In electric power distribution, a busbar is a metallic strip or bar housed inside switchgear, panel boards, busway enclosures for local high current power distribution. They are used to connect high voltage equipment at electrical switchyards, low voltage equipment in battery banks, they are uninsulated, have sufficient stiffness to be supported in air by insulated pillars. These features allow sufficient cooling of the conductors, the ability to tap in at various points without creating a new joint; the term busbar is derived from the Latin word omnibus, which translates into English as "for all", indicating that a busbar carries all of the currents in a particular system. The material composition and cross-sectional size of the busbar determine the maximum amount of current that can be safely carried. Busbars can have a cross-sectional area of as little as 10 square millimetres, but electrical substations may use metal tubes 50 millimetres in diameter or more as busbars. An aluminium smelter will have large busbars used to carry tens of thousands of amperes to the electrochemical cells that produce aluminium from molten salts.
Busbars are produced in a variety of shapes, such as flat strips, solid bars, or rods, are composed of copper, brass, or aluminium as solid or hollow tubes. Some of these shapes allow heat to dissipate more efficiently due to their high surface area to cross-sectional area ratio; the skin effect makes 50–60 Hz AC busbars more than about 8 millimetres thickness inefficient, so hollow or flat shapes are prevalent in higher current applications. A hollow section has higher stiffness than a solid rod of equivalent current-carrying capacity, which allows a greater span between busbar supports in outdoor electrical switchyards. A busbar must be sufficiently rigid to support its own weight, forces imposed by mechanical vibration and earthquakes, as well as accumulated precipitation in outdoor exposures. In addition, thermal expansion from temperature changes induced by ohmic heating and ambient temperature variations, as well as magnetic forces induced by large currents, must be considered. In order to address these concerns, flexible bus bars a sandwich of thin conductor layers, were developed.
These require a structural cabinet for their installation. Distribution boards split the electrical supply into separate circuits at one location. Busways, or bus ducts, are long busbars with a protective cover. Rather than branching from the main supply at one location, they allow new circuits to branch off anywhere along the route of the busway. A busbar may either be supported on insulators, or else insulation may surround it. Busbars are protected from accidental contact either by a metal earthed enclosure or by elevation out of normal reach. Power neutral busbars may be insulated because it is not guaranteed that the potential between power neutral and safety grounding is always zero. Earthing busbars are bare and bolted directly onto any metal chassis of their enclosure. Busbars may be enclosed in a metal housing, in the form of bus duct or busway, segregated-phase bus, or isolated-phase bus. Busbars may be connected to each other and to electrical apparatus by bolted, clamped, or welded connections.
Joints between high-current bus sections have precisely-machined matching surfaces that are silver-plated to reduce the contact resistance. At extra high voltages in outdoor buses, corona discharge around the connections becomes a source of radio-frequency interference and power loss, so special connection fittings designed for these voltages are used. 110 kV busbars in electrical substations Electrical busbar system Bus Bus duct Walter A. Elmore. Protective Relaying Theory and Applications. Marcel Dekker Inc. ISBN 978-0-8247-9152-0. Paschal, John. "Ensuring a Good Bus Duct Installation". Electrical Construction & Maintenance. Retrieved 2009-04-06. ·Assessment Of Bus Duct And Their Relevance·
An electrical conduit is a tube used to protect and route electrical wiring in a building or structure. Electrical conduit may be made of metal, fiber, or fired clay. Most conduit is rigid. Conduit is installed by electricians at the site of installation of electrical equipment, its use and installation details are specified by wiring regulations, such as the US National Electrical Code and other building codes. Some early electric lighting installations made use of existing gas pipe serving gas light fixtures, converted to electric lamps. Since this technique provided good mechanical protection for interior wiring, it was extended to all types of interior wiring and by the early 20th century purpose-built couplings and fittings were manufactured for electrical use. However, most electrical codes now prohibit the routing of electrical conductors through gas piping, due to concerns about damage to electrical insulation from the rough interiors of pipes and fittings used for gas. Electrical conduit provides good protection to enclosed conductors from impact and chemical vapors.
