Installation of Cable
2.11 Preparation
Prior to installing cable in the tray or ladder, examine the cable paths to ensure all areas are free of debris that may interfere with the cable’s installation. Surface areas of tray or ladder components likely to come into contact with cables shall not cause damage to the cables when installed according to the manufacture’s instruction or this guide.
Cable tray or cable ladder should never be used as a walkway.
2.12 Wiring Regulations
The installation of cables shall meet the requirements of BS 7671 (The Wiring Regulations) or other national requirements as applicable.
2.13 Power Cables
2.13.1 Pulling Considerations
Where cables are large or cable runs are long, their installation may require pulling tools (Figures 23 and 24); in such cases the following is recommended.
• On horizontal straight runs, the cables generally ride on rollers mounted in or on the cable tray or cable ladder (Figure 23a). These rollers should be properly spaced dependent on the size and weight of the cable to prevent the cable from sagging and dragging in the cable tray or cable ladder during the pull. Contact the cable manufacturer for information regarding proper roller spacing.
• On horizontal bends and vertical pulls, the cables are generally run through rollers or pulleys to maintain a minimum bending radius (Figures 23b and 23c). Rollers and pulleys must be of sufficient diameter to prevent pinching the cable between the roller/ pulley flanges. Each cable will have a minimum bending radius that must be maintained to prevent damage to the cable. Information on cable bending radii can be obtained from Table 1. Multiple pulling tools may be required at one bend to maintain this radius. Care should be taken with the entry and exit angle of the cable at the pulling tool, as this angle can exceed the bending radius.
• Due to the length of some cable pulls and the cable weight, a great deal of force can be applied to the pulleys on horizontal and vertical bends. These pulleys should be anchored to the structural steel and not to the cable tray or cable ladder due to the force that can be applied during pulling. Do not pull down on the horizontal rollers, as they are not designed for this purpose, and damage could result to the cable, cable tray or cable ladder.
• Due to the length of some cable pulls and the cable weight, a great deal of force can be applied to the pulleys on horizontal and vertical bends. These pulleys should be anchored to the structural steel and not to the cable tray or cable ladder due to the force that can be applied during pulling. Do not pull down on the horizontal rollers, as they are not designed for this purpose, and damage could result to the cable, cable tray or cable ladder.
2.13.2 Pulling the cable
Larger cables will usually require a pulling sock (basket grip) and/or pulling eye to be attached to the leading end of the cables metallic conductor(s). If the cable does not have a pulling eye attached by the manufacturer, the cable manufacturer should be contacted for information on field installation of a pulling sock and/or pulling eye (Figures 24a and 24b). Where pulling attachments are used on the conductors, they should be covered with protective tape or similar to prevent scoring of the cable trays, cable ladders and installation pulleys.
Cables generally have pulling tension restrictions, so a dynamometer may be installed at the pulling end in order to ensure that the cable’s maximum pulling tension is not exceeded. The cable should be pulled at a constant speed. The maximum pulling tension and cable pulling speed cable can be obtained from the cable manufacturer. Cables should be placed and not dropped in to the cable tray or cable ladder.
2.13.3 Fastening
• Cables should be fastened to the cable ladder and/or cable tray using cable cleats or cable ties to prevent movement of the cables under normal use and during fault conditions (Figures 25a and 25b). Generally the spacing between cable fastenings should not exceed the dimensions stated in Table 2. For some applications where the fault current level requires it, spacing between cable fastenings may be less than those stated in Table 2. Where this applies details should be obtained from the electrical installation designer and/or the supplier of the fastenings. Cable cleats and cable ties should be correctly sized and only tightened enough to secure the cable without indenting the insulation sheath.
• On vertical runs the fastenings must be able to withstand the forces exerted by the weight of the cable. The cable weight should be supported in such a manner as to prevent damage to the cable ladder, cable tray or cable.
• Where possible it is best practice to position cable cleats on alternate rungs of the cable ladder in order to evenly spread the load along the length of the cable ladder as illustrated in Figure 25a.
2.14 Data Cables
2.14.1 Installation
There are some general rules that apply to the installation of all data cable bundles, regardless of containment type, and they are:
Cable ties must not be too tight. Any cable within a tied bundle must be able to be moved through that tie with slight resistance. Data and optical cables cannot stand the same heavy-duty ‘lashing’ as power cables. The tie must not be too thin as it may cut into the sheath of the cable.
The minimum bend radius shall not be less than that specified by the cable manufacturer. Manufacturers generally specify six to eight times the cable diameter as the cable bend radius.
There is no exact or correct figure for the amount of cables allowed in any one bundle, typically a figure of between 24 and 48 cables is used.
2.14.2 Segregation
Where power and data cables are installed within the same containment system or within close proximity to each other, suitable segregation shall be used. Guidance on segregation can be found from BS 6701 and BS EN 50174.
2.15 Expansion
Where expansion joints are present in the cable tray or cable ladder installation, provision must be made for the cable to expand and contract correspondingly. This is usually achieved with a loop in the cable at the expansion joint position.
2.16 Electro Mechanical Effects Electrical Short Circuits
When an electrical short circuit occurs under fault conditions the current that flows can in some instances reach tens of thousands of amps which can last from a few milliseconds to several seconds depending on the electrical installation requirements. Such short circuit currents produce high magnetic fields which can interact to produce large mechanical forces. These forces can cause significant displacement of the cables and therefore some form of restraint must be provided to prevent damage to the cables. For large diameter cables the most common form of restraint is by the use of cable cleats which hold the cables to the cable ladder or cable tray. Some of the force may therefore be transferred to the cable ladder or cable tray via the cable cleat, and could be sufficient to cause damage to the ladder or tray.
The calculation of the forces is complex and the effect on a cable ladder or cable tray can only be fully determined by testing. See photographs below showing the effects of testing.
Where such large electrical fault currents could possibly occur then the cable ladder/cable tray/cable cleat manufacturers should be consulted.
For reference the calculation of the forces between two conductors can be carried out using the formula given in BS EN 61914:2009:
For reference the calculation of the forces between two conductors can be carried out using the formula given in BS EN 61914:2009:
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