PROT 401 provides an overview of the principles and schemes for protecting power lines, transformers, buses, generators, and motors. Compact current relay and. FDPro 401 protection series are designed to electrical power distribution systems from overvcurrent faults to quickly isolate the grid and provide comprehensive and reliable protection. FDPro 401 80 provides the following protective functions: What is an overcurrent relay? What are the usage areas. Selectivity is a mandatory requirement for all protection, but the importance of it depends on the application. For example, unselective protection operation during a medium voltage network fault will cause an outage for an unnecessarily large number of consumers.