Varying numbers and types of conductors can be pulled into a conduit, which simplifies design and construction compared to multiple runs of cables or the expense of customized composite cable. Wiring systems in buildings may be subject to frequent alterations. Frequent wiring changes are made simpler and safer through the use of electrical conduit, as existing conductors can be withdrawn and new conductors installed, with little disruption along the path of the conduit. A conduit system can be made submersible. Metal conduit can be used to shield sensitive circuits from electromagnetic interference, can prevent emission of such interference from enclosed power cables. Non-metallic conduits are light-weight, reducing installation labor cost; when installed with proper sealing fittings, a conduit will not permit the flow of flammable gases and vapors, which provides protection from fire and explosion hazard in areas handling volatile substances. Some types of conduit are approved for direct encasement in concrete.
This is used in commercial buildings to allow electrical and communication outlets to be installed in the middle of large open areas. For example, retail display cases and open-office areas use floor-mounted conduit boxes to connect power and communications cables. Both metal and plastic conduit can be bent at the job site to allow a neat installation without excessive numbers of manufactured fittings; this is advantageous when following irregular or curved building profiles. Special tube bending equipment is used to bend the conduit without denting it; the cost of conduit installation is higher than other wiring methods due to the cost of materials and labor. In applications such as residential construction, the high degree of physical damage protection may not be required, so the expense of conduit is not warranted. Conductors installed within conduit cannot dissipate heat as as those installed in open wiring, so the current capacity of each conductor must be reduced if many are installed in one conduit.
It is impractical, prohibited by wiring regulations, to have more than 360 degrees of total bends in a run of conduit, so special outlet fittings must be provided to allow conductors to be installed without damage in such runs. Some types of metal conduit may serve as a useful bonding conductor for grounding, but wiring regulations may dictate workmanship standards or supplemental means of grounding for certain types. While metal conduit may sometimes be used as a grounding conductor, the circuit length is limited. For example, a long run of conduit as grounding conductor may have too high an electrical resistance, not allow proper operation of overcurrent devices on a fault. Conduit systems are classified by the wall thickness, mechanical stiffness, material used to make the tubing. Materials may be chosen for mechanical protection, corrosion resistance, overall cost of the installation. Wiring regulations for electrical equipment in hazardous areas may require particular types of conduit to be used to provide an approved installation.
Rigid metal conduit is a thick-walled threaded tubing made of coated steel, stainless steel or aluminum. Galvanized rigid conduit is galvanized steel tubing, with a tubing wall, thick enough to allow it to be threaded, its common applications are in industrial construction. Intermediate metal conduit is a steel tubing heavier than EMT but lighter than RMC, it may be threaded. Electrical metallic tubing, sometimes called thin-wall, is used instead of galvanized rigid conduit, as it is less costly and lighter than GRC. EMT itself can be used with threaded fittings that clamp to it. Lengths of conduit are connected to equipment with clamp-type fittings. Like GRC, EMT is more common in commercial and industrial buildings than in residential applications. EMT is made of coated steel, though it may be aluminum. Aluminum conduit, similar to galvanized steel conduit, is a rigid tube used in commercial and industrial applications where a higher resistance to corrosion is needed; such locations would include food processing plants, where large amounts of water and cleaning chemicals would make galvanized conduit unsuitable.
Aluminum can not be directly embedded in concrete. The conduit may be coated to prevent corrosion by incidental contact with concrete. Aluminum conduit is lower cost than steel in
Ducts are conduits or passages used in heating and air conditioning to deliver and remove air. The needed airflows include, for example, supply air, return air, exhaust air. Ducts also deliver ventilation air as part of the supply air; as such, air ducts are one method of ensuring acceptable indoor air quality as well as thermal comfort. A duct system is called ductwork. Planning, optimizing and finding the pressure losses through a duct system is called duct design. Ducts can be made out of the following materials: Galvanized mild steel is the standard and most common material used in fabricating ductwork because the zinc coating of this metal prevents rusting and avoids cost of painting. For insulation purposes, metal ducts are lined with faced fiberglass blankets or wrapped externally with fiberglass blankets; when necessary, a double walled duct is used. This will have an inner perforated liner a 1–2" layer of fiberglass insulation contained inside an outer solid pipe. Rectangular ductwork is fabricated to suit by specialized metal shops.
