Copper busbars are normally selected according to the current they need to carry during normal operation. However, a properly designed busbar must also withstand the much higher current that can occur during a short circuit.
A short circuit creates two major stresses on a copper busbar: thermal stress caused by rapid heating and estresse mecânico caused by strong electromagnetic forces between conductors.
This means that a copper busbar can be large enough for its normal operating current but still require additional design verification for short-circuit conditions.
This article explains how short-circuit current affects copper busbar design, what I2t means, why busbar supports matter, and how joints and conductor spacing affect short-circuit performance.
Copper busbar short-circuit withstand is the ability of a busbar assembly to tolerate the electrical, thermal, and mechanical stresses produced by a fault current for the specified duration.
During normal operation, the busbar carries its rated current continuously. During a short circuit, the current can increase dramatically for a short period before the protection device disconnects the fault.
The busbar therefore needs to withstand the fault until the protective device clears it.
For low-voltage switchgear assemblies, IEC 61439 defines requirements and verification methods for assembly characteristics, including short-circuit withstand verification. The standard also includes verification requirements related to temperature rise and short-circuit withstand strength. IEC 61439-1 provides the general rules, while the applicable assembly part must also be considered.
The rated current is the current that the electrical system is designed to carry continuously under specified operating conditions.
Short-circuit current is a fault condition and can be many times higher than the normal operating current.
For example, a distribution board operating normally at a few thousand amperes can experience a much higher fault current for a short period following a phase-to-phase or phase-to-ground fault.
The protection system is designed to clear the fault quickly, but the busbar must survive the electrical and mechanical stress during that short interval.
Therefore, two separate questions should be asked when designing a copper busbar:
A short circuit produces several effects at the same time.
The large fault current produces heat according to the relationship:
P = I2R
Because the current increases substantially during a fault, the instantaneous heating effect can become very large.
The fault usually lasts for a relatively short time, so the thermal evaluation is different from normal continuous operation. The short-circuit duration and the starting temperature of the conductor are important design parameters.
A second effect is mechanical force between conductors carrying high fault currents.
Parallel conductors carrying current in opposite directions can experience significant electromagnetic forces during a short circuit.
These forces can push conductors apart or pull them together depending on the conductor arrangement and current direction.
As a result, the busbar supports, insulators, fasteners, joints, and conductor geometry must be strong enough to withstand the resulting mechanical stress.
ABB’s IEC 61439 guidance specifically notes that busbars should be dimensioned and arranged to withstand the short-circuit loads that can occur on the supply side of the busbars. ABB IEC 61439 practical guidance.
The duration of the fault is extremely important.
The same fault current can create very different thermal stress depending on whether the protection device clears the fault in a fraction of a second or after several seconds.
For a simplified adiabatic thermal calculation, the relationship can be expressed as:
Isc = k × A / √t
Onde:
The relationship shows why faster fault clearing can significantly reduce the thermal stress placed on the busbar.
For preliminary engineering calculations, this type of formula can be useful. However, the final short-circuit rating of a complete busbar assembly should be verified according to the applicable assembly standard, design verification, and manufacturer requirements rather than relying only on a generic formula.
Not necessarily.
Increasing the copper cross-sectional area can improve thermal withstand because more conductive material is available to absorb the heat generated during the short circuit.
However, short-circuit performance is not determined by cross-sectional area alone.
The complete design also needs to consider:
A very large copper bar with insufficient mechanical support can still experience excessive movement during a high short-circuit event.
During a short circuit, electromagnetic forces act on the conductors. The supports resist these forces and keep the busbars in their designed positions.
If the distance between supports is too large, the mechanical stress on the conductor and support structure can increase.
Closer support spacing can improve mechanical stability, but it also affects the physical design, insulation arrangement, installation space, and cost of the assembly.
For this reason, support spacing should be designed together with the conductor arrangement and expected short-circuit level.
A busbar system can contain several joints where conductors are connected to other busbars or electrical equipment.
These joints need to withstand both normal operating current and short-circuit forces.
Important joint parameters include:
Schneider Electric notes that short-circuit withstand design can depend on parameters including the number of bolts, bolt class, tightening torque, washers, and the overlap between busbars. Schneider Electric short-circuit withstand guidance.
A joint should therefore be treated as part of the complete short-circuit mechanical system rather than as a simple electrical connection.
The physical arrangement of conductors affects the electromagnetic forces generated during a fault.
Important variables include:
Changing the conductor spacing or arrangement can change the mechanical force acting on the busbar supports.
This is one reason short-circuit design should not be performed only by selecting a copper bar based on its cross-sectional area.
| Fator | Thermal Withstand | Mechanical Withstand |
|---|---|---|
| Main concern | Temperature rise during fault | Electromagnetic force |
| Main variables | Current, area, material, fault duration | Current, conductor spacing, support spacing, geometry |
| Potential failure | Excessive heating or material damage | Busbar deformation, support damage, or joint failure |
| Key design elements | Copper cross-section and fault duration | Supports, fasteners, spacing, and conductor arrangement |
A reliable copper busbar assembly must satisfy both requirements.
Flexible copper connectors can be used in applications where mechanical movement needs to be accommodated, but their suitability for a particular short-circuit duty must be verified as part of the complete electrical assembly.
A flexible connector has a different mechanical structure from a rigid copper busbar. Its number of copper layers, joining method, terminal structure, support arrangement, and overall geometry all affect its performance.
Therefore, a flexible copper connector should not be assigned a short-circuit rating simply because it has the same cross-sectional area as a rigid copper bar.
The complete connection and its mechanical support need to be evaluated for the required fault conditions.
When a copper busbar will be installed in a high-short-circuit environment, the manufacturer should receive both electrical and mechanical information.
Providing these details allows the copper busbar design to be evaluated as part of the complete electrical assembly rather than based on current capacity alone.
Short-circuit current affects copper busbar design in two major ways: it produces rapid thermal stress in the conductor and strong electromagnetic forces within the busbar assembly.
A copper busbar therefore needs to be designed for both normal operating current and the specified fault conditions. Increasing conductor cross-section can improve thermal withstand, but support spacing, conductor arrangement, joints, fasteners, and mechanical strength are equally important for short-circuit performance.
For low-voltage assemblies, the final design should be evaluated against the applicable IEC 61439 requirements and the verified characteristics of the complete assembly.
GRL Copper provides customized copper busbars and copper electrical connection components, including rigid, flexible, and multi-layer constructions manufactured according to customer drawings, current requirements, mechanical conditions, and application requirements.
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