Polymer insulators have high dielectric strength. When the voltage applied to both sides of an insulator reaches a critical value, local conduction occurs, the current suddenly increases, and permanent damage occurs. This phenomenon is called dielectric breakdown. Dielectric strength is the limit that characterizes the voltage resistance of an insulating material, expressed as the ratio of the breakdown voltage of the dielectric in a uniform electric field to the thickness of the dielectric. Under standard conditions, the dielectric strength of air is approximately 32 kV/cm, while the dielectric strength of polymer insulators is generally between 200 and 1500 kV/cm.