For ease of handling, it most comes in 4' sections. Round duct is made using a continuous spiral forming machine which can make round duct in nearly any diameter when using the right forming die and to any length to suit, but the most common stock sizes range evenly from 4" to 24" with 6"-12" being most used. Stock pipe is sold in 10' joints. There are 5' joints of the non-spiral type pipe available, used in residential applications. Aluminium ductwork is quick to install. Custom or special shapes of ducts can be fabricated in the shop or on site; the ductwork construction starts with the tracing of the duct outline onto the aluminium preinsulated panel. The parts are typically cut at 45°, bent if required to obtain the different fittings and assembled with glue. Aluminium tape is applied to all seams where the external surface of the aluminium foil has been cut. A variety of flanges are available to suit various installation requirements. All internal joints are sealed with sealant. Aluminum is used to make round spiral duct, but it is much less common than galvanized steel.
Traditionally, air ductwork is made of sheet metal, installed first and lagged with insulation. Today, a sheet metal fabrication shop would fabricate the galvanized steel duct and insulate with duct wrap prior to installation. However, ductwork manufactured from rigid insulation panels does not need any further insulation and can be installed in a single step. Both polyurethane and phenolic foam panels are manufactured with factory applied aluminium facings on both sides; the thickness of the aluminium foil can vary from 25 micrometres for indoor use to 200 micrometres for external use or for higher mechanical characteristics. There are various types of rigid polyurethane foam panels available, including a water formulated panel for which the foaming process is obtained through the use of water and CO2 instead of CFC, HCFC, HFC and HC gasses. Most manufacturers of rigid polyurethane or phenolic foam panels use pentane as foaming agent instead of the aforementioned gasses. A rigid phenolic insulation ductwork system is listed as a class 1 air duct to UL 181 Standard for Safety.
Fiberglass duct board panels provide built-in thermal insulation and the interior surface absorbs, helping to provide quiet operation of the HVAC system. The duct board is formed by sliding a specially-designed knife along the board using a straightedge as a guide; the knife automatically trims out a groove with 45° sides which does not quite penetrate the entire depth of the duct board, thus providing a thin section acting as a hinge. The duct board can be folded along the groove to produce 90° folds, making the rectangular duct shape in the fabricator's desired size; the duct is closed with outward-clinching staples and special aluminum or similar metal-backed tape. Flexible ducts are made of flexible plastic over a metal wire coil to shape a tube, they have a variety of configurations. In the United States, the insulation is glass wool, but other markets such as Australia, use both polyester fibre and glass wool for thermal insulation. A protective layer surrounds the insulation, is composed of polyethylene or metalised PET.
It is sold as boxes containing 25' of duct compressed into a 5' length. It is available in diameters ranging from as small as 4" to as big as 18", but the most used are sizes ranging from 6" to 12". Flexible duct is convenient for attaching supply air outlets to the rigid ductwork, it is attached with long zip ties or metal band claps. However, the pressure loss is higher than for most other types of ducts; as such and installers attempt to keep their installed lengths short, e.g. less than 15 feet or so, try to minimize turns. Kinks in flexible ducting must be avoided; some flexible duct markets prefer to avoid using flexible duct on the return air portions of HVAC systems, however flexible duct can tolerate moderate negative pressures. The UL181 test requires a negative pressure of 200 Pa; this is an air distribution device and is not intended as a conduit for conditioned air. The term fabric duct is therefore somehow misleading. However, as it replaces hard ductwork, it is easy to perceive it as a duct.
