Principles of Operation
AWS is part of the signalling system and warns the driver about the aspect of the next signal. These warnings are normally given 200 yards (180 metres) before the signal. Information about the signal aspect is conveyed electromagnetically to the moving train through equipment fixed in the middle of the track, known as AWS inductors. Each inductor contains a permanent magnet and an electromagnet which 'cancels' the effect of the permanent magnet. The system is fail-safe because the electromagnet is required to be energised to give the 'clear' indication, the 'warning' indication being given by the permanent magnet alone.
The system basically consists of a relay control box, EP valves connected to the braking system, a 'sunflower' indicator mounted in a promient position in the driver's cab, a magnetically operated 'receiver' underneath the locomotive/multiple unit driving car, a driver's reset plunger and a static voltage-converter providing operating voltages of 12 & 40V from the unit supply, plus an isolating handle that is both an electrical and vacuum pipe/air isolator.
When the AWS inductor is reached, the AWS sets a visual indicator in the driver's cab and gives an audible indication. If the signal being approached is displaying a 'clear' aspect, the AWS will sound a bell (modern locomotives/multiple units use an electronic device that gives a distinctive 'ping' that can be heard in Pacers and some of the more recent DMU/EMU stock) and leave the visual indicator black. This lets the driver know that the next signal is showing 'clear' and that the AWS system is working. If the signal being approached is displaying a restrictive aspect (red, yellow or double yellow in colour-light installations or a distant semaphore at caution (horizontal) in mechanically signalled installations), the AWS will sound a horn continuously until the driver pushes and releases a button to acknowledge it. The AWS will also give a warning horn on the approach to certain permanent speed restrictions and all temporary and emergency speed restrictions. When the warning is acknowledged, the horn stops and the visual indicator changes to a pattern of black and yellow spokes, which persists until the next AWS inductor and reminds the driver that they have cancelled the AWS and therefore have full responsibility for controlling the train. If the button is not pressed and released within six seconds, a full brake application brings the train to a halt. The system has a fail-safe mechanism built in if the driver collapses onto the AWS cancel button whereby it is not the press of the button that cancels the warning, it is when the driver lets go after the downward motion.
Read more about this topic: Automatic Warning System
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