Due to possible equipment failure or human error, it is necessary to provide protection devices. These devices minimize system damage and limit the extent and duration of service interruption when failure occurs. The main goal of protection coordination is to isolate the affected portion of the system quickly while at the same time maintaining normal service for the remainder of the system. It is necessary to show on the one-line diagram the protective relaying required and the breakers affected in order to complete the power distribution system. This chapter illustrates typical applications of protective devices. It presents the characteristics of electromechanical overcurrent protective relays. The overcurrent relays are current sensitive and require a seal-in contact to keep them energized after activation. The chapter also illustrates a typical tripping circuit and the residual grounding scheme. Partial differential/summation relaying is commonly used to detect and isolate faults without affecting other portions of the system.