AAR AUTOMATIC BLOCK SIGNALING
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There are two systems of operating automatic block signals in general use, the normal clear and the normal danger
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The normal clear system is a term used to express the normal indication of the signals in an automatic  block system in which an indication to proceed is always displayed except  when the block is occupied
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The normal danger system is a term used to express the normal indication of t _,fignals in an automatic block system in which the indication to proceed is given only upon the approach of a train to an unoccupied block
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Signaling on multiple-track roads is generally of the two-block indication type, although considerable three-block indication signaling has been installed on roads of heavy traffic density or where necessary to provide proper braking distance.   There are some installations of four-block indication
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A fixed signal for a two-block signaling system displays a proceed, approach, or "stop; then proceed" indication, depending upon block conditions
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TWO BLOCK INDICATION SIGNAL SYSTEM
A fixed signal for a two-block signaling system displays a proceed, approach, or "stop; then proceed" indication, depending upon block conditions. This fixed signal may  have one three-position unit or two two-position units operating in either the upper or lower quadrant. The two-block indication system is so named for the reason that a clear aspect indicates that at least two blocks in advance of such a signal are unoccupied. A fixed signal dis­ playing an approach aspect indicates that only the block immediately in advance of the signal is unoccupied and that a train must be prepared to stop at the next signal. The two-block indication signals are generally spaced braking distance apart as a minimum, but this spacing is frequently increased where traffic is light
·Three-Block Indication System
A fixed signal for three-block systems usually displays proceed, approach­ medium, approach, or "stop; then proceed" indications, depending upon block conditions. This fixed signal has two operating units, one three-position unit above a two-position, or a three-position unit. The three-block indication system is so named for the reason that a "proceed indication cannot be given until at least three blocks in advance of such a signal are unoccupied. A fixed signal cannot display an approach-medium indication until at least two blocks in advance of such a signal are unoccupied. A fixed signal can display an approach indication only when the block immediately in advance of the signal is unoccupied; a train accepting such indication must prepare to stop at the next signal.
It is evident that a proceed indication would be displayed approximately1 miles in the rear of a stop and proceed signal, whereas in two-block indi cation signaling the proceed indication would be displayed 2 miles in the rear of a stop and proceed signal.  
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.  Therefore, in the three-block indication⦁ system trains spaced 1   miles apart will receive proceed indications, whereas in the two-block indication system trains must be spaced 2 miles apart in order to receive pr0ceed indications.
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The signal repeating relay (HDPR) for each signal is energized when the signal displays the approach, approach-medium or proceed indication
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Slow-release relays are used for signal repeating relays so that the front contacts will remain closed while the indication changes from approach or approach-medium to proceed, or vice versa.
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The approach relay at any signal is connected in series with the signal con­trol relay (HDR) for the A unit of the first signal in the rear; thus it is ener­gized only when all the track circuits between the signal where it is located and the first signal in the rear are unoccupied. When the approach relay is energized, one of its coils is ·shunted by a front contact and the resistance in series with the signal control relay is reduced accordingly
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fixed signals operate in three positions and display proceed, approach, and "stop; then proceed" indications. The "stop; then proceed" indication displayed by each  signal is controlled through two blocks instead of one.
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Automatic train stop control circuits. An automatic train stop assumes the non-tripping position when two track circuits in advance are unoccupied, and assumes the tripping position when either of these two track circuits is occupied except that it will not change from the non-tripping position to the tripping  position  while  the  track  in the rear is occupied. This insures that a train passing a signal will not cause the train stop at that signal to change from the non-tripping position to the tripping position until the entire train has passed.
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The  signal  control  relay  (HR)  which  controls  the  approach  indication circuit of a signal is energized when the train stop at that signal is in the non­ tripping  position  and  two  track  sections in advance of  that  signal  are un · occupied
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On single track, tram movements are in both directions and signals are arranged to permit following movements and also opposing movements. For opposing movements the signal control circuits must be so arranged to prevent the display of signal indications that would permit opposing trains to enter the same block at the same time. On the overlap system this is accomplished by extending the control of a signal beyond the next signal in advance govern­Ing in the same direction
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In the absolute permissive system of signaling for single track, the block for opposing train movements is from siding to siding and the signals govern­ing entrance to this block are "absolute" or "stop" signals. For following train movements the sections between sidings are divided into two or more blocks, as traffic conditions may require, and train movements into these blocks, except the first one, are governed by intermediate signals, commonly called "permissive," usually displaying "stop; then proceed" as their most restrictive indication.
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