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WIRING SAFETY CODES In industrialized countries installation of wiring is governed by national or local regulations. While in certain countries a single national body is responsible for electrical safety and installations code, in some other countries a national technical standards-setting organization will produce only a model electrical code, which is then adopted, perhaps with local amendments, by state/provincial or city regulations. The intention of wiring safety codes is to provide technical, performance and material standards that will allow efficient distribution of electrical energy and communication signals, at the same time protecting persons in the building from Electric Shock and preventing Fire or Explosion . Electrical codes arose in the 1880's with the early commercial introduction of electrcal power, since many conflicting standards existed for the selection of wire sizes and other design rules for electrical installations. COUNTRY SPECIFIC North America The first electrical codes in the United States originated in New York in 1881 to regulate installations of electric lighting. The U.S. National Fire Protection Association, a private nonprofit association formed by insurance companies, produced the first draft of the U.S. National Electrical Code in 1885 . Since 1927, the Canadian Standards Association has produced the Canadian ''Safety Standard for Electrical Installations'', which is the basis for provincial electrical codes. Although these two national standards deal with the same physical phenomena and broadly similar objectives, they differ significantly in technical detail. As part of the NAFTA program, US and Canadian standards are slowly converging towards each other, in a process known as harmonization. In the United States local governing bodies such as counties or cities often include the National Electrical Code in their local building codes by reference along with any local differences. The NEC is published by NFPA.org; publication 70; 2005 is the latest edition. This is a consensus code -- written by many individuals of many disciplines on many panels, based upon inputs from sources not limited to panel members. Because of Copyright law one most obtain both the local codes and the National Electrical Code separately. Europe In European countries, an attempt has been made to harmonize national wiring standards in an IEC standard, IEC 60364 ''Electrical Installations for Buildings''. However, this standard is not written in such language that it can readily be adapted as a national wiring code. Neither is it designed for field use by electrical tradesmen and inspectors for verification of compliance to national wiring standards. National codes, such as the NEC or CSA C22.2, exemplify the common objectives of IEC 60364, and provide rules in a form that allows for guidance of persons installing and inspecting electrical systems. The 2006 edition of the Canadian electrical code references IEC 60364 and states that the code addresses the fundamental principles of electrical protection in Section 131. The Canadian code reprints Chapter 13 of IEC 60364 and it is interesting to note that there are no numerical criteria listed in that chapter whereby the adequacy of any electrical installation can be assessed. Germany '' DKE '' - German Commission for Electrical, Electronic & Information Technologies of DIN and VDE - is the German organisation responsible for the elaboration of electrical standards and safety specifications. United Kingdom In the United Kingdom wiring installations are regulated by the produced by the IEE ''Requirements for Electrical Installations: IEE Wiring Regulations, BS 7671: 2001'' which is now in its 16th edition. WIRING METHODS Materials for wiring interior electrical systems in buildings vary depending on:
Wiring systems in a home, for example, are simple, with relatively low power requirements, infrequent changes to the building structure and layout, usually with dry, moderate temperature, and noncorrosive environmental conditions. In a light commercial environment, more frequent wiring changes can be expected, large apparatus may be installed, and special conditions of heat or moisture may apply. Heavy industries have more demanding wiring requirments, such as very large currents and power ratings, frequent changes of equipment layout, corrosive, wet or explosive atmospheres. Early wiring methods The very first interior power wiring systems used conductors that were bare or covered with cloth, which were secured by staples to the framing of the building or on running boards. Where conductors went through walls, they were protected with cloth tape. Splices were done similarly to telegraph connections, and soldered for security. Underground conductors were insulated with wrappings of cloth tape soaked in pitch, and laid in wooden troughs which were then buried. Such wiring systems were unsatisfactory due to the danger of electrocution and fire, and due to the high labour cost for installation. Knob and tube The earliest standardized method of wiring in buildings, from about 1880 to the 1930s, was single cloth-insulated copper conductors run across interior walls or within ceiling cavities, passing through joist and stud drill-holes via protective porcelain insulating tubes, and supported along their length on nailed-down porcelain "knob" insulators. This system is known as "knob-and-tube" from the insulators used. Where conductors entered a wiring device such as a lamp or switch, they were protected by flexible cloth insulating sleeving. Wire splices in such installations were twisted for good mechanical strength, then soldered and wrapped with "friction" tape (asphalt saturated cloth), or made inside metal junction boxes. Historically, the standards for installing electrical wiring were less stringent in the age of knob-and-tube wiring than they are today. Compared to modern electrical wiring standards, the main shortcomings of knob-and-tube era wiring are: knob-and-tube wiring never included a safety ground conductor; knob-and-tube wiring did not confine switching to the hot conductor; knob-and-tube wiring permitted the use of in-line-splices in walls without using an accessible junction box to contain the splice. Older homes may have knob-and-tube wiring for all or part of their electrical system. Such wiring systems may require replacement and modernization, as it may be inadequate for modern levels of power use. Wiring may have been damaged by renovations done in the building, and insulation covering the wires may be brittle due to age or may be damaged by rodents or carelessness (for example, hanging objects off wiring running in accessible areas like basements). Other historical wiring methods Other methods of securing wiring that are now obsolete include:
Cables (see also " Power Cable ") The first cables for building wiring were introduced in 1922. These were two or more solid copper wires, with woven cloth and paper insulation, sometimes impregnated with tar as a protection from moisture wicking. The advantage was that cables require less labor. Just before World War II, the cost and other advantages of cable resulted in a decline in new knob-and-tube installations. Next, the insulation was rubber; through mid-1960 in the US. Homeowners should be advised that rubber-insulated cables have a tendency to become brittle over time, so they must be handled with extreme care, and should be replaced with modern cabling whenever drywall is removed for renovations. When switches, outlets or light fixtures are replaced, the simple act of tightening Marrette s may cause insulation to flake off the conductors. From the mid-1960s to the mid-1980s, many buildings used asphalt-impregnated wiring, which appears to have retained the flexibility of its insulation. Asphalt-impregnated wiring will typically have a cloth-like appearance on the outer jacket, but must be checked for markings like "Al" or "Cu". Aluminum Wiring was common in residential wiring from the mid 1960s to mid 1970s, due to the rising cost of copper. The outer jacket should be checked for markings like "Al" or "Cu". Due to the greater "resistivity" of aluminum, aluminum wiring will typically use one wire gauge larger conductors than would be required of copper - instead of 14 awg ( American Wire Gauge ) for most lighting circuits, aluminum wiring would typically be 12awg on a typical 15 amp circuit, though local building codes may vary. Aluminum conductors were originally used with wiring devices intended for copper wires. Because of the wide expansion characteristics of the indium and steel and brass metals commonly used in receptacle devices of the time, a few residential installations of aluminum wiring failed early, causing fires due to poor connections. Later wiring devices were designed compatible with both aluminum and copper wiring, but because aluminum screw connection joints require more care and attention in installation, aluminum wiring became unpopular and, with moderation of copper prices, small solid aluminum wiring was no longer used in residential construction. Many residential installations are still working without problem after 25 years! Aluminium conductors are still used in industrial installations and for power distribution because it costs less than copper wiring, especially in large sizes needed for heavy current loads. Attention must be given to prevent aluminum flow due to too much or too little torque in mechanical connections, and to continual production of an oxidized layer on the surface. From the mid-1970s, asphalt cables were replaced with PVC -jacketed cables which are still in use. |
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