Cable tray load curves
Structural forms of trough-type cable trays
Structural forms of modular cable trays
Structural forms of truss-type ladder trays and long-span cable trays
Selection of cable trays
In engineering design, the layout of cable trays should be determined by comprehensively comparing factors such as economic rationality, technical feasibility, and operational safety to select the optimal scheme; requirements for construction and installation, maintenance and inspection, and cable laying must also be fully met.
When installed horizontally, the height of the cable tray above the ground should generally be no less than 2.5 m; for vertical installations, the section below 1.8 m from the ground should be protected by a metal cover plate, except when installed in dedicated electrical rooms. If a cable tray is installed horizontally in an equipment mezzanine or on a walkway at a height of less than 2.5 m, protective grounding measures should be implemented.
Cable trays, wire ducts, and their supports/hangers used in corrosive environments should be manufactured from corrosion-resistant rigid materials or undergo anti-corrosion treatment; the treatment method must meet the requirements of the project environment and durability. Aluminum alloy cable trays are recommended for locations requiring high corrosion resistance or cleanliness.
In sections with fire protection requirements, materials such as fire-resistant or flame-retardant boards or mesh may be added to cable ladder trays or perforated trays to form an enclosed or semi-enclosed structure; measures such as applying fire-retardant coatings to the surfaces of the trays and their supports/hangers should also be taken. The overall fire resistance performance must comply with relevant national codes or standards. Aluminum alloy cable trays should not be used in locations with high fire protection requirements.
Solid-bottom (non-perforated) tray-type cable trays should be selected for cable circuits requiring electromagnetic interference shielding or for environments necessitating protection against external factors such as outdoor sunlight, oil, corrosive liquids, or flammable dust.
In locations prone to dust accumulation, cable trays should be fitted with cover plates; in public passageways or sections crossing roads outdoors, backing plates should be added to the bottom-most tray, or solid-bottom trays should be used. 1.7 Cables of different voltage levels or intended for different purposes should not be laid on the same tier of a cable tray system:
(1) Cables rated above 1 kV and cables rated 1 kV or below;
(2) Dual-circuit cables supplying power to a Class I load along the same route;
(3) Emergency lighting cables and general lighting cables;
(4) Power cables, control cables, and telecommunication cables.
If cables of different categories are laid in the same cable tray, a partition plate must be installed to separate them.
When the length of a straight steel cable tray section exceeds 30 m, or an aluminum alloy cable tray section exceeds 15 m, or when the cable tray crosses a building expansion (or settlement) joint, a compensation allowance of 0–30 mm should be provided; expansion connection plates should be used for the connection.
The selection of width and height for cable ladders and trays must comply with fill rate requirements. Generally, the fill rate for power cables in ladders or trays may range from 40% to 50%, and for control cables from 50% to 70%; additionally, a reserve capacity of 10% to 25% should be allowed for future expansion.
When selecting the load class for a cable tray, the actual uniformly distributed working load must not exceed the rated uniformly distributed load of the selected class. If the actual span of the cable tray supports or hangers is not 2 m, the working uniformly distributed load must satisfy the following condition:
Where: qG - working uniformly distributed load (kN/m);
qE - rated uniformly distributed load (kN/m);
LG - actual span (m).
