Corona inception voltage due to a water droplet on epoxy resin in an electrode gap is high under DC voltage compared to AC voltage. It is observed that, as the contact angle of the epoxy nanocomposite material becomes higher, the corona discharge inception voltage increases. The droplet movement is observed, using a high-speed camera, on application of the voltage. It is seen that a droplet moves toward the ground electrode under an AC or a negative DC voltage, whereas it moves toward the high-voltage electrode under a positive DC voltage. It is also observed that carbonization occurs near the ground electrode under AC and negative DC voltages, and near the high-voltage electrode under a positive DC voltage. During the evaporation of the water droplet (during arcing) on the surface of the insulating material under AC and DC voltages, carbonization of material occurs and is high both in pure epoxy resin and in nanocomposites with 5 wt% epoxy clay. The magnitude of the arcing current is nearly the same irrespective of the percentage of clay in the epoxy nanocomposites. The magnitude of discharge current flow is high under negative DC voltage compared to positive DC/AC voltages. The rise time of injected current pulses, at the time of corona inception and during arcing, under AC/DC voltages, is a few nanoseconds. Ultrahigh-frequency signals were emitted as a result of the corona discharge from the water droplet on epoxy nanocomposites and at the time of arcing between the droplets and the electrodes, both under AC and DC voltages, with its dominant frequency in the range 1-2 GHz. © 2013 Institute of Electrical Engineers of Japan.