One of the most complex disciplines in electrical engineering is power system protection which requires not only the proper understanding of the different components of a power system and their behavior but also a good knowledge and analysis of the abnormal circumstances and failures that can occur in any element of a power system. Protective relaying is an integral part of any electrical power system. Relays are not required to operate during normal operation, but must immediately activate to handle intolerable system conditions. This immediate availability criterion is necessary to avoid serious outages and damages to parts of or the entire power network. The fundamental objective of system protection is to quickly isolate a problem so that the unaffected portions of a system can continue to function. Protective relays are the decision-making devices in the protection scheme. These relays underwent, through more than a century, important changes in their functionalities and technologies. In fact, the accuracy of relays depends not only on their hardware components but also on the manner of information processing to evolve the decision signal; this is what is called the data processing algorithm or the processing method. So, the research of the optimal method to obtain the most accurate decision in the fastest way is one major challenge in the numerical protective relay design. Power Supply Devices and Systems of Relay Protection provides a survey of protective relaying technology and its associated communications technology used in today's power transmission systems. The various protocols and network topologies used for protective relaying purposes are explained. Associated communication standards are outlined. The aim of this book is to provide background on the communication technologies used by protection system. Moreover, the rapid changing and development in relays principles as well as in their technologies are additional factors that oblige those people working in the field to expand and update continuously their knowledge. In this book, we shed light in the evolution of protective relays since the onset of electrical energy to currently. We try also to foresee the future prospects and trends in this area.