Recently the polymer composite insulators covered with the silicone protective shell (PCI) of the nominal voltage of 35 kV have become more widespread, instead of porcelain and glass insulators. As early as at the stage of production, PCIs form specific defects that lead to their destruction and as a consequence to the emergency modes of the power system. In order to control PCI, the present study proposes a nondestructive method of testing based on an analysis of partial discharges. The method is effective for detecting the internal defects and for detecting the defects which arise due to moistening insulator shells.