Product Expertise: Monitoring leakage resistance in railway signalling systems


In Railway Signaling systems the earth faults are the most troublesome faults because of two reasons; first, they are not easy to locate in a shortest possible time and second, their sources are invisible. Therefore, it is critical to employ advanced technology to meticulously monitor and diagnose faults to sustain the railway traffic and prevent railway mishaps. In this technical document, an overview of detecting earth faults in railway signalling systems using two measuring instruments is explained.

Overview of Power Supply and Railway Signalling Systems:

In Railways, the power supply used for signaling is an un- grounded type and in such systems all active parts are insulated from ground/earth. In this system, exposed non- current carrying conductive parts of the electrical installations are earthed individually or collectively.

Figure-1 Block diagram of simplified Railway Signaling System

In the above template, you may notice that a floated type of power supply and signaling circuit is connected to a load through cable pairs. All current carrying parts are insulated from the Earth and the exposed non-current carrying conductive parts are Earthed. The sources of earth leakage are multifarious, they could be single or multiple in the resultant leakage. At this juncture, as a solution Earth Leakage Detector is developed to diagnose such earth leakage and maintain cable health.

On-line Leakage Resistance Monitoring using Earth Leakage Detector:

Earth Leakage Detector (ELD) was developed by Anuvidyut Pvt. Ltd., Roorkee, India as per Indian Railway specification RDSO/SPN/256/77 Rev 2002. It is used for online monitoring of Leakage Resistance of railway signaling systems.

Working Principle: ELD superimposes a measuring signal generated inside the equipment. On the occurrence of a new insulation fault, the measuring circuit between system and earth completes through the low insulation resistance which causes a voltage drop, which is sensed by an electronic circuit. If the drop of insulation (leakage Resistance) exceeds the pre-set value, an alarm is activated. ELD only works with ungrounded system. ELD is such designed that it is neither affected by external operations, load, disturbances etc., nor it influences any internal circuits. ELD helps in effective preventive maintenance ensuring good health of signaling systems.

Insulation Measurement Using Earth Cable Pair Insulation Tester:

Earth Cable Pair Insulation Tester (ECPIT) is a handy measuring instrument that complements the ELD to ensure cable health of railway signaling circuits. Normally ELD is connected to busbars of the signaling supply, which connects all the circuits powered from these busbars (Bx and Nx or B+ and B-) and Leakage Resistance is measured of busbars w.r.t ground. It does not measure the IR of the individual pair with respect to ground. The function of ECPIT is to measure the IR (Insulation Resistances) of the signaling pair to ground when the pair is isolated, that is, when the signaling pair is unenergized. Basic operating principle of ECPIT conforms to RDSO/SPN-256/77 Rev 2002/clause 3.1 .5.

Working Principle: Measurement of insulation of a cable pair (with respect to earth) in an unenergized condition without disconnection is possible through CT RACK. One terminal (green) is connected to Earth and other two (red) to the cable pair under test.

Fig-2 shows positions of both ELD and ECPIT in Railway signaling system. In case if pair is energized by any signal potential, then the instrument isolates itself and displays Pair Energized. ECPIT has a measuring signal generated inside the equipment and the insulation value can be directly read on VDU. These two diagnostic instruments if used diligently, not only help in preventive maintenance of cable health and railway signaling systems, but also avert railway mishaps resulting from earth faults.

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Product Expertise Source: Anuvidyut Private Limited | This Product Expertise is a part of our Latest Magazine: Subscribe to our Magazine Today!