Electrical Products

Why use cable cleats for securing cables?

Cable cleats are designed to fix, retain and support cables. In addition, where short-circuit faults are anticipated, correct cleating will result in the containment of the cables during a fault and enable the circuit to be restored once the fault has been repaired.

When adjacent cables carrying three phase current suffer a short circuit fault, the induced magnetic fields result in the cables experiencing significant opposing forces, a safe installation requires well designed and thoroughly tested cable cleats.

Short circuits and short circuit testing

Short circuit current is given either as a “peak” or an “rms” value. The peak current is the maximum current experienced by any of the phases and it occurs once within the first few milliseconds of the start of the fault.
The rms current is a calculated value for the initial cycles of the fault. The relationship between peak current and rms current varies from installation to installation.

The forces experienced by a cleat during a short circuit are a function of short circuit current, cleat spacing and the distance between the cable centres (in the case of trefoil arrangements this is the cable diameter). When comparing short circuit test results for different products all three factors must be taken into consideration to compare the relative aggressiveness of the tests. The following formula taken from the Cenelec standard for cable cleats EN 50368 calculates the force experienced by a cleat with cables arranged in trefoil formation:

Where:

Ft = maximum force on the cable conductor in trefoil formation for a three phase short-circuit (N/m)

i_p = the peak short-circuit current (kA)

S = the cable diameter (m)

This formula gives the static load instantaneously created by the short circuit, however the phase changes of the alternating current mean that the forces on the cable and their direction are changing continuously.
No simple formula and no static test can assess the effect of these dynamic forces. The ultimate proof of any product to withstand a short-circuit is to undertake a short-circuit test.

Ellis Patents carry out short circuit tests on a regular basis at various testing stations including The British Short Circuit Testing Station, Hebburn (BSTS). This comprehensive testing program enables us to produce empirical results when developing new products as well as to ensure that our existing products meet the appropriate International Standards. Recent tests have been conducted in accordance with the requirements of the current draft European Standard.

Ellis Patents are happy to allow their cable cleats to be subjected to any comparative tests with competitors’ products.

Copies of test reports and video footage of tests on CD Rom are available on request.

The selection of trefoil cable cleats

When the cable diameter and anticipated maximum fault level are known it is easy to select a cleat and the appropriate spacing to give the most economical installation. However, other constraints such as space on the ladder or the need to fit the base before the strap may mean other alternatives must be considered.
The following tables give recommended cleat spacing along the cable run for the Ellis Patents range of cleats for more popular cable sizes at different peak fault levels.

Table of Cleat Features

LSF Materials

Our LSF plastics contain no halogen or phosphorus components and, in the event of fire, emit low smoke & fumes.

We use 2 grades as appropriate:

a. LSF Nylon - A rigid plastic with VL94 accreditation to V0 at 1.6mm and a Limited Oxygen index of 31%

b. LSF Polymeric - A fire resistant flexible plastic to BS 7878:7 and DIN VDE 0207 Part 24 Type HM2 (Germany)

and a Limited Oxygen index of 35%