Development of test instrumentation to measure the leakage resistance of a single fastener in trackwork

During normal operation of transit vehicles, currents drawn by traction motors in the vehicle cause a voltage to develop on the running rails, causing leakage currents to flow to ground over the rail fastener surfaces which may be contaminated with dirt and moisture. These leakage currents can cause extensive damage, including erosion of the rails, fasteners and structural metals. The total leakage current in a system may be readily measured, and may be tens to hundreds of amperes. However, determining the electrical resistance of a selected fastener in situ requires measuring the leakage current of that individual fastener, which is extremely difficult since the fasteners are electrically connected by the low impedance of the continuously welded running rails. Commercial equipment for measuring the leakage resistance of one fastener in the field is currently unavailable. A previous paper presented a development procedure and system analysis to measure the leakage current of one fastener in situ in the BART system without any modification of the rails, and presented computer simulations to optimize circuit parameters including test voltage and frequency. This paper describes a method to measure the leakage current of a single fastener in trackwork in the BART system without any modification of the rails. The paper also presents a test instrument using state-of-the-art current transducers to perform this function, with no cutting of the track or use of insulated joints. The proposed test instrumentation is bench tested, and the relatively high accuracy obtained in measuring leakage resistance of one fastener in place using the proposed test instrument, clearly demonstrates the feasibility of such an instrument.