Tree growth rate in epoxy/layered silicate (1 wt%) nanocomposite at a constant electrical field of 781.4 kV/mm (60 Hz) at 30℃. The inset was for the neat epoxy resin at the same conditions. The morphology of electrical tree corresponding to photos (a)-(d) was collected during HV applying for (a) 5,270 min and (b) 15,680 min in the epoxy/layered silicate (1 wt%) nanocomposite, and (c) 135 min and (d) 181 min in the neat epoxy.