Made of polyester material, fabric ducts can provide a more distribution and blending of the conditioned air in a given space than a conventional duct system. They may be man
The Earth–ionosphere waveguide refers to the phenomenon in which certain radio waves can propagate in the space between the ground and the boundary of the ionosphere. Because the ionosphere contains charged particles, it can behave as a conductor; the earth operates as a ground plane, the resulting cavity behaves as a large waveguide. Low frequency and low frequency signals can propagate efficiently in this waveguide. For instance, lightning strikes launch a signal called radio atmospherics, which can travel many thousands of miles, because they are confined between the Earth and the ionosphere; the round-the-world nature of the waveguide produces resonances, like a cavity. Radio propagation within the ionosphere depends on frequency, angle of incidence, time of day, Earth's magnetic field, solar activity. At vertical incidence, waves with frequencies larger than the electron plasma frequency of the F-layer maximum fe = 9 1/2 kHz can propagate through the ionosphere nearly undisturbed. Waves with frequencies smaller than fe are reflected within the ionospheric D-, E-, F-layers.
Fe is of the order of 8–15 MHz during day time conditions. For oblique incidence, the critical frequency becomes larger. Low frequencies, low frequencies are reflected at the ionospheric D- and lower E-layer. An exception is whistler propagation of lightning signals along the geomagnetic field lines; the wavelengths of VLF waves are comparable with the height of the ionospheric D-layer. Therefore, ray theory is only applicable for propagation over short distances, while mode theory must be used for larger distances; the region between Earth's surface and the ionospheric D-layer behaves thus like a waveguide for VLF- and ELF-waves. In the presence of the ionospheric plasma and the geomagnetic field, electromagnetic waves exist for frequencies which are larger than the gyrofrequency of the ions. Waves with frequencies smaller than the gyrofrequency are called hydromagnetic waves; the geomagnetic pulsations with periods of seconds to minutes as well as Alfvén waves belong to that type of waves. The prototype of a short vertical rod antenna is a vertical electric Hertz dipole in which electric alternating currents of frequency f flow.
Its radiation of electromagnetic waves within the Earth-ionospheric waveguide can be described by a transfer function T: Ez = T Eo where Ez is the vertical component of the electric field at the receiver in a distance ρ from the transmitter, Eo is the electric field of a Hertzian dipole in free space, ω = 2πf the angular frequency. In free space, it is T = 1. Evidently, the Earth–ionosphere waveguide is dispersive because the transfer function depends on frequency; this means that phase- and group velocity of the waves are frequency dependent. In the VLF range, the transfer function is the sum of a ground wave which arrives directly at the receiver and multihop sky waves reflected at the ionospheric D-layer. For the real Earth's surface, the ground wave becomes dissipated and depends of the orography along the ray path. For VLF waves at shorter distances, this effect is, however, of minor importance, the reflection factor of the Earth is Re = 1, in a first approximation. At shorter distances, only the first hop sky wave is of importance.
The D-layer can be simulated by a magnetic wall with a fixed boundary at a virtual height h, which means a phase jump of 180° at the reflection point. In reality, the electron density of the D-layer increases with altitude, the wave is bounded as shown in Figure 2; the sum of ground wave and first hop wave displays an interference pattern with interference minima if the difference between the ray paths of ground and first sky wave is half a wavelength. The last interference minimum on the ground between the ground wave and the first sky wave is at a horizontal distance of ρ1 ≈ 2 f h2/c with c the velocity of light. In the example of Figure 3, this is at about 500 km distance; the theory of ray propagation of VLF waves breaks down at larger distances because in the sum of these waves successive multihop sky waves are involved, the sum diverges. In addition, it becomes necessary to take into account the spherical Earth. Mode theory, the sum of eigen-modes in the Earth–ionosphere waveguide is valid in this range of distances.
