Editing Automatic Warning System (section)

== Network Rail ==
{{unreferenced section|date=August 2018}}
[[Network Rail]] (NR) AWS consists of:
* A permanent magnet set centrally between the rails and usually positioned such that it is encountered {{cvt|200|yd|0}} before the signal to which it relates. The top of the magnet casing is nominally level with the running surface of the rails (to within {{cvt|12|mm|frac=2|disp=sqbr}}).<ref name=rssb/>
* An electromagnet between the rails (with opposite polarity to the permanent magnet) positioned after the permanent magnet. Again the top of the casing is nominally level with the running surface of the rails (to within {{cvt|12|mm|frac=2|disp=sqbr}}).<ref name=rssb>{{cite book |title=AWS and TPWS Interface Requirements |date=March 2018 |publisher=[[Rail Safety and Standards Board]] |location=London |page=10 |edition=3}}</ref>
* A cab indicator that can show a black disk or a yellow and black "exploding" disk, known as the "AWS sunflower"
* A control unit that connects the system to the brakes on the train
* A driver's AWS acknowledgement button
* An AWS control panel

The system works on a set/reset principle.
 
When the signal is at 'clear' or green ("off"), the electromagnet is energised. As the train passes, the permanent magnet sets the system. A short time later, as the train moves forward, the electromagnet resets the system. Once so reset, a bell is sounded (a chime on newer stock) and the indicator is set to all black if it is not already so. No acknowledgement is required from the driver. The system must be reset within one second of being set, otherwise it behaves as for a warning indication.

An additional safeguard is included in the distant-signal control wiring to ensure the AWS "clear" indication is only given when the distant is proved "off" – mechanical semaphore distants have a contact in the electromagnet coil circuit closed only when the arm is raised or lowered by at least 27.5 degrees. Colour-light signals have a current sensing relay in the lamp lighting circuit to prove the signal alight, this is used in combination with the relay controlling the green aspect to energise the AWS electro-magnet. In a Solid State Interlocking the signal module has a "Green-Proved" output from its driver electronics that is used to energise the electromagnet.

[[File:BR AWS inductor 1.JPG|thumb|BR Standard Strength AWS track equipment]]

When the distant signal is at 'caution' or yellow (on), the electro-magnet is de-energised. As the train passes, the permanent magnet sets the system. However, since the electromagnet is de-energised, the system is not reset. After the one-second delay within which the system can be reset, a horn warning is given until the driver acknowledges by pressing a plunger. If the driver fails to acknowledge the warning within 2.75 seconds, the [[brakes]] are automatically applied. If the driver does acknowledge the warning, the indicator disk changes to yellow and black, to remind the driver that they have acknowledged a warning. The yellow and black indication persists until the next signal and serves as a reminder between signals that the driver is proceeding under caution. The one-second delay before the horn sounds allows the system to operate correctly down to speeds as low as {{cvt|1+3/4|mph}}. Below this speed, the caution horn warning will always be given, but it will be automatically cancelled when the electromagnet resets the system if the driver has not already done so. The display will indicate all black once the system resets itself.

The system is [[fail-safe]] since, in the event of a loss of power, only the electro-magnet is affected and therefore all trains passing will receive a warning. The system suffers one drawback in that on single track lines, the track equipment will set the AWS system on a train travelling in the opposite direction from that for which the track equipment is intended but not reset it as the electromagnet is encountered before the permanent magnet. To overcome this, a suppressor magnet may be installed in place of an ordinary permanent magnet. When energised, its suppressing coil diverts the magnetic flux from the permanent magnet so that no warning is received on the train. The suppressor magnet is fail-safe since loss of power will cause it to act like an ordinary permanent magnet. A cheaper alternative is the installation of a lineside sign that notifies the driver to cancel and ignore the warning. This sign is a blue square board with a white [[Saltire|St Andrew's cross]] on it (or a yellow board with a black cross, if provided in conjunction with a temporary speed restriction).

