To evaluate the condition of insulation during operation the following parameters: are used moisture content, breakdown voltage during a smooth rise in voltage of commercial frequency, as well as the volumetric and surface resistivity. They were determined both for individual characteristic segments of the fiber glass insulator, and along its entire length. It is concluded that destruction of fiber glass insulators during operation occurs as a result of destruction of airtightness of the end terminals, penetration of moisture into the fiber glass and breakdown of the fiber glass near the end terminal. Reliable sealing of the end ensures reliable operation of fiber glass insulators in electrical networks.