The wave modes have fixed vertical structures of their vertical electric field components with maximum amplitudes at the bottom and zero amplitudes at the top of the waveguide. In the case of the fundamental first mode, it is a quarter wavelength. With decreasing frequency, the eigenvalue becomes imaginary at the cutoff frequency, where the mode changes to an evanescent wave. For the first mode, this happens at fco = c / ≈ 1 kHz below; the attenuation of the modes increases with wavenumber n. Therefore only the first two modes are involved in the wave propagation The first interference minimum between these two modes is at the same distance as that of the last interference minimum of ray theory indicating the equivalence of both theories As seen in Figure 3, the spacing between the mode interference minima is constant and about 1000 km in this example; the first mode becomes dominant at distances greater than about 1500 km, because the second mode is more attenuated than the first mode. In the range of ELF waves, only mode theory is appropriate.
The fundamental mode is the zeroth mode. The D-layer becomes here an electric wall (Ri
In telecommunications, an atmospheric duct is a horizontal layer in the lower atmosphere in which the vertical refractive index gradients are such that radio signals are guided or ducted, tend to follow the curvature of the Earth, experience less attenuation in the ducts than they would if the ducts were not present. The duct acts as an atmospheric dielectric waveguide and limits the spread of the wavefront to only the horizontal dimension. Atmospheric ducting is a mode of propagation of electromagnetic radiation in the lower layers of Earth’s atmosphere, where the waves are bent by atmospheric refraction. In over-the-horizon radar, ducting causes part of the radiated and target-reflection energy of a radar system to be guided over distances far greater than the normal radar range, it causes long distance propagation of radio signals in bands that would be limited to line of sight. Radio "ground waves" propagate along the surface as creeping waves; that is, they are only diffracted around the curvature of the earth.
This is one reason. The best known exception is; the reduced refractive index due to lower densities at the higher altitudes in the Earth's atmosphere bends the signals back toward the Earth. Signals in a higher refractive index layer, i.e. duct, tend to remain in that layer because of the reflection and refraction encountered at the boundary with a lower refractive index material. In some weather conditions, such as inversion layers, density changes so that waves are guided around the curvature of the earth at constant altitude. Phenomena of atmospheric optics related to atmospheric ducting include the green flash, Fata Morgana, superior mirage, mock mirage of astronomical objects and the Novaya Zemlya effect. Sky wave Thermal fade Temperature inversion Tropospheric ducting Earth-Ionosphere waveguide
Duct tape called duck tape, is cloth- or scrim-backed pressure-sensitive tape coated with polyethylene. There are a variety of constructions using different backings and adhesives, the term'duct tape' is used to refer to all sorts of different cloth tapes of differing purposes. Duct tape is confused with gaffer tape. Another variation is heat-resistant foil duct tape useful for sealing heating and cooling ducts, produced because standard duct tape fails when used on heating ducts. Duct tape is silvery gray, but available in other colors and printed designs. During World War II, Revolite developed an adhesive tape made from a rubber-based adhesive applied to a durable duck cloth backing; this tape was used as sealing tape on some ammunition cases during that period. The first material called "duck tape" was long strips of plain cotton duck cloth used in making shoes stronger, for decoration on clothing, for wrapping steel cables or electrical conductors to protect them from corrosion or wear. For instance, in 1902, steel cables supporting the Manhattan Bridge were first covered in linseed oil wrapped in duck tape before being laid in place.
In the 1910s, certain boots and shoes used canvas duck fabric for the upper or for the insole, duck tape was sometimes sewn in for reinforcement. In 1936, the US-based Insulated Power Cables Engineers Association specified a wrapping of duck tape as one of many methods used to protect rubber-insulated power cables. In 1942, Gimbel's department store offered venetian blinds that were held together with vertical strips of duck tape. All of these foregoing uses were for plain cotton or linen tape that came without a layer of applied adhesive. Adhesive tapes of various sorts were in use by the 1910s, including rolls of cloth tape with adhesive coating one side. White adhesive tape made of cloth soaked in rubber and zinc oxide was used in hospitals to bind wounds, but other tapes such as friction tape or electrical tape could be substituted in an emergency. In 1930, the magazine Popular Mechanics described how to make adhesive tape at home using plain cloth tape soaked in a heated liquid mixture of rosin and rubber from inner tubes.
