Optimum parameter selection to aid in developing a method for electrically testing 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. This paper presents a development procedure and system analysis to measure the leakage current of one fastener in situ in the Bay Area Rapid Transit (BART) system without any modification of the rails. The paper further presents the magnitude of the electrical parameters of the rail fastener system as a function of frequency, for various operating conditions including dry or wet fasteners.