Without it, a minor electrical issue can snowball into a system-wide outage or dangerous event. Protective relaying aims to stop that chain reaction before it starts, detecting problems instantly, cutting off the affected section, and keeping the rest of the system stable and safe. Here's why power system. Protective relays and devices have been developed over 100 years ago to provide “lastline”of defense for the electrical systems. The term is also used for a branch of electrical power engineering that deals with. Selectivity is a mandatory requirement for all protection, but the importance of it depends on the application. While this is bad, It's not a.