In 1923, Richard Gurley Drew working for 3M invented masking tape, a paper-based tape with a mildly sticky adhesive. In 1925 this became the Scotch brand masking tape. In 1930, Drew developed a transparent tape based on cellophane, called Scotch Tape; this tape was used beginning in the Great Depression to repair household items. Author Scott Berkun has written that duct tape is "arguably" a modification of this early success by 3M. However, neither of Drew's inventions was based on cloth tape; the idea for what became duct tape came from Vesta Stoudt, an ordnance-factory worker and mother of two Navy sailors, who worried that problems with ammunition box seals would cost soldiers precious time in battle. She wrote to President Franklin D. Roosevelt in 1943 with the idea to seal the boxes with a fabric tape, which she had tested at her factory; the letter was forwarded to the War Production Board, who put Johnson on the job. The Revolite division of Johnson & Johnson had made medical adhesive tapes from duck cloth from 1927 and a team headed by Revolite's Johnny Denoye and Johnson & Johnson's Bill Gross developed the new adhesive tape, designed to be ripped by hand, not cut with scissors.
Their new unnamed product was made of thin cotton duck coated in waterproof polyethylene with a layer of rubber-based gray adhesive bonded to one side. It was easy to apply and remove, was soon adapted to repair military equipment including vehicles and weapons; this tape, colored in army-standard matte olive drab, was nicknamed "duck tape" by the soldiers. Various theories have been put forward for the nickname, including the descendant relation to cotton duck fabric, the waterproof characteristics of a duck bird, "Water off a duck's back", the name of the 1942 amphibious military vehicle DUKW, pronounced "duck". According to etymologist Jan Freeman, the story that duct tape was called duck tape is "quack etymology" that has spread "due to the reach of the Internet and the appeal of a good story" but "remains a statement of faith, not fact." She notes that duct tape is not made from duck cloth and there is no known primary-source evidence that it was referred to as duck tape. Her research does not show any use of the phrase "duck tape" in World War II, indicates that the earliest documented name for the adhesive product was "duct tape" in 1960.
The phrase "duck tape" to refer to an adhesive product does not appear until the 1970s and was not popularized until the 1980s, after the Duck brand became successful and after the New York Times referred to and defined the product under the name "duct tape" in 1973. After the war, the duck tape product was sold in hardware stores for household repairs; the Melvin A. Anderson Company of Cleveland, acquired the rights to the tape in 1950, it was used in construction to wrap air ducts. Following this application, the name "duct tape" came into use in the 1950s, along with tape products that were colored silvery gray like tin ductwork. Specialized heat - and cold-resistant tapes were developed for air-conditioning ducts. By 1960 a St. Louis, Missouri, HVAC company, Albert Arno, Inc. trademarked the name "Ductape" for their "flame-resistant" duct tape, capable of holding together at 350–400 °F. In 1971, Jack Kahl renamed it Manco. In 1975, Kahl rebranded the duct tape made by his company; because the used gene
In anatomy and physiology, a duct is a circumscribed channel leading from an exocrine gland or organ. Examples include: As ducts travel from the acinus which generates the fluid to the target, the ducts become larger and the epithelium becomes thicker; the parts of the system are classified as follows: Some sources consider "lobar" ducts to be the same as "interlobar ducts", while others consider lobar ducts to be larger and more distal from the acinus. For sources that make the distinction, the interlobar ducts are more to classified with simple columnar epithelium, reserving the stratified columnar for the lobar ducts. Ductal carcinoma Endocrine gland Anatomy photo: termscells&tissues/epithelial/exocrinegland/exocrinegland1 - Comparative Organology at University of California, Davis - "Exocrine gland" Overview at uwa.edu.au Overview at siumed.edu