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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
The choice of wiring, methods of laying wires and cables. Electrical components. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Electric installation work The main elements of any electrical wiring are wires, cables and cords. What is common between them and how they differ from each other will become clear from a brief definition of each of them. A wire is one or more bare or insulated conductors. The cores can be enclosed in a metal sheath, winding or braiding with fibrous materials or wire (depending on the conditions of laying and operation). A cable is already several insulated wires covered with a sealed metal or non-metal sheath. On top of this shell, there may be one or more protective covers, including armor (depending on the laying and operation conditions). A cord is two or more flexible strands interconnected by a twist or a common non-metallic braid. Depending on the conditions of laying and operation, the cord may have a non-metallic sheath or a protective coating. The cores of wires, cables and cords can be aluminum or copper. Copper wires and cables conduct 1,5 times the current density than aluminum; contact connections of copper wiring are stronger, therefore, the wiring does not overheat; copper wires withstand more deformation and are more resistant to corrosion, therefore, the wiring is more durable. But copper wires and cables are quite expensive: they cost 2-2,5 times more than similar aluminum ones. Therefore, the question of preference for one or another type of material is decided independently. In order to competently choose the right cable or wire in the store, you must: firstly, understand their marking; secondly, to know which brand of wire or cable is used for laying electrical wiring in specific conditions. Depending on the environmental conditions, all rooms are divided into 9 groups. The first group includes dry heated or unheated rooms, if the relative humidity of the air in them does not exceed 60%, the temperature is 30 ° C, there is no conductive dust and no chemically active environment. These areas include the living rooms of the house. The second group includes dusty rooms, if conductive or non-conductive dust is released in the room in such an amount that it can settle on wires, penetrate inside machines, devices. These include premises where bulk non-combustible materials (for example, cement) are used. The third group includes wet rooms, kitchens, stairwells, unheated pantries, etc. where vapors or condensing moisture are released temporarily in small quantities, relative humidity of 60-75% (but not more). The fourth group includes damp rooms, in which the relative humidity of the air exceeds 75% for a long time, but less than 100%, for example, storage for vegetables. The fifth group includes rooms that are especially damp, when the relative humidity of the air is close to 100% (the ceiling, walls, floor and objects are covered with moisture droplets). These include greenhouses, sheds, outdoor installations under a canopy, greenhouses, utility unheated premises. The sixth group is premises with a chemically active or organic environment (for example, mineral fertilizer stores, stables, poultry houses, other premises for animals). The seventh group - hot rooms with a constant temperature above 35 ° C, and in baths, steam rooms and saunas - more than 100 ° C. The eighth group includes fire hazardous areas of premises where flammable liquids with a flash point of vapor above 45 ° C are stored (category II-I), combustible dust or fibers are emitted (category II-II), solid or fibrous combustible substances are contained (category II-IIa ). This group includes outdoor areas with flammable liquids (vapour flash point above 61 °C) or solid flammable substances. All buildings made of wooden structures are classified as fire hazardous - II-IIa. The ninth group includes explosive zones with installations containing explosive gases or vapors of flammable liquids. Wires should be selected appropriate brands necessarily depending on environmental conditions. The methods of their laying inside and outside the premises are selected after determining all the conditions (Tables 2, 3). Table 2. Types of electrical wiring, methods of laying wires and cables
Table 2 (continued). Types of electrical wiring, methods of laying wires and cables
Table 2 (continued). Types of electrical wiring, methods of laying wires and cables
Table 3. Types of electrical wiring, cables and methods for their laying in compliance with fire safety conditions
Table 3 (continued). Types of electrical wiring, cables and methods of their laying in compliance with fire safety conditions
