General Information on Track Circuits - RGS Online

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Railway Group Approved Code of Practice GK/RC0752 Issue Two Date December 1998 Page B10 of 25 Withdrawn Document Uncontrolled When Printed <strong>General</strong> <strong>Information</strong> on Track Circuits The mechanical strength of light rust films is much reduced by the presence of moisture when the contaminant tends to be squeezed out from the wheel–rail contact patch and doesn’t cause any shunting problems. Therefore, lightly rusted rails will only be a problem when dry. This problem is most severe when conditions are: • showery weather accompanied by drying wind; or • prolonged periods without trains. Special precautions need to be taken after relaying operations, when track circuits must not be restored to full operation until a reasonable surface has been created. 8.2 Leaf Residue This problem is confined to particular areas where the extent of lineside afforestation is significant. It is also limited to autumn when trees are shedding their leaves. Leaves are drawn into the wheel–rail interface by the passage of a train, where they are squashed into a pulp which contaminates both rail and tyre. The severity of this problem in particular years is connected to the general weather situation. In simple terms, reasonably dry weather with little wind will cause the leaves to fall gradually over a long time period and to be reasonably sap free when they do fall. Conversely, gale conditions will lead to a sudden massive fall of sap laden leaves. It is the latter situation which gives rise to the worst conditions. In terms of track circuit operation, the electrical characteristics of severe leaf residue are similar to very heavy rust. Fortunately, the sites suffering such problems are generally known and special arrangements can be made. 8.3 Coal Dust and Sand Problems with coal dust on the rail head tend to be confined to colliery areas, where coal deposited on the wagon chassis after loading/unloading is subsequently shaken off. Sand contamination is usually associated with slow moving locomotives using their sanders excessively. In each case, the effect is similar to heavy rust films. 8.4 Composition Tread Brake Blocks Certain types of rolling stock are fitted with a composite type of tread brake block instead of the traditional cast iron variety, the intention being to improve brake performance. This is found to deposit a contaminant film on the steel tyre, which tends to insulate the train from the rails. 8.5 Tread and Disc Brakes When considering the electrical contact between two pieces of metal separated by a thin film of insulation, it can be appreciated that surface roughness of the metal can permit high spots to penetrate the film. Where this occurs, the insulation will be ineffective. Tread brakes cause the tyres to be roughened at each brake application, whereas disc brakes allow the tyres to be rolled into a very smooth surface condition. This can be observed visually as tread braked tyres have a matt appearance, whilst disc braked tyres show a mirror–like quality. Therefore, tread braked vehicles provide a better train shunt than disc braked vehicles. 8.6 Axle Weight and Suspension Design The pressure applied to any contaminant film is proportional to the downward force of the wheel on the rail and this is proportional to vehicle axle weight. B10 RAILTRACK
<strong>General</strong> <strong>Information</strong> on Track Circuits 9 Detection of “Lightweight” Vehicles Withdrawn Document Uncontrolled When Printed Railway Group Approved Code of Practice GK/RC0752 Issue Two Date December 1998 Page B11 of 25 In practice, wheels do not roll smoothly and friction free. There is a guidance force continually pulling the wheelset into the correct trajectory and this guidance force is associated with microscopic slippage between wheel and rail. Advances in bogie design have tended to reduce this guidance force and slippage, giving a smoother ride for the passenger as well as reducing the wear rate of both rail and tyre. Unfortunately, these qualities reduce the ability of the tyres to penetrate any film, as well as reducing their ability to clean the rail by abrasion. 8.7 Track Geometry Vehicle guidance force and wheel rail slippage are increased in curved track. Therefore, train shunt will be improved when the vehicles are travelling on curved track. If all rails and tyres were clean and wheel–rail contact was perfect, any type of vehicle would satisfactorily operate any type of track circuit. However, secondary lines in particular have suffered a fall in traffic leading to regular formation of light rust films. At the same time, the vehicles using such lines have been increasingly of the modern DMU variety, which magnify the train shunt difficulties because of their suspension design, brake type, weight (which, although still heavy, is relatively light) and small number of vehicles in a train. When a vehicle is static on a light rust film, the track circuit voltage will usually break it down and the track circuit will occupy. This is because the track clear rail voltage is higher than the film breakdown voltage. However, when that vehicle is moving, the wheels are continually rolling onto new film which requires to be repeatedly broken down. Consider the following sequence of events: When a wheel first enters the track circuit, the track clear rail voltage is presented across the film. The film breaks down resulting in the rail voltage collapsing towards zero. As the wheel moves on to new surface, there is insufficient voltage available to break through the new film. The train shunt is removed and the rail voltage rises towards the clear value. When the rail voltage attains the breakdown level, the film is punctured, the train shunt re–applies and the rail voltage once again plummets toward zero. The result is a high frequency noise voltage across the rails which can be observed with a suitably sensitive instrument. Where the threshold breakdown voltage is less than the rail voltage at which the relay drops away, the noise will not result in track circuit malfunction. This parameter is used to assess the performance of various track circuit types relative to their ability to detect lightweight vehicles. To assist vehicles to shunt track circuits, a device known as the “Track Circuit Assister” has been fitted to modern diesel multiple units. RAILTRACK B11
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