Laying cables and wires in cable trays and ducts
The laying of cable lines in trays and ducts is one of the most commonly used methods of installing cable lines in the premises of various industrial enterprises and electrical installations.
As a rule, it is carried out in an open way on walls and ceilings in dry, humid, hot and fire-hazardous rooms, as well as in rooms with chemically aggressive environments. It is mainly used in industrial buildings and structures, technical rooms, basements, warehouses, workshops, as well as in outdoor installations in the surrounding areas.
With the open method of cable laying, trays and ducts are used to structure cable and low-current systems, and at the same time they provide unhindered access and visual control of cable routes.
Cable ducts can be used for both hidden and open electrical wiring. Cable trays in residential and administrative premises are mainly used for laying the electrical network in a closed way (behind walls, suspended ceilings or raised floors). And the open method of laying the electrical network using cable trays is allowed only for industrial highways.
The cable tray is an open, non-combustible trough-like structure made of various materials and used for cable and wiring inside buildings and outdoor installations. The trays are mounted on supporting structures along walls and ceilings, as a result of which a kind of shelves are formed, where wires or power cables are laid in one or more layers with a given distance.
Trays do not protect the laid cable from damage, but form the frame of the cable network and fix the location. And their main task is to ensure easy, safe and orderly routing of power and signal cables and wires.
By design, the trays can be solid, perforated, ladder type and wire.
Cable ducts differ from trays in that they are closed. They are hollow sections of rectangular, triangular, square or other cross-section with a flat base with removable covers or blind, with perforated surfaces or not.
Ducts with removable covers are used for open electrical wiring. Blind ducts are a non-disassembled hollow structure with solid walls and without a removable cover and are used in case of hidden laying.
Like trays, ducts are attached to walls and ceilings and, unlike trays, they are designed to protect the cables laid in them from mechanical damage.
According to rules for electrical equipment installation, cable trays for laying cables in cable structures are made of metal, non-metal or composite. And cable trays for laying cables in the ground or on the surface of the soil are made of reinforced concrete.
Trays are made only from non-combustible materials. The most common metal cable trays are made of galvanized or stainless steel, galvanized iron or aluminum. Galvanized steel trays have the best performance and corrosion resistance. Thanks to this, they can be used for laying cables both outside and inside buildings, for laying lines in any premises and mounting on a vertical and horizontal surface.
Ducts, as well as trays, are made of metal, but unlike trays, they can also be made of various plastics. Steel ducts are produced with solid or perforated walls and lids and are used in rooms and outdoor installations instead of steel pipes for lighting and power networks. An open laying of steel ducts is allowed in dry, humid, hot and fire hazardous premises, where laying in steel pipes is not required. And it is prohibited in damp, especially damp, with a chemically active environment and explosive rooms.
Non-metallic (plastic) products are used for laying low-voltage cables indoors, mainly in residential and office buildings. Relatively inexpensive, small in size, light enough, they are not afraid of moisture and fit well into the interior. At the same time, they are distinguished by low strength and heat resistance, short life during operation and are prone to deformation when cables are heated, therefore, they are not used for external installation.
Cable ducts made of PVC plastic are the most widely used in residential and office buildings.
Composite trays and ducts are made of polymeric materials (synthetic polyester resins) and fiberglass by hot pressing or pultrusion. They have high mechanical strength and rigidity, increased resistance to vibration, moisture resistance and frost resistance, resistance to any corrosion, ultraviolet radiation, aggressive chemical environments and temperature extremes. They are distinguished by exceptional insulating properties, low thermal conductivity and lightness of construction, which makes them easy to transport and install. They also provide a long service life.
Composite products can be successfully used for organizing cable routes outside and inside premises, for above-ground and underground laying. They are recommended for the creation of cable routes in aggressive environments and the most difficult operating conditions. Available in solid and perforated, open and closed types.
Reinforced concrete trays are used when laying cable routes underground or on the surface of the ground. They are able to withstand heavy loads, are durable, do not allow water to pass through, retain their integrity during temperature fluctuations and soil movement (due to which they can be used in seismically active zones and wet soils).
After installing the wiring or other elements inside the tray, it is closed with a lid and, in case of laying underground, the lid is covered with the soil, after which the tray provides complete safety to the structures and highways inside.
The use of trays simplifies and speeds up the installation process. And cables laid in reinforced concrete trays can be easily revised and repaired after opening the cover.
Perforated trays and ducts are products with perforated holes in the base. This design reduces the weight of the product and facilitates the installation of cables, as it allows the tray or duct to be attached to the ceiling or wall directly through the enclosure. It provides ventilation of the cable route, preventing overheating of the cable, removes moisture, but at the same time it worse protects the line from the dust.
Solid trays and ducts are closed-type non-perforated products with the solid base and surface. They are used when laying lines that require a high level of protection from the environment on the streets, in mines, public places. They protect the line from the negative external influences, dust and precipitation. At the same time, it excludes the ventilation of the cables, and, consequently, prevents their natural cooling.
The ladder trays consist of stamped sidewalls, connected by jumpers at a given distance, and outwardly resemble a regular ladder. They are able to withstand heavy loads and are used for laying the most massive highways. They are used for vertical cable laying, when installing open cable routes and laying wiring inside the mine. They do not impede natural ventilation of cables and leave free access to the route.
