This is a guide for the application of digital communication for protective relaying systems and schemes@ including transmitting and receiving equipment@ digital channels@ application principals@ performance@ installation@ troubleshooting@ testing and maintenance. Reflected in this guide are the knowledge and experience of equipment manufacturers and power utility users. This guide is not intended to supplant specific or general instructions contained in manufacturers' books nor any contractual agreements. Purpose The primary purpose of this document is to guide the power system user in applying@ installing and operating digital communication channels for the purpose of protective relaying. The guide is prepared not only for those considering digital communication relaying for the first time@ but also as a reference for the experienced user.