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Knob-and-tube wiring (sometimes abbreviated K&T) was an early standardized method of electrical wiring in buildings, in common use in North America from about 1880 to the 1930s. It consisted of single-insulated copper conductors run within wall or 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. Where conductors entered a wiring device such as a lamp or switch, or were pulled into a wall, they were protected by flexible cloth insulating sleeving called loom. The first insulation was asphalt-saturated cotton cloth, then rubber became common. Wire splices in such installations were twisted together for good mechanical strength, then soldered and wrapped with rubber insulating tape and friction tape (asphalt saturated cloth), or made inside metal junction boxes. Knob and tube wiring was eventually displaced from interior wiring systems because of the high cost of installation compared with use of power cables, which combined both power conductors of a circuit in one run (and which later included grounding conductors). At present, new knob and tube installations are permitted in the US only in a few very specific situations listed in the National Electrical Code, such as certain industrial and agricultural environments. ==Elements== Ceramic knobs were cylindrical and generally nailed directly into the wall studs or floor joists. Most had a circular groove running around their circumference, although some were constructed in two pieces with pass-through grooves on each side of the nail in the middle. A leather washer often cushioned the ceramic, to reduce breakage during installation. By wrapping electrical wires around the knob, and securing them with tie wires, the knob securely and permanently anchored the wire. The knobs separated the wire from potentially combustible framework, facilitated changes in direction, and ensured that wires were not subject to excessive tension. Because the wires were suspended in air, they could dissipate heat well. Ceramic tubes were inserted into holes bored in wall studs or floor joists, and the wires were directed through them. This kept the wires from coming into contact with the wood framing members and from being compressed by the wood as the house settled. Ceramic tubes were sometimes also used when wires crossed over each other, for protection in case the upper wire were to break and fall on the lower conductor. Ceramic cleats, which were block-shaped pieces, served a purpose similar to that of the knobs. Not all knob and tube installations utilized cleats. Ceramic bushings protected each wire entering a metal device box, when such an enclosure was used. Loom, a woven flexible insulating sleeve, was slipped over insulated wire to provide additional protection whenever a wire passed over or under another wire, when a wire entered a metal device enclosure, and in other situations prescribed by code. Other ceramic pieces would typically be used as a junction point between the wiring system proper, and the more flexible cloth-clad wiring found in light fixtures or other permanent, hard-wired devices. When a generic power outlet was desired, the wiring could run directly into the junction box through a tube of protective loom and a ceramic bushing. Wiring devices such as light switches, receptacle outlets, and lamp sockets were either surface-mounted, suspended, or flush-mounted within walls and ceilings. Only in the last case were metal boxes always used to enclose the wiring and device. Image:Knob-corner.jpg|Knob supporting a wire change in direction. Image:Splice-knob.jpg|Knob supporting a splice Image:Support-knob.jpg|Knobs supporting long runs of wire Image:Splice-knob-corner.jpg|Knobs serving multiple functions Image:Tubes-no-knobs.jpg|Ceramic tubes protecting wires passing through ceiling joists. Image:Tube-socket.jpg|Ceramic junction for suspended light socket. Note deteriorated wire insulation. Image:Splice-socket.jpg|Splice with more modern power outlet, a probable code violation as seen here. Image:Knob-and-tubes.jpg|Wiring running through tubes, and turning a corner supported by a knob. Notice the direct splice with more modern (1950s-era) NonMetallic sheathed cable. This type of connection is forbidden by the National Electrical Code, and a junction box should have been used. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Knob-and-tube wiring」の詳細全文を読む スポンサード リンク
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