The principle of impedance measurement and the engineering of basic impedance measuring elements into schemes of distance protection whether they be switched, nonswitched, carrier-aided etc., has always formed a very important part of power system feeder protection. The importance of its role is reflected in the fact that this form of protection more than any other has long been the subject of rapid and continuous development necessary to meet the ever increasing demands typical of modern power systems. Such development has, for many years, seen little change in operating principle (discounting more recent computer-based techniques) although the methods of implementation vary considerably largely because of the degrees of freedom allowed by modern developments in technologies of all kinds (component, computer, communications etc.). However, although implementation of the principle is subject to continuous change through development, there are basic design concepts which are almost universally established and which are important in understanding the principles of application. For this reason, this chapter is concerned mainly with the theoretical basis for design of distance protection and the more fundamental aspects of application.

Overvoltages due to lightning have their cause external to the system, therefore they are often referred to as external overvoltages.The mechanism of the lightning discharge has been studied with rotating cameras, it illustrates a typical record by Schonland obtained with such a camera. It has been found that the discharge is normally initiated from the cloud at a point of high electric stress, and in the majority of cases, a negative charge from the cloud proceeds towards the ground in a series of jerks of 'steps'. This is the 'leader stroke'. When the leader stroke approaches the ground, high electric stresses develop above the ground and an upward streamer of positive charges develops from the ground.