Cable cleats — short-circuit withstand per IEC 61914

The guide on electrodynamic forces in a busbar short circuit per IEC 60865-1 covers how the force between conductors during a short circuit is calculated. This article covers the fixing that actually has to absorb that force on cables: the cable cleat, whose mechanical short-circuit withstand is tested and specified per IEC 61914 ("Cable cleats for electrical installations").

Why a cable cleat is more than a fixing point

A cable resting loosely in a cable tray or on a ladder is loaded only by its own weight during normal operation. During a short circuit, however, a brief but enormous, pulsating electrodynamic force arises between the cables of a group — the same physics as with busbars, but between flexible cable conductors that want to move apart under that force. A cable cleat must absorb that force for the full duration of the protection's tripping time without failing, releasing, or damaging the cable sheath — a completely different requirement from simply carrying the static cable weight.

What IEC 61914 tests and specifies

Short-circuit test of 0.1 s (five cycles at 50 Hz): the standard prescribes a test duration of 0.1 second, during which the cleat must withstand a specified test current without failure of the fixing to the support or unacceptable damage to the cable itself. Within that 0.1 s the peak of the electrodynamic force already occurs within the first half-cycle.
Mechanical retention, axial and lateral: besides short-circuit withstand, the standard also tests how much pull-out force (axial, along the cable) and side force (lateral, across the cable) a cleat can take without the cable slipping out or the cleat deforming.
Environmental influences: UV resistance, temperature range and fire behaviour of the cleat material (often plastic or with a metal bracket) are also tested, since a cleat used outdoors, in a cable basement, or along a fire-rated route must meet different environmental requirements than an indoor cleat.
Manufacturer documentation of the mounting surface: because a cleat's short-circuit withstand depends partly on how the cleat itself is fixed to the support (cable tray, ladder, wall bracket), the manufacturer specifies the mounting method for which the tested short-circuit level is valid.

Note: a cable cleat's short-circuit withstand level applies only in combination with the cable configuration, cleat spacing and mounting method with which the cleat was tested. The same cleat with a larger spacing, a different cable diameter range, or a different mounting to the support can have a substantially lower actual short-circuit withstand than the value stated by the manufacturer.

Cleat spacing and the relation to prospective short-circuit current

The maximum allowable spacing between two cable cleats is directly linked to the prospective short-circuit current expected at that point in the installation (see the [guide on prospective short-circuit current and Icu sizing](/guides/nen-1010/prospectieve-kortsluitstroom-icu-dimensionering)): the higher the prospective short-circuit current, the shorter the cleat spacing must be to keep the electrodynamic deflection of the cables between two cleats within the limits tested by the manufacturer. Cleat spacing based only on cable weight — without accounting for the short-circuit current at that point — can lead to excessive deflection and possible cleat failure or sheath damage during a severe fault.

Practical relevance

When designing a cable route with a high prospective short-circuit current — for example close to a transformer or a main distribution board — it is important to select not only the cable cross-section and the type of tray or ladder, but also a cable cleat with a manufacturer short-circuit withstand level demonstrably tested per IEC 61914 that matches the actual short-circuit current and the chosen cleat spacing at that point.

Common mistakes

Using an arbitrary, generic cable cleat without checking what short-circuit withstand level the manufacturer has demonstrated per IEC 61914, assuming that any cleat that mechanically holds the cable can also withstand a short circuit.
Basing cleat spacing only on cable weight, without factoring in the prospective short-circuit current at that point in the installation.
Increasing cleat spacing beyond the manufacturer-tested configuration to save material, without checking whether the stated short-circuit withstand level is still valid.
Assessing a cleat used outdoors or on a fire-rated route on mechanical strength alone, without checking the UV resistance, temperature range, or fire behaviour of the cleat material.