As polymer powders have high resistivity, they can easily be charged due to the repeated collision and separation of particles along with particle-wall friction in a pneumatic conveying system. This study experimentally investigated the electrostatic charges and discharges inside a conical-cylindrical silo during the loading of polypropylene (PP, 2-3 mm) powders. The silo was continuously loaded with PP powders at 0.68 kg/s to a total mass of approximately 800 kg. To measure electrostatic charges of the PP powder being loaded into the silo, a Faraday cage with a cover was set at a distance of 50 mm from the center of the loading pipe inside the silo. To observe electrostatic discharges inside the silo, an image-intensifier unit was set on the windowpane of the silo roof. The results show that the charge-to-mass ratio of the loading PP powder remained at a constant value of approximately -12 mu C/kg. The ring-shaped light, which is the electrostatic discharge, appeared clearly at the center of the silo approximately 7 s after initial loading. The diameter of the ring-shaped light grew larger as time passed after loading. This was because the diameter of the accumulated PP powders increased in the silo during the running time, meaning that electrostatic discharge occurred between settled PP powders and the grounded metal silo wall. Additionally, the electrostatic discharges during the loading of powder in this study were clearly observed and classified into three kinds of discharges: brush, linear, or broad bulk surface discharges. (C) 2018 Published by Elsevier B.V.