TheContinuous Automatic Warning System (CAWS) is a form ofcab signalling andtrain protection system used inIreland to helptrain drivers observe and obey lineside signals.
CAWS has been in use onIarnród Éireann diesel trains since 1984. AllNIR trains operating cross-border must also be fitted with CAWS. Codedtrack circuits, which form the basis of the system (see below), are installed on allDublin Suburban and Intercity routes toCork,Limerick, andAthlone and as far as the border on theBelfast line.
This system is of American origin, being originally supplied byUS&S, then a division ofWestinghouse Air Brake Company (WABCO). Current equipment is supplied by SASIB, a division ofAlstom.
CAWS repeats the aspects shown by the lineside colour lightsignals on anaspect display unit (ADU) inside the driver's cab. The ADU continuously displays the aspect that was shown by the previous signal until updated about 350 metres before the next signal. The ADU then displays the aspect shown by that signal.
A change of ADU display to a less restrictive aspect (e.g., double yellow to green) is termed anupgrade, while a change to a more restrictive aspect (e.g., single yellow to red) is called adowngrade. Any change of ADU display is accompanied by an audible indication. A momentary audible ‘warble’ sound indicates an upgrade. A downgrade is accompanied by a continuous audible tone and the illumination of theAcknowledge Switch that must be pressed by the driver within seven seconds to prevent an automatic brake application occurring for one minute. This is not recoverable until the time has expired. Acknowledgement by the driver within the first seven seconds immediately silences the tone.
The system consists of codedtrack circuits and on-board equipment. The coded track circuits transmit information about the signal aspect to the on-board equipment via two pick-up coils mounted on the front of the train, one over each rail.
The 50 Hzcarrier signal ismodulated with pulsedsquare wave codes according to the signal aspect. No code is generated for a red aspect. The system is thereforefail-safe, since loss of code will result in a red indication on the ADU.
In electrifiedDART areas, a differentcarrier frequency of 83.3 Hz is used to avoid interference from50 Hzpower line frequency. There is also a greater range of codes because they also control theautomatic train protection on the DART units.
Diesel drivers have to select the appropriate carrier frequency when moving to or from an electrified area. "Carrier 1" (50 Hz) is used in non-electrified areas and "Carrier 2" (83.3 Hz) is used in electrified areas.
Because CAWS is a continuous system, an upgrade can occur anywhere between signals. The driver may then accelerate in the knowledge that the signal ahead has changed to a better aspect, even though it may not yet be visible. Similarly, the driver can quickly act upon an unexpected downgrade, which may be the result of thesignalman having returned the signal ahead to red in an emergency.
Since CAWS is dependent on track circuits for its operation, it cannot be applied whereaxle counters are used for train detection (i.e., "mini-CTC" areas) without the additional cost.
CAWS does not act in the event of a signal beingpassed at danger if the Red aspect has been acknowledged.