Wire and cable marking consists of a series of letters and numbers that carry information about the material of the conductive cores, insulation and sheath, the nature of the insulation and sheath, the number of cores in the wire and cable and the size of their cross section: - the first in the marking is a capital letter indicating the material of the conductive cores: aluminum - A, copper - the letter is omitted; - the following group of capital letters (one or two) in the marking of wires characterizes their type: P - wire or PP - flat wire; in the marking of cables, the insulation material is indicated at this place: B - polyvinyl chloride, P - polyethylene, R - rubber, H - neurite; - the third place in cable marking is occupied by the sheath material (the meaning of the letter symbols of the sheath material is similar to the value of the letter symbols of the insulation material); - further additional information is encrypted: G - flexible, H - non-combustible; - in the last place there are digital symbols indicating the number of cores and their cross section. Characteristics of the brands of wires and cables most commonly used for laying electrical wiring in domestic conditions are given in Table. 4. Table 4. Brands of wires and cables for electrical wiring
To select a brand of wire or cable for specific purposes, you can use the data not only in Table. 2, 3, but also tab. 5. Table 5. Brands of wires and cables for specific conditions
Among the recommended materials for laying electrical wiring, cords do not appear by chance - they have a different purpose: cords are used mainly to connect household electrical appliances to the network. Cords, as well as wires and cables, have their own marking, in which the capital letter Ш (cord) is in the first place, the rest of the alphabetic characters characterize the insulation material and additional information and have meanings similar to the meanings of the alphabetic characters in the marking of wires and cables. Currently, manufacturers of household electrical equipment install on their products mainly cords of the ShV-1, ShV-2 and ShVVP brands, the PVC insulation of which is pressed into a non-separable plug. Heating electrical appliances (irons, tiles) are equipped with cords with rubber insulation of the ShRS and ShTR brands. To power light fixtures with electric current, ShPS brand cords are used, which at the same time act as a supporting structure, therefore they are equipped with a special load-bearing sheath. Now about how to choose the right wires and cables for laying electrical wiring according to the size of the cross section of the cores based on specific conditions: the maximum value of the current that heats the insulation; mechanical loads on the wire, including in the terminal clamps of the wiring terminal devices. The fact is that the operating temperature of wires and cords should not exceed 65 ° C if their sheath is rubber, and 70 ° C if the sheath is plastic (the temperature values are given taking into account the ambient temperature, that is, if the room temperature is 20 ° C, then the allowable overheating of the insulation is 45 and 50 °C, respectively). Thus, there is a direct relationship between the cross-sectional values of the conductors of wires (cables) and the current passing through them (Table 6), which must be taken into account when constructing the circuit and directly when laying electrical wiring. Table 6. Permissible current values
It should be noted that the data in Table. 6 are acceptable if the wiring is exposed. If hidden wiring is being installed, and even from several wires in one channel (pipe), then the value of the permissible current in them should be reduced by 10-20%. Such an amendment is accepted on the basis that in this case the wires will heat each other. In addition, in conditions of hidden wiring, cooling is much worse. As for the dependence of the allowable current passing through the cords, here the following ratios should be taken as a basis (core cross section (mm2): allowable current strength (A): 0,35: 7; 0,5: 10; 0,7: 14; 1:22; 1,5:26 and 2:31. Core cross section (not to be confused with the cross-sectional area) is determined by the formula: S = 0,78d2, where S is the cross section of the core in mm2, d is the diameter of the core in mm. The core diameter is easiest to measure with a caliper (measurement accuracy is quite high - up to 0,1 mm, so a possible minor error can be neglected). If this measuring tool was not at hand, then to measure the diameter of the core, you can use this simple method: wind a small piece of the core cleaned from insulation on a thick nail, screwdriver or any other rod and tightly squeeze the turns of the wire; measure the resulting spiral with a metal ruler (with a division value of 0,5 mm) and divide this length by the number of turns. But this method of measurement already gives a large error. To determine the cross section of a stranded wire or cable, it is enough to calculate the cross section of one core and multiply the resulting value by the number of cores. The cross section of the cores at low currents, especially in screw terminals, is determined by the mechanical strength of the conductor. It should not be less than 2 mm2 for an aluminum conductor and 1 mm2 for a copper conductor. If the open wiring inside the room is made on rollers, the cross section of the aluminum core should not be less than 2,5 mm2. Author: Korshevr N.G.
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