection Criteria
- All technical articles
In Devpan busbar systems the support pitch and insulator selection are calculated for the short-circuit level of the project and applied within verified system rules. Contact our engineering team for mechanical verification of your busbar design. Related Devpan solution: DevPanel main distribution board.
