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A pocket device to warn switchyard personnel of imminent insulation breakdown in a nearby HV plant has been developed. The detector is based on the well-known UHF technique for PD detection. However, it is much smaller, lighter, and cheaper than a conventional PD detector. It can discriminate against communication, broadcasting, and external PD sources present in substation switchyards and thus avoid false alarms. During the design stage, an analysis of the electromagnetic emission spectra of several substations was carried out. Continuous recording of frequency and amplitude enabled the identification of several discharge sources, whose characteristics were very dependent on climatic conditions. During laboratory and field testing, several RF sensors and pulse processing algorithms were tested using an oil tester to simulate PD in oil. Final tests in outdoor substations were carried out to verify the immunity of the detector to external back ground emissions while retaining an acceptable detection range for targeted internal discharges. The developed UHF pocket alarm has much greater sensitivity than AM radio, the current detection method. It provides simple light and sound warnings and thereby eliminates the need for continuous interpretation of interference sounds on a detuned radio.