Wire trays are made from galvanized steel wire pieces that are joined by welding. They provide lightness of construction, excellent ventilation of cables and wires, free access to conductors for inspection and maintenance, require a minimum of accessories and tools for installation.
They are used for the installation of open lines in places where it is necessary to lay a large number of cables of different types, laying cable routes and electrical wiring inside various rooms, buildings and structures. They allow you to quickly branch the cable system in the right places, as well as connect the necessary electrical devices to the conductors.
At the same time, they are not designed for a significant load and are used in the construction of lightweight cable shafts and horizontal routes.
The type and material of the tray or duct is selected depending on the laying conditions and the room in which the cables are planned to be laid. When choosing a profile, the type and size of the cable are also taken into account, since the size of the duct or tray depends on this. The height and width of the bead are selected taking into account the diameter of the cable or wire bundle to be laid, providing for a certain margin in height and width. The greater the weight of the laid wires or cables, the thicker the walls of the duct should be.
Installation of electrical wiring in trays and ducts is carried out in the following sequence:
- The marking of the route in compliance with the places of installation of the supporting and holding structures, as well as the points of their attachment to the building elements of the premise.
- Installation of supporting structures, in the form of racks, brackets or hangers on walls, ceilings and under ceilings at a height of at least 2 m from the floor or service platform. In rooms accessible only to electrical personnel, the height of the trays and ducts is not standardized.
- Fastening trays and ducts to supporting structures.
- Connection of elements of trays and ducts to each other. The trays are connected to each other by bolted overlays or by welding. And the connection of the sections of the ducts is carried out using connectors and bolts. In dry, dust-free rooms, in which there are no vapors and gases that adversely affect the insulation and sheath of wires and cables, it is allowed to connect ducts without sealing. In rooms that contain vapors or gases that negatively affect the insulation or sheaths of wires and cables, in outdoor installations and in places where oil, water or emulsion can get into the ducts, as well as in dusty rooms, the ducts should be connected to each other and to the housings electrical equipment with a seal. Ducts in these cases should be with solid walls and with sealed solid covers or blind.
- Roll out cables and wires using a winch or manually (with a cable length of up to 50 m), using rolling rollers.
- Laying of wires and cables. After broaching, wires and cables from the rolling rollers are transferred to cable trays and ducts by hand and fixed.
- Connection and fastening of wires and cables.
On the trays, it is allowed to lay unarmored power cables with a voltage of up to 1 kV and a cross-section of no more than 16 mm2. Cables and wires are laid on trays in four different ways:
– in rows with 5 mm intervals between them;
– bundles in one row with a distance between the bundles of about 20 mm (the number of wires in the bundle should not exceed 12, and the outer diameter of the bundle should not be more than 0.1 m);
– packages with a distance of 20 mm between them;
– multilayer without gaps.
Bundles of cables and wires are fastened with bandages: on horizontal straight sections at a distance of no more than 4.5 m and on vertical sections – no more than 1 m. Fastening of individual wires and cables on straight sections of the route is not required for horizontal installation of trays. In places where the route turns and at branches, wires and cables are fixed at a distance of no more than 0.5 m from the turn or branch.
In ducts, wires and cables are laid single- and multi-layer with an arbitrary arrangement so that the layer height in one duct does not exceed 0.15 m. When the ducts are horizontally arranged with the lid upwards, the fastening of wires and cables to the duct is not required. When the duct cover is located on the side, the wires and cables are attached to the duct after 3 m, when laying the duct with the cover down, they are fixed after 1.5 m and with the vertical location of the duct – after 1 m.
Also, cables and wires are fixed at their extreme points, on couplings, line bends and other connection points.
Laying of cables and wires is carried out in the form of measured segments or directly from drums and coils. The cables are laid in trays and ducts in such a way as to provide a margin in length necessary to compensate for changes in the length of the conductor due to temperature fluctuations. When laying, no more than half of the volume of the tray or duct is filled for subsequent easy access for repair and maintenance of the line and for air cooling of the wiring.
The way of wiring installation in ducts, due to the closed structure, should not allow moisture and condensation to accumulate in them. For this purpose, to inspect the condition of the ducts, they provide for opening hatches and removable covers.
At the ends of trays and ducts, turns of the track and branches, as well as at the points where wires are connected to electrical equipment, marking tags are installed. Trays or ducts connected to the trunk are grounded.
Summing up our review, it should be noted that the organization of cable routes using special cable trays and ducts by open laying has a number of significant advantages. This is primarily:
– simplicity of line support due to free access to conductors for repair and maintenance;
– economical installation (in comparison with the hidden type of laying or installation in pipes);
– reduction of time-consuming operations of fastening wiring;
– ensuring good cooling conditions for cables;
– the possibility of using in rooms with a chemically unfavorable environment, high humidity and temperature.
Trays provide organized routing and cooling of large accumulations of cables and remove them at a safe distance from the aggressive environment, heat sources or hazardous devices. They allow you to easily equip branches in any area and increase the number of cables in the tray. Their disadvantages include weak protection of cables from external influences and a restriction on the laying of electrical wiring in an open way in damp rooms.
The ducts, in turn, provide protection of cables from external influences, but at the same time worsen the conditions for their cooling, which leads to a decrease in their current load.
In addition, the installation of cable trays and trunks requires a lot of space and has rather low aesthetic characteristics.