it is my understanding that all dieletrics are insulators but not all insulators are dielectrics. A dielectric is an insulator that will polarize in the presence of an electric field. So is it safe to say that a vaccum would be considered a perfect insulator or a perfect dialectric? Furthermore, this makes me wonder why then oil circuit breakers and sulfur hexaflouride circuit breakers are commonly used with high voltage transmission lines and vaccum circuit breakers are only used on medium voltage applications. Can someone explain why this is the case?