ELECTRICAL SYSTEMS(James M.Bannon) .pdf
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1、15.1 SECTION FIFTEEN ELECTRICAL SYSTEMS James M. Bannon Chief Electrical Engineer STV Incorporated Douglassville, Pennsylvania Design of the electrical installations in a building used to be simple and straight- forward. Such installations generally included electrical service from a utility com- pa
2、ny; power distribution within the building for receptacles, air conditioning, and other electrical loads; lighting; and a few specialty systems, such as fi re alarm and telephone. There were, of course, some specialized installations for which this sim- ple description did not apply, but such buildi
3、ngs were uncommon. Now, however, design of electrical systems has become more complex and sophisticated. This development has been driven by rapid advances in technology, availability of computers and computerized equipment, more enlightened life-safety and secu- rity concerns, and changes in the ph
4、ilosophical outlook of workers toward their workplace and their need for a comfortable environment. To meet these needs, a new building will likely include in its electrical installation an access control sys- tem, intrusion detection system, an extensive computer data network, Internet ac- cess, un
5、interruptible power supply, and numerous other systems not commonly installed in the past. Corollary to the advent of these new building systems is the need for suitable power quality to support them. Though highly sophisticated and capable, these systems can easily be disrupted or damaged by power
6、system anom- alies such as sags, surges, noise, and power outages. Electrical design elements to protect against these disturbances must be included and must be designed to be appropriately sensitive, fast, and robust. The introduction of electrical competition in some states adds further complexity
7、 to the electrical system design problem. Not only have these systems become common but the basic electrical systems have undergone drastic changes. Advances in electrical-power-distribution materials and methods, which have occurred at a nearly uniform rate since the turn of the century, have accel
8、erated rapidly under the infl uence of computers and microproc- essor controls. New light sources give designers added opportunities to improve lighting and energy effi ciency. Microprocessor-based fi re-alarm systems with ad- dressable devices offer greatly improved protection, fl exibility, and ec
9、onomy. And establishment of more local telecommunication operating companies and competi- tion between them, encouraging innovation, has brought designers new choices and challenges with respect to telecommunication systems for buildings. 15.2SECTION FIFTEEN Nevertheless, the basic principles of ele
10、ctrical design still apply, and they are described in this section. In addition, the section was developed to be helpful to those who must assume responsibility for applying, coordinating, integrating, and installing the many electrical systems now available for buildings. 15.1ELECTRICAL POWER In ma
11、ny ways, transmission of electricity in buildings is analogous to water-supply distribution. Water fl ows through pipes, electricity through wires or other conduc- tors. Voltage is equivalent to pressure; wire resistance, to pipe friction; and electric current, or fl ow of electrons, to water drople
12、ts. The hydraulic analogy is limited to only very elementary applications with elec- tric fl ow like direct current, which always fl ows in the same direction. The analogy does not hold for alternating current, which reverses fl ow many times per second without apparent inertia drag. Direct-current
13、systems are simple two-wire circuits, whereas alternating current uses two, three, or four wires and the formulas are more complex. Any attempt to apply the hydraulic analogy to alternating currents would be more confusing than helpful. The mathematical concepts are the only guides that remain true
14、over the whole area of application. Ampere (abbreviated A) is the basic unit for measuring fl ow of current. The unit fl owing is an electric charge called a coulomb. An ampere is equivalent to a fl ow of one coulomb per second. One source of direct current is the battery, which converts chemical en
15、ergy into electric energy. By convention, direct current fl ows from the positive terminal to the negative terminal when a conductor is connected between the terminals. The voltage between battery terminals depends on the number of cells in the battery. For a lead-plate-sulfuric-acid battery, this v
16、oltage is about 1.5 to 2 V per cell. For high voltages, a generator is required. A generator is a machine for con- verting mechanical energy into electrical energy. The basic principle involved is illustrated by the simple experiment of moving a copper wire across the magnetic fi eld between a north
17、 pole and south pole of a magnet. In a generator, the rotor is wound with coils of wire and the magnets are placed around the stator in pairs, two, four, six, and eight. When the coil on the rotor passes through the magnetic fi eld under a south pole, current fl ows in one direction. When the same c
18、oil passes through the north-pole fi eld, the current reverses. For this reason, all generators produce alternating current. If direct current is required, the coils are connected to contacts on the rotor, which transfer the current to brushes arranged to pick up the current fl owing in one directio
19、n only. The contacts and brushes comprise the com- mutator. If the commutator is omitted, the generator is an alternator, producing alternating current. See also Conversion of AC to DC in Art. 15.3. 15.2DIRECT-CURRENT SYSTEMS Resistance of fl ow through a wire, measured in units called ohms (?), dep
20、ends on the wire material. Metals like copper and aluminum have low resistance and are classifi ed as conductors. ELECTRICAL SYSTEMS15.3 FIGURE 15.1Types of electric circuits: (a) series; (b) parallel. Resistance for a given material varies inversely as the area of the cross section and directly as
21、the length of wire. Ohms law states that the voltage E (volts) required to cause a fl ow of current I (amperes) through a wire with resistance R (ohms) is given by E?IR(15.1) Power P is measured in watts and is the product of volts and amperes: 2 P?EI?(IR)I?I R(15.2a) or 2 EE P?EI?E?(15.2b)? ? RR La
22、rge amounts of power are measured in kilowatts (kW), a unit of 1000 W, or megawatts (MW), a unit of 1,000,000 W. Electric Energy.The energy expended in a circuit equals the product of watts and time, expressed as watt-seconds or watt-hours (Wh). For large amounts of energy, a unit of 1000 watt-hours
23、, or kilowatt-hours, kWh, is used. Charges for electric use are usually based on two separate items. The fi rst is total energy used per month, kWh, and the second is the peak demand, or maximum kW required over any short period during the month, usually 15 to 30 min. Power Transmission.Power is usu
24、ally transmitted at very high voltages to min- imize the power loss over long distances. This power loss results from the energy consumed in heating the transmission cables and is equal to the square of the current fl owing I, times a constant representing the resistance r of the wires,?/ft, times t
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