The origin and the consequences of the electric failure of in-service composite hollow insulators used in railway transportation were investigated. Direct electrical measurements made it possible the detection of partial discharges (PD) and UV emissions allowed of identifying the location of the PD at the interface between metal braids and a silicone liner. To investigate the effects of the damaging PD activity, physicochemical properties of the different polymers were investigated and compared to new samples. The thermomechanical properties of the HTV and RTV silicones showed important alterations in the polymers structure. The chemical analyses of the silicones and nitrile rubber conducted using ATR-FTIR spectroscopy revealed that the polymers were oxidized after electrical failure. The results indicated that the ozone production and UV emission were due to the surface electrical activities. The breakdown of alumina trihydrate (ATH) in the inner HTV silicone liner demonstrated that undesired electrical discharges induced an significant increase of temperature and the formation of moisture in the air present in the hollow insulator. Laboratory artificial aging performed on the silicones and nitrile rubber showed similar degradation as in service. Further electrical experiments conducted on empty hollow insulators enabled the detection of internal PD located in micro-voids present in the adhesive used to bond the HTV silicone liner to the inner composite tube.