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1 Methods for detecting DC leakage and fault currents in transit systems are categorized as (1) nonintrusive injection-based measurement techniques and (2) radio frequency (RF) emission- based detection techniques. However, no prior sensor exists that can detect low-level leakage currents in DC transit systems. This guidebook focuses on the methods the research team used during the course of TCRP Project D-17 to develop sensors to be used in these techniques and test them in the field. This guidebook provides specifications for these sensors and outlines their capabilities and appropriate operating conditions. The frequency, voltage, and current limitations are specified for the injection-based sensors. The data processing and linkage to failures along with the frequency of operation are provided for the RF emission-based sensors. C H A P T E R 1 Introduction