Leakage current detection is a promising method for measuring pollution levels on insulators of overhead transmission lines. This paper describes the development of a leakage current sensor based on a fiber-coupled light-emitting diode, as well as the development of a processing module that detects, stores, and classifies leakage current peaks that are directly related to partial discharges on the surface of the high-voltage insulators. The relative humidity is also detected and stored for further analysis. The processing module uses a PIC 16F877A microcontroller and can register the rate of partial discharges for two months on three different levels.