Busbar short-circuit withstand: Icw, Ipk and what they mean

Two different fault checks

A short circuit loads a busbar in two ways at once. The current heats the conductor, and the magnetic field between conductors pushes them apart. The heating is a slow effect that depends on the current squared and the duration. The force is a fast effect that depends on the square of the instantaneous current and the spacing.

The standards describe these as two separate ratings. The short-time withstand current, usually written Icw, is the RMS current the assembly can carry for a stated time without exceeding a temperature limit. The peak withstand current, usually written Ipk, is the peak current the assembly can survive without damage from the magnetic force.

A design can pass one and fail the other. A bar with a large cross-section may absorb the thermal duty easily, while its supports are too far apart for the peak force. A short, well-braced bar may survive the force, while a thin section overheats before the protection clears.

Icw: the thermal rating

Icw is stated as an RMS current and a duration, for example 25 kA for one second. The duration matters because the heating is proportional to the current squared times the time. A bar rated for 25 kA for one second is not rated for 25 kA for three seconds.

The rating belongs to the assembly, not just the conductor. The test or calculation has to include the joints, the supports and any insulation that touches the bar. A joint with a higher resistance than the bar runs hotter, and a support that traps heat changes the cooling. The temperature limit is usually set by the insulation, the plating or the risk of annealing the copper.

For a first estimate, an adiabatic calculation is often used. It assumes that all the heat stays in the conductor for the duration of the fault, which is conservative for short faults. The temperature rise depends on the current density squared and the time. A higher current density or a longer clearing time needs a larger section.

Ipk: the peak rating

Ipk is the peak value of the fault current, including the asymmetric component that appears in the first cycles. It is the current that produces the maximum force. The relationship between Ipk and Icw depends on the circuit power factor and the point on the wave where the fault starts. Standards and manufacturers use a factor, often between 1.5 and 2.2, and the factor should be stated with the rating.

The peak current is not simply the RMS fault current multiplied by the square root of two. The DC offset can make the first peak much higher. That is why a busbar rated for a certain Icw may need a separate Ipk rating, and why the two numbers are usually quoted together.

The force between bars

Two parallel conductors carrying current in the same direction attract each other. In a fault, the current is large and the force is large. For a pair of long parallel bars, the force per metre is proportional to the square of the current and inversely proportional to the spacing.

For illustration, two bars carrying a peak current of 50 kA with a centre spacing of 50 mm see a force of about 10 kN per metre. That is roughly the weight of a tonne acting on every metre of bar. Move the bars to 100 mm apart and the force halves. The spacing is therefore a design variable, not just a clearance detail.

The force acts on the supports as a repeated load. The bar bends between supports, and the support has to take the reaction. A support that is strong enough but too flexible will let the bar move enough to reduce the clearance or fatigue the fixing.

Support spacing and stiffness

The support spacing sets the bending moment in the bar. For a uniformly loaded span, the bending moment is proportional to the force per metre times the square of the span. Double the span and the moment goes up by four. That is why a small change in support spacing can have a large effect on the stress in the bar.

The bar section resists the moment through its second moment of area. A bar bent about its strong axis is much stiffer than the same bar bent about its weak axis. The orientation of the bar therefore affects the fault performance as well as the current capacity.

The support material matters too. A plastic support that creeps under a sustained load may be fine for the continuous case and marginal for a fault. A support that is brittle may crack under the impact. The fault check should include the support, the fixing and the surrounding structure, not just the bar.

• Keep the support span short where the peak current is high.
• Orient the bar to bend about its strong axis where possible.
• Check the support material for impact and creep.
• Allow for the reaction at each support, not just the bar stress.
• Keep the clearance to other parts under the deflected shape.

How the rating is verified

A short-circuit withstand rating can be verified by test or by calculation, depending on the standard and the assembly. A test applies the stated current for the stated time and checks the assembly afterwards. The pass criteria usually include no deformation that affects clearance, no damage to the insulation, no loosening of the joints and no failure of the supports.

A calculation can be used for a design check, but it has to model the actual assembly. The bar section, the support spacing, the joint detail and the enclosure all affect the result. A calculation for a bare bar in free air is not a verification of an assembly in a cabinet.

For a new design, ask for the test report or the calculation basis. Check the duration, the peak current, the spacing and the condition of the sample after the test. A report that states only the current, without the duration and the pass criteria, is not enough to compare with another supplier.

What to put in the specification

State the prospective fault current, the protection clearing time, the required Icw and the required Ipk. Add the ambient, the mounting and the enclosure, because the thermal rating depends on them. If the assembly will be tested as part of a switchgear assembly, state the standard and the test sequence.

Then check the design against both ratings. The thermal check sets the section and the joint detail. The peak check sets the support spacing and the strength of the fixings. If either check is marginal, change the design before the test, not after.

1. 01

   Get the fault study

   Obtain the prospective current, the peak and the clearing time from the protection study.

2. 02

   Set the ratings

   State Icw with its duration and Ipk with its basis.

3. 03

   Check the section

   Confirm the thermal withstand for the current density and the duration.

4. 04

   Check the force

   Calculate the force at the actual spacing and support span.

5. 05

   Check the supports

   Confirm the support, fixing and surrounding structure can take the reaction.

6. 06

   Verify

   Use a test or a calculation that models the actual assembly.