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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
Fundamentals of electrical work. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Electric installation work When assembling and installing electrical devices, electric installation work, which should be understood as cable and overhead lines, closed and open substations, power and lighting equipment, etc. The production and organization of electrical work implies compliance with the requirements of the system of regulatory documents in construction and the standardization system. The main documents of the system of regulatory documents are the Construction Norms and Rules (SNiP), Electrical Installation Rules (PUE), fire protection rules, safety regulations, departmental instructions, as well as instructions from electrical equipment manufacturers. The installation of electrical devices is carried out in accordance with the working drawings and according to the relevant documentation of the factories - manufacturers of technological equipment. In the production of electrical installation and electrical repair work, they operate with the following concepts: Voltage To transmit electricity over long distances, voltages of several tens and even hundreds of thousands of volts are used. In most cases, 220 V electricity is used in everyday life. Compared to the voltage of electrical networks (6-220 kV) and high-voltage power lines (330-750 kV), the voltage of 220 V is small, therefore it is sometimes called low voltage, although "low" is not means "safe": Improper use of equipment and appliances can result in life-threatening injuries. If you touch bare wires or other live parts that are energized with 220 V, an electric current will pass through the human body, which can lead to death. For the safe use of electricity in cramped conditions (basements, etc.) and with an increased risk of electric shock, a low voltage is used - 12 or 36-42 V. The voltage of 12 V is considered safe, and 36-42 V in rooms with conductive (earth, cement) floors or walls is allowed only for connecting permanently installed lamps in a protective design. In garages and other utility rooms with non-conductive floors and walls made of stone, concrete or non-conductive materials finished inside, voltages up to 42 V can be used for power tools and portable lamps with a protected lamp. To obtain low voltage, special transformers are used, for example, a transformer for household needs with a voltage of 220/36 or 220/12 V. Voltage deviation The passage of electric current through the wires is accompanied by losses, as a result of which the voltage at the end of the line is somewhat lower than at the beginning. In order for all consumers connected to the line to be supplied with electricity with a reliable voltage level, at the beginning of the line at a transformer substation (TS) it has to be increased by 5-8% relative to the nominal 380/220 V. In rural areas, according to the quality standards of electric energy for most consumers voltage deviation up to 7,5% of the nominal is allowed. In other words, with a nominal voltage of 220 V, a rural consumer can actually have a voltage of 200 to 240 V. It is assumed that power receivers designed for a voltage of 220 V should operate satisfactorily. For electric motors and luminaires with fluorescent lamps, there are usually no difficulties in this regard due to their low sensitivity to voltage fluctuations. For electric heaters, with a decrease in voltage, the heat output noticeably drops, and with an increase, the service life is reduced. Semiconductor devices (TVs, sound-reproducing devices, household office equipment, etc.) may become inoperable if the voltage fluctuates. Sometimes voltage stabilization devices are built into the equipment, providing insensitivity to voltage deviations in a fairly wide range. If the instructions do not contain any data on permissible voltage deviations, a permissible deviation of 5% is assumed and it is considered that the power receiver must operate properly at a voltage of 210-230 V. In rural areas, the voltage at consumers often goes beyond the specified limits, so you have to use special autotransformers or voltage stabilizers. They are selected according to the power of the electrical receiver, which requires a stabilized voltage. Voltage fluctuations affect electric incandescent lamps quite noticeably: when the voltage decreases, their luminous flux decreases significantly, and when the voltage increases, the service life is reduced. To increase the efficiency of incandescent lamps, they are produced with a voltage of 215-225 to 235-245 V. Lamps marked 220-230 V are designed to operate with small voltage fluctuations. If they last less than a year, lamps for 230-240 or 235-245 V should be used, and when, with year-round operation, their service life exceeds two years, lamps marked 215-225 V should be used. Power In everyday life, electrical receivers with power ranging from fractions of a watt (chargers) to several thousand watts (floor electric stoves) are used. The power actually consumed by the electrical receiver from the network does not always correspond to its rated power, which is indicated on the label. The power consumed by incandescent lamps