The erosion resistance of materials against partial discharges (PD) is a useful first indicator of their suitability for use as high voltage insulators. Epoxy resin-based composites loaded with different concentrations of silica nanoparticles were exposed to high voltage stress via an IEC(b) electrode. Laser scanning analysis was used to investigate the eroded volume and the erosion depth. The aged samples were investigated by transmission electron microscopy (TEM) analysis and scanning transmission electron microscopy (STEM), which show the formation of a protective layer of partially fused nanoparticles. The results suggest that improved erosion resistance against PD using the IEC(b) electrode is caused by the in situ formation of a protective layer consisting of sintered nanoparticles.