The book introduces a new concept for EHV networks protection via digital protective relays. the new concept being named Cos-Sin is strictly examined against all normal and abnormal conditions in EHV networks. The Cos-Sin concept could be applied on both the voltage and current signals, where it relies on a simple mathematical approach that can be easily processed. The Cos-Sin based protection succeeded in detecting single phasing in large induction motors as well as all fault types on EHV transmission lines and bus bars. The new Cos-Sin Tool proved to be simple, reliable and accurate.