Dictionary of National Biography, 1901 Supplement/Bright, Charles Tilston > 자유게시판

The wheels, being steel, take it to the running rails. In the German cities we have seen that many of the telegraph-wires are carried under-ground from the center of the city to connect with cables running to other cities. At the other end of the scale, a tram depot may have just a single wire hung directly from insulated supports. Figure 8: Insulated neutral section in an overhead line. This was well understood in the late 19th Century and was one of the reasons why London’s Underground railways adopted a fully insulated DC system with a separate negative return rail as well as a positive rail - the four-rail system. Provided precautions are taken to prevent the voltage getting too high above the zero of the ground, it works very well and has done so for the last century. It is easier to boost the voltage of AC than that of DC, so it is easier to send more power over transmission lines with AC. However, the choice of AC or DC power transmission system along the line is important. Apart from only requiring a simple control system for the motors, the smaller size of urban operations meant that trains were usually lighter and needed less power.

It doesn’t really matter whether you have AC or DC motors, nowadays either can work with an AC or DC supply. It should be mentioned at this point that corrosion is always a factor to be considered in electric supply systems, particularly DC systems. Up to 3000 volts overhead is used by DC main line systems (e.g., parts of France, Belgium and Italy) but below 1500 volts, a third rail can be used. However, it was also used on a number of main line railway systems, and still is in some parts of continental Europe, for example. Nevertheless, some embarrassing incidents in Asia with disintegrating manhole covers near a metro line as recently as the early 1980s means that the problem still exists and isn’t always properly understood. A waving wire will cause another problem. The wire is zigzagged relative to the centre line of the track to even the wear on the train's pantograph as it runs underneath. The contact wire has to be held in tension horizontally and pulled laterally to negotiate curves in the track. Figure 9: To reduce the arcing at a neutral section in the overhead catenary, some systems use track magnets to automatically switch off the power on the train on the approach to the neutral section.

Small-scale solar is decentralized power production taken to its extremes. Illustrative of these usages and of the special conditions to be met are cables for use in steel mills and boiler rooms (high temperature), on mobile equipment (vibration and excessive flexing), in chemical plants (corrosion), for submarines and mines (mechanical abuse), near nuclear reactors (high radiation), and on artificial satellites (pressure extremes). Therefore, installing a few large power plants and then distributing the power over large distances across a region makes sense. If you're unsure which type of electrical wire you need, it's helpful to consult a professional electrician before installing any sort of electrical lines in your home. Generally, it’s a question of what sort of railway you have. Overhead catenary systems, called "catenary" from the curve formed by the supporting cable, have a complex geometry, nowadays usually designed by computer. This was overcome by placing substations at close intervals - every three or four kilometres at first, nowadays two or three on a 750 volt system - compared with every 20 kilometres or so for a 25 kV AC line. DC overhead wires are usually thicker and, in extreme load cases, double wires are used, as in Hong Kong Mass Transit’s 1500 v DC supply system.

The wire must be able to carry the current (several thousand amps), remain in line with the route, withstand wind (in Hong Kong typhoon winds can reach 200 km/h), extreme cold and heat and other hostile weather conditions. Although 3rd rail is considered a suburban or metro railway system, 750 volt DC third rail supply has been used extensively over southern England and trains using it run regularly up to 145 km/h. Simple - use the steel rails the wheels run on. Broadly speaking, the higher speeds, the more complex the "stitching", although a simple catenary will usually suffice if the support posts are close enough together on a high speed route. This line failed in exactly the same way as the American lines, and the pipes were dug up and placed on short posts six inches above the ground. Another splice, followed by a fresh start, was made during the same afternoon; but when some fifty miles were payed out of each vessel, the current which kept up communication between them suddenly failed owing to the cable having snapped in the sea. The Agamemnon, a British man-of-war fitted out for the purpose, took in the section made at Greenwich, and the Niagara, an American warship, that made at Liverpool.

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