In this article, we'll explain how protective relays work, review some of the most common relay functions for solar and energy storage systems, and provide best practices for relay programming during project development. Image courtesy Schweitzer Engineering. Electrical relays, protective devices used to switch power