An experimental study has been carried out to\ud investigate the effect of sanding on the electrical\ud isolation of a wheel/rail contact. Sand is applied to the\ud wheel/rail interface to increase adhesion in both braking\ud and traction. Train detection, for signalling purposes,\ud can be by means of track circuits. Signalling block\ud occupancy is triggered by the wheelset of the train\ud ‘shorting out’ the track circuit. Sand in the wheel/rail\ud interface means that contact between the wheelsets and\ud the track may be compromised, inhibiting train\ud identification.\ud \ud Static tests were performed using sections cut\ud from wheels and rail and dynamic tests on a twin disc\ud machine where rail and wheel steel discs are loaded\ud together and driven under controlled conditions of\ud rolling and slip. The electrical circuit used was a\ud simplified simulation of the TI21 track circuit.\ud \ud The application of sand was carried out under a\ud range of mild and severe test conditions. The results\ud indicated that a transition exists in the amount of sand\ud applied, below which there is a measurable, but not\ud severe, change in voltage, but above which the contact\ud conductance decreases by an order of magnitude. A\ud model of electrical isolation has been developed\ud assuming either full disc separation by a sand layer or\ud partial disc contact with some sand present.\ud \ud Idealisations inherent in both test methods mean\ud that they represent a severe case. Given these\ud limitations, it is likely that the test methods, at their\ud present stage of development, should be used as a\ud means to qualitatively assess the relative effects on\ud electrical isolation of different contaminants
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