1．Cable Tray Structure Type Selection
In the engineering design documents are usually generalized cable tray called “bridge”, did not point out the specific structural features, different types, different production materials, cable tray price difference is large, and the structure of the type of confusion will bring about the work site heat dissipation, mechanical protection problems. Therefore, at the design stage, designers should be required to choose the structural characteristics of cable trays according to the characteristics of the project environment and technical requirements, and clearly express them in the model labeling and material table of the plan.
2．Cable Tray Material Selection
The material of cable tray is another common problem in cable tray engineering design. According to the material division, cable trays are mainly steel, FRP and aluminum alloy. FRP cable trays are characterized by light weight, specific gravity is only 1/4 of carbon steel; good water resistance and corrosion resistance, suitable for chemical plants. Not easy to burn, flame retardant FRP cable tray oxygen index ≥ 32. long service life, general design life of 20 years, but the price is three times the steel cable tray. The superiority of the construction is easy to cut, flexible assembly, installation without fire, which for the explosion hazardous environment and the tight project schedule of the chemical plant project is particularly significant, because in the explosion hazardous environment, the project must be shut down when the installation of fire, the chemical plant, the economic benefits will be affected. Aluminum alloy cable tray weight is also very light, due to aluminum, steel specific gravity is different (Al = 2.7, Fe = 7.86), according to the weight calculation, the ratio of aluminum to steel is about 1: 3. Aluminum alloy cable tray dimensions, load characteristics are basically similar to the steel bridge. As far as the cost is concerned, the cost of aluminum alloy bridge is 20% higher than that of galvanized steel cable tray, and the service life is more than 5 times of that of steel cable tray.
3．Cable tray surface corrosion layer category selection
The third common problem in engineering design is that the cable bridge model is not labeled corrosion layer categories, there is no uniform text description. This problem in reality there are lessons, such as our country to undertake general contracting of a project in Indonesia, steel cable tray surface anti-corrosion treatment did not carry out salt spray test, completed shortly after the bridge corrosion is quite serious, had to be replaced. Cable bridge surface anti-corrosion layer categories are mainly hot dip galvanized, galvanized nickel, cold galvanized, powder electrostatic spraying, etc., manufacturer information shows: hot dip zinc process life of not less than 40 years, applicable to outdoor heavy corrosion environment, high cost; galvanized nickel process life of not less than 30 years, also applies to outdoor heavy corrosion environment, high cost; cold galvanized process life of not less than 12 years, applies to light corrosion environment, the cost is general; powder electrostatic spraying, the cost is general; the surface corrosion of steel cable tray has to be replaced, the bridge had to be replaced. General cost; powder electrostatic spraying process life is not less than 12 years, applicable to indoor dry environment at room temperature, general price. Designers should reasonably select the surface anticorrosive layer category of cable tray according to the environmental conditions of the project, and express it clearly in the design documents.
4．Cable Tray Fire Rating Selection
Cable tray in the fire requirements of the section, can be added in the cable ladder frame, tray with fire-resistant or flame-retardant properties of the board, net and other materials to form a closed or semi-closed structure, and to take in the bridge and its support hanger surface brush fireproof coating and other measures, and its overall fire resistance should meet the requirements of the relevant national norms or standards. It is not suitable to use aluminum alloy cable trays in places with high fireproofing requirements of the project.
Under the same specification, the flame retardant cable tray is 2.2 times more expensive than the steel cable tray, the service life is more than 5 times of the steel cable tray, and the weight is 30% heavier than the steel cable tray. Fire-retardant cable tray is slightly more expensive than steel cable tray, the service life is more than 3 times that of steel cable tray, and the weight is basically the same as steel cable tray.
5．Selection of fill rate for cable trays
Cable ladder racks, tray width and height selection should be in line with the requirements of the fill rate, the cable ladder racks and trays within the fill rate in general, power cables can be taken as 40% to 50%, control cables can be taken as 50% to 70%, and it is desirable to reserve 10% to 25% of the project development margin.
6．Selection of cable tray load rating
When selecting the load level of cable tray, the working uniform load of cable tray should not be greater than the rated uniform load of the selected load level of cable tray, and the working uniform load should meet the requirements if the actual span of the support hanger of cable tray is not equal to 2m. Various components and support hangers in the conditions to meet the corresponding load, its specifications and dimensions should be matched with the tray, ladder frame straight section, bend through the series.
7．Cable Tray Specification Size Selection
In engineering practice, the problem in the selection of the specification size of the cable tray is either on the large side or on the tight side. How to reasonably select the specification size of the cable tray? “Electrical Design Code for Civil Buildings” (JGJ16-2008) in the 8, 10, 7 provisions: ”the ratio of the total cross-sectional area of the cable to the cross-sectional area in the tray, power cables should not be greater than 40%; control cables should not be greater than 50%.”
8．Select cable tray according to cable bending radius
When selecting the bend-through or lead-on and lead-off devices for cable trays, they should not be smaller than the minimum allowable bending radius of the cables in the cable trays.
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