This guide presents factors to be considered in the planning and design of transition facilities between overhead and underground transmission lines. These include the system implications of a hybrid installation as they relate to the transition facility. While this document focuses on transmission lines only@ some of the considerations listed in this guide are common to both transmission and distribution installations. Purpose The purpose of this guide is to list and describe typical factors that should be considered in the planning and designing of transition facilities between overhead and underground transmission lines. Some of these factors relate to the installation@ operation@ and maintenance of the transmission system. This guide is intended to be comprehensive@ but may not be all inclusive.