Book excerpt: Distance protection provides the basis for network protection in transmission systems and meshed distribution systems. Initially this book covers the fundamentals of distance protection and the special features of numerical distance relays in distribution and transmission systems. This book is aimed at students and engineers who wish to familiarise themselves with the subject of power system protection, as well as the experienced user, entering the area of numerical distance protection. Furthermore it serves as a reference guide for solving application problems. For the third edition all contents, especially the product descriptions and the very useful appendix, have been revised and updated.