A secure and uninterrupted supply of electricity is only possible with the help of comprehensive protection and control functions, which ensure the reliable operation of the power system. As the complexity and ratings of electrical power systems increase, so do also the demands on the protective devices and systems which have to protect them from damage and preserve power system stability. Protection relays in conjunction with communication links provide the best possible means of selectively isolating faults on HV lines, transformers, reactors and other important items of electrical plants.