Short-circuit forces in plain terms

Radial hoop stress, axial compression and thermal withstand are three separate failures with three separate causes. Passing one says nothing about the other two.

3 July 2026 · 8 min read · Engineering

What actually happens in the first cycle

During a through fault the current rises to many times rated value, and because force goes with the square of current, the mechanical loading rises by a much larger factor still. The peak asymmetrical current in the first cycle is the design case, not the steady-state fault current.

The forces split into two directions. Radial forces push the outer winding outward and squeeze the inner winding inward. Axial forces compress the windings towards their electrical centre, or, when the ampere-turns are unbalanced, push them apart unevenly.

The outer winding is in tension, the inner in compression

The outer winding experiences hoop tension, and the check is a straightforward stress comparison against the conductor's proof strength, with the work-hardening state of the copper mattering a great deal. Fully annealed copper and hardened copper are different materials for this purpose.

The inner winding is in compression and fails differently. It buckles, either freely between supports or by collapsing onto the core. That makes the number and stiffness of the axial supports part of the calculation, not a detail of the drawing.

Axial force is a balance problem

If the ampere-turns of the two windings were distributed identically along the height, the axial force would be pure compression towards the centre and comparatively benign. They never are. Taps, unequal winding heights and end effects create an imbalance, and the imbalance produces axial thrust that the clamping structure has to hold.

This is the failure mode that damages the clamping and the pressboard rather than the conductor, and it is the one most sensitive to how the tapping section is arranged.

The third check is thermal, not mechanical

Even if nothing moves, the fault current heats the conductor almost adiabatically for its duration. The check is whether the winding reaches a temperature at which the insulation is damaged or the copper anneals, computed from the initial temperature, the current density and the fault duration.

A design can be mechanically comfortable and thermally marginal, typically when the designer has pushed current density hard to save copper. The three checks are independent, and a short-circuit report that reports one number is reporting a third of the answer.