This paper proposes a hierarchical fault detection method for transmission lines using a microphone array to detect the location of a fault and thermal imaging and charge coupled device (CCD) cameras to verify the fault and store the image, respectively. There are partial arc discharges on faulty insulators which generate specific patterns of sound. By detecting these patterns using the microphone array, the location of the faulty insulator can be estimated. A sixth-order bandpass filter and an autocorrelation scheme were applied to remove the noise signals caused by the wind, bird chirpings, or other external influences. When a mobile robot carries the thermal CCD cameras to the possible location of the fault, the faulty insulators or power transmission wires can be detected by the thermal images. The CCD camera then captures an image of the faulty insulator for the record. This detection scheme has been proved to be effective through experimentation. As a result of this research, it will be possible to use a mobile robot with integrated sensors to detect faulty insulators instead of using a human being.