The same three single-core cables installed in trefoil formation versus flat formation can experience several times less resultant electromagnetic force during a fault. That is not a rule of thumb — it follows directly from three-phase symmetry and the geometry of conductor spacing. Understanding why trefoil works determines which cleat type to specify and why a substitution with three individual cleats negates the advantage entirely.
§ 01 Why three-phase single-core cables are bundled
Three-phase AC currents are 120° apart in time. In a balanced system, their instantaneous sum is always zero: i_A + i_B + i_C = 0 at every moment. This means the combined magnetic field of all three conductors, at sufficient distance from the group, is very nearly zero. The closer the cables are grouped together, the more completely this cancellation operates.
Trefoil formation — all three cables in contact, arranged in an equilateral triangle — maximises the grouping symmetry and therefore the extent of field cancellation. The practical consequences are lower losses in surrounding metalwork and, critically, lower resultant electromagnetic forces on the cables themselves during a fault.
§ 02 Trefoil vs. flat: force behaviour compared
Three cables in a line
- Centre conductor receives forces from both outer phases — maximum resultant
- Asymmetric force distribution: outer cables driven outward, centre compressed or deflected
- During a fault all three fly apart; no geometric cancellation
- Highest cleat strength requirement for a given fault level
Three cables in equilateral triangle
- Equal spacing between all pairs; three-fold rotational symmetry
- Resultant force on each conductor directed toward the bundle centre — partial cancellation by symmetry
- Net resultant substantially lower than flat formation at the same fault current
- External magnetic field largely cancelled; lower eddy-current heating in nearby metalwork
§ 03 Why the cleat must lock the geometry
The force advantage of trefoil formation depends entirely on the cables remaining in the equilateral triangle arrangement throughout the fault. If the cables separate — even partially — the symmetry breaks, force asymmetry increases sharply, and the rating calculated for trefoil becomes invalid.
A trefoil cable cleat is a three-pocket design that holds all three conductors in the correct relative position simultaneously. It performs two functions:
- Geometry lock: prevents the cables from separating under the electromagnetic impulse, preserving the cancellation benefit;
- Force transfer: carries the residual net force to the supporting structure.
Substituting three individual single-core cleats — even positioned in approximate trefoil geometry — provides no geometry lock. During a fault, the cables immediately diverge and the force conditions revert to an uncontrolled state.
§ 04 Material requirement for trefoil cleats
A trefoil cleat encircles three single-core AC conductors. If the cleat material is ferromagnetic (mild steel, galvanised steel), it forms a magnetic circuit around three-phase conductors. Even though the net external field is small, proximity heating from eddy currents and hysteresis losses in the cleat body can be significant.
The rule for single-core installations applies to trefoil cleats too: use non-magnetic material — aluminium alloy, austenitic stainless steel (304/316), or engineering nylon. The heating mechanism is explained in detail in Cable Cleat Materials.