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Gas insulated switchgears (GIS) are widely used as major substation equipment because of their excellent insulation and interruption performance. However, while a GIS is highly reliable, it is a concealed device, meaning diagnostic technology using various sensors is required to detect any insulation abnormality. Among various potential defects inside a GIS, the most problematic is caused by the existence of fine metallic particles, and thus the ability to detect harmful particles in tests at a factory and on site as well as in operation is becoming an important subject. This paper examines, on an experimental basis, the UHF sensor's characteristics of detecting partial discharge (PD) signals generated from particles inside the GIS with those placed freely on the bottom surface of the tank, having adhered to the insulating spacer's surface, and fixed on the center conductor, respectively. Based on previous reports, breakdown characteristics with the existence of particles were arranged in terms of the particle length, and the harmful particle length was estimated. Both results were compared with each other to evaluate the possibility of detecting harmful particles in each location with the UHF sensor. Consequently, it emerged that free particles were detected comparatively easily. However, while those having adhered to the spacer's surface and fixed on the center conductor could be detected in insulation tests at 2 pu at a factory, detection was difficult in many cases in the field.