and electric heaters depends significantly on the voltage: if its value is 5-7% higher than the nominal value, the power will also increase, but by 10-15%, and if the voltage drops, it will decrease accordingly. For mechanical power tools and electric pumps, the power consumption depends mainly on the force that they overcome during operation and should not exceed the nominal one. Electric current strength The value of the current in the wires is determined by the power of the electrical receivers connected to them. To determine the current strength for single-phase receivers, the power consumption in watts is divided by the voltage applied to them in volts and by the power factor - a dimensionless value that does not exceed one. For incandescent lamps and electric heaters, the power factor is equal to one, and for electric motors and transformers it is always less. Its value depends not only on the design of the machine or apparatus, but also on the conditions of their operation. Usually, the power factor is sought to be brought to 0,9-0,92, but there are power receivers in which its value is close to 0,6. What does this mean for the consumer who pays for electricity? The lower the power factor, the more current flows through the wires, therefore, the energy losses in the wires increase. Capacitors connected in parallel with the load are used to improve the power factor. The current in the wires is calculated, assuming the power of the electrical receivers and the voltage applied to them as nominal. In this case, a discrepancy between the current strength and its actual value is possible. For example, at a nominal voltage of 220 V, a 100 W lamp consumes a current of 0,45 A; at a voltage of 250 V, the power of the same lamp will be approximately 120 W, and the current will be 0,5 A; at a voltage of 200 V, respectively, 80 W and 0,4 A, i.e., with voltage deviations, the error in determining the current strength will not exceed 12%. Electrical load The highest value of the current that passes through the wire for a long time (30 minutes or more) is considered to be its electrical load. We present the values of the current strength for electric incandescent lamps, electric heaters and other electrical receivers with a power factor equal to one, at a nominal voltage of 220 V (Table 1). Table 1
If you need to calculate the electrical load of several power receivers, you can sum up their rated currents, when all power receivers have the same power factor or close enough to one. If this is not the case, find the average value of the power factor (approximately 0,8-0,9 can be taken) and calculate the current strength based on the sum of the rated powers. The electrical load on the phase wire from a three-phase power receiver is calculated based on the fact that each phase accounts for one third of the power and that the phase voltage is 1,73 times less than the linear voltage: the power of the three-phase power receiver is divided by the rated line voltage, power factor and 1,73 . Consumers using three-phase current, one of the phases is allocated to power single-phase electrical receivers. The current strength in this phase wire is found by summing up the loads of three- and single-phase electrical receivers. Single-phase power receivers do not affect the current in other phase wires, but they determine the current in the neutral wire. If only three-phase electrical receivers are turned on, then there is no current in the neutral wire. Electrical resistance If a voltage of 220 V is applied to the electrical receiver and a current of 1 A flows, then the resistance of the circuit is 220 ohms. If the resistance is increased, the current will decrease proportionally. Using the relationship between current strength and rated power, we calculate that the resistance of a 220 V power receiver with a power of 15 W is 3200 Ohms, and the resistance of a 1500 W power receiver is only 32 Ohms. The resistance of the wires of the electrical network is usually in the range from fractions of an ohm to 1-2 ohms. The heating of wires by electric current depends on the resistance and current strength. If the electrical connection is made carelessly (screws are not tightened enough, wires are loosely twisted or the insulation is poorly stripped), its resistance is greater than with a quality workmanship, dangerous overheating occurs and there is a risk of fire. In the event of a short circuit, the mains voltage is applied to the wires closed to each other (low resistance) and the current strength reaches hundreds of amperes, which is several times higher than the permissible value. If the necessary protective measures are not taken, there is a risk of ignition of the wires due to their excessive heating. Electric Energy Measured with electricity meters. If the power of electrical receivers is 1 kW, then 1 kWh will be consumed for 1 hour of operation. Electric receivers with a power of 500 W (0,5 kW) will consume the same amount of electricity in 2 hours, and electric lamps with a power of 25 W in almost two days (40 hours), i.e., the power consumption in kilowatt-hours is determined by the product of the power consumption in kilowatts by operating time in hours. Author: Bannikov E.A.
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