Vertical cable support in shafts — cleat spacing per IEC 61914

The guide on pulling cables — pulling tension and bend radius covers the forces that occur while installing a cable: the pulling tension that may be applied to the conductor and the minimum bend radius that must not be undershot during pulling-in. This article covers a different, permanent load: what a vertically installed cable in a shaft or riser carries after installation, and how cable cleats per IEC 61914 absorb that load.

Two different loads, two different moments

While a cable is being pulled in, the tension on the conductor is temporary and ends once the cable is in place. In a vertical shaft, a different, permanent load then arises: the cable's own weight, which continuously pulls on the lowest fixing and on every intermediate cleat for as long as the cable is in service. Without sufficient intermediate supports, the full weight of a long vertical run is carried by the top fixing or by the terminal connections at the end — a load those connection points are not designed for.

Function of the cable cleat per IEC 61914

IEC 61914 specifies requirements and test methods for cable cleats that mechanically support cables along their route length and hold them in place. A cable cleat has two distinct functions:

Static support: carrying the cable's own weight between two fixing points, so no single point in the run becomes permanently overloaded.
Dynamic fixation during short-circuit: restraining the electrodynamic forces that occur between phase conductors during a short-circuit, so the cables are not thrown apart or displaced (see the guide on electrodynamic forces during short-circuit for the underlying physics of those forces, explained there for busbars but essentially the same for cables in trefoil or flat formation).

IEC 61914 therefore tests cleats both for static clamping force and for withstanding a defined short-circuit current for the test duration, and assigns cleats a classification based on which short-circuit current and which cable configuration (trefoil or flat) they can withstand.

Cleat spacing: no universal figure

The maximum permissible spacing between cable cleats in a vertical shaft is not a fixed figure from the standard itself, but is determined by the cleat manufacturer in combination with the cable weight per metre, the cable diameter, and — for medium-voltage cables — the expected short-circuit current. Practical points to note:

Manufacturer tables typically give a maximum support spacing per cable diameter and weight; for a vertical run this spacing is often smaller than for a horizontal run, because in a shaft the cleat carries the full weight of the section beneath it rather than merely supporting part of it.
On ladder trays, cleat spacing is often tied to a multiple of the ladder's rung spacing (for example every 300 mm), so each cleat can be fixed to a fixed rung rather than at an arbitrary point.
Extra cleats close to the top fixing point of a long vertical section are often applied, since the cumulative pulling load from the cable weight hanging below is greatest there.
For medium-voltage cables in trefoil formation, cleat spacing must also be checked against the electrodynamic short-circuit force: a higher prospective short-circuit rating requires closer cleat spacing to prevent the cables being thrown apart during a fault.

Practical inspection relevance

During periodic inspection of a cable shaft or riser, it is worth checking for:

Sagging cable between two cleats, indicating a support spacing too large for the actual cable weight.
Deformed or cut-into sheath at a cleat location, indicating a cleat that is over-tightened or incorrectly sized.
Loose or displaced cleats, particularly after a short-circuit event elsewhere in the installation — this can indicate a prior electrodynamic load that the cleat barely, or only just, withstood.
Missing cleats in the lower part of a long vertical run, where the cumulative load is greatest.

Common mistakes

Applying horizontal-run spacing to a vertical run, while the self-weight in a shaft requires a different, often smaller, maximum spacing.
Selecting cable cleats without accounting for the prospective short-circuit rating for medium-voltage cables in trefoil formation, so the cleat cannot withstand a short-circuit.
Confusing the pulling-tension and bend-radius requirements from installation with the permanent support requirements after installation — these are two different assessments against different standards.
Not applying extra cleats near the top fixing point of a long vertical section, leaving the cumulative load concentrated on a single connection point.