The worldwide growth in demand for electricity has forced the pace of developments in electrical power system design to meet consumer needs for reliable, secure and cheap supplies. Power system protection, as a technology essential to high quality supply, is widely recognised as a specialism of growing and often critical importance, in which power system needs and technological progress have combined to result in rapid developments in policy and practice in recent years. In the United Kingdom, the need for appropriate training in power system protection was recognised in the early 1960s with the launch of a correspondence course from which these books emerged and have since developed designed to meet the needs of protection staff throughout the world. The Electricity Training Association, in response to the important recent developments in the field of protection, have now commissioned an additional volume covering digital technology. The existing three volumes, of which this is the second, have been reviewed by leading authorities within the electricity supply industry and electrical manufacturing companies in the UK and, with the new fourth volume, the new edition gives a comprehensive and up-to-date treatment of the subject, covering theory, analytical and design principles, equipment design and application and protection management.