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  Signalli Signalling systems Q. Why are multiple aspect signalling systems used? What was  wrong with the older systems which had two aspects? Multiple aspect signals, by providing several intermediate speed stages between 'clear' and 'on', allow high-speed trains sufficient time to brake safely if required. This becomes very important as train speeds rise. Without multiple-aspect signals, the stop signals have to be placed very far apart to allow sufficient braking distance, and this reduces track utiliation. !t the same time, slower trains can also be run closer together on track with multiple aspect signals. Q. What kinds of signals (semaphores, lamps, etc.) does IR use? # uses several kinds of signals. $emaphore signals have generally given way to colour-light signals although there are still many places with semaphore signalling in use. %&()* Semaphore  signals are the older style signals seen widely throughout the country, where each signal has an assembly with an arm  mounted on a mast, where the arm can move through two or three different positions at different angles, each position providing a distinct signalling aspect. +ery early in ndia's railway history, two-position lower-quadrant semaphore signals werethe most prevalent. !round the &(s, however, the introduction of !merican style power signalling equipment in some areas resulted in three-position upper-quadrant signalling being introduced as well, although both systems continued in use for many decades afterwards. t is not clear when distant signals were introduced. Colour-light  signals are assemblies of lamps that indicate different aspects by means of different colours of lamps that are lit. olour-light signals were introduced in &)/ but were slow to take off. n recent years many older semaphore signals have been replaced by colour-light signals. Position-light signals are assemblies of lamps where the signal aspect is indicated not by colour but rather by the combination of the lamps that are lit. Disc  signals are in the form of a vertical disc with a pattern such as a bar painted on it, which rotates about its centre to different positions to indicate different signal aspects. These are usually mounted on poles but may be close to ground level. Target signals  have a vertical disc 0or two parallel vertical discs1 which can rotate about a vertical a2is so as to present the disc either face-on or edge-on to an observer along the track. 3sually a lamp is provided behind the disc 0or between the parallel discs1 which is visible only when the discs are oriented edge-on. The centres of the discs usually also have lamps. The two  aspects of this type of signal are indicated by the two orientations of the discs. This type of signalis almost always at ground level. In the following, 'on' refers to that position of a signal which shows its most restrictive indication (in accordance with IR's terminology). However, we use 'clear' for the position that shows the least restrictive indication instead of the word 'off' because the latter is used by IR to refer to any signal position other than the on position. Q. What types of signalling systems are used on IR? # uses several forms of signalling. n # manuals reference is made usually only to 4 main types of systems, 5ower 6uadrant semaphore, Modified 5ower 6uadrant semaphore, Multiple !spect 3pper 6uadrant semaphore, and multiple-aspect colour-light signalling. 7ut in practice there are some variations in the kinds of colour-light signalling seen, so for ease of analysis, the following classification is used here. 0!bbreviations in parentheses given for ease of reference in the te2t that follows.1  Two-aspect 5ower 6uadrant semaphore signalling 0)561  Modified 5ower 6uadrant semaphore signalling 0M561  Multiple !spect 3pper 6uadrant semaphore signalling 0M!361  Two-aspect olour-5ight signalling 0)51  Three-aspect olour-5ight signalling 051  8our-aspect olour-5ight signalling, normally known 9ust as Multiple !spect olour-5ight signalling 0M!51
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