With mechanical signalling, the AWS system was installed only at distant signals but, with multi-aspect signalling, it is fitted at all main line signals. All signal aspects, except green, cause the horn to sound and the indicator disc to change to yellow on black.

AWS equipment without electromagnets is fitted at locations where a caution signal is invariably required or where a temporary caution is needed (for example, a temporary speed restriction). This is a secondary advantage of the system because temporary AWS equipment need only contain a permanent magnet. No electrical connection or supply is needed. In this case, the warning indication in the cab will persist until the next green signal is encountered.

To verify that the on-train equipment is functioning correctly [[Traction maintenance depot|motive power depot]] exit lines are fitted with a 'Shed Test Inductor' that produces a warning indication for vehicles entering service. Due to the low speed used on such lines the size of the track equipment is reduced from that found on the operational network.
[[File:BR_AWS_Test_Shed_Inductor.jpg|thumb|BR AWS Test Shed Inductor]]
'Standard Strength' magnets are used everywhere except in DC [[third rail]] electrification areas and are painted yellow. The minimum field strength to operate the on-train equipment is 2 [[Tesla (unit)|milliteslas]] (measured {{cvt|125|mm|0|disp=sqbr}} above the track equipment casing). Typical track equipment produces a field of 5&nbsp;mT (measured under the same conditions). Shed Test Inductors typically produce a field of 2.5&nbsp;mT (measured under the same conditions). Where DC third rail electrification is installed 'Extra Strength' magnets are fitted and are painted green. This is because the current in the third rail produces a magnetic field of its own which would swamp the 'Standard Strength' magnets.

AWS is provided at most main aspect signals on running lines, though there are some exceptions:<ref name="Awsrules"/>
* At through stations where the permitted speed is {{cvt|30|mph}} or less and the layout is complex. Where this occurs, these are called ''AWS gap'' areas.
* AWS magnets are not provided at semaphore stop signals (which can only display clear or stop).
* Where a line is not fitted with AWS magnets, it is shown in the [[Sectional Appendix]].
[[File:Temporary Speed Restriction Warning Board Assembly.svg|thumb|75px|Temporary speed restriction warning board]]

=== Expansion of AWS application ===
* From 1971, an AWS permanent magnet was fitted ahead of the [[UK railway signalling#Permissible speed indicators|warning indicator]] where the permissible speed of a line dropped by more than a third.<ref>{{cite web| title =Master Rulebook: Signals, hand signals, indicators and signs handbook. Section 7.2 "Warning indicators"| url =https://www.rssb.co.uk/rgs/rulebooks/GERM8000-master-module%20Iss%201.pdf| publisher =Network Rail| access-date =2017-03-07| archive-date =10 August 2018| archive-url =https://web.archive.org/web/20180810143338/https://www.rssb.co.uk/rgs/rulebooks/GERM8000-master-module%20Iss%201.pdf| url-status =dead}}</ref> This was a recommendation of the inquiry into the [[Rail accidents at Morpeth#1969 derailment|derailment at Morpeth]] on 7 May 1969.
* From 1977, a portable AWS permanent magnet was fitted ahead of the warning board on the approach to [[UK railway signalling#Speed restrictions|temporary speed restrictions]] (TSRs). This was a recommendation of the inquiry into the [[Nuneaton rail crash|derailment at Nuneaton]] on 6 June 1975, which occurred when the driver missed a TSR warning board due to its lights being extinguished.
* From 1990, AWS permanent magnets were installed immediately ahead of certain 'high risk' stop signals, as a [[Signal passed at danger|SPAD]] mitigation measure. This additional AWS magnet was suppressed when the associated signal showed a 'proceed' aspect. Since the introduction of the [[Train Protection & Warning System]] (TPWS) it is no longer current practice to use AWS for this purpose. [[Signal passed at danger#Collision prevention systems|SPAD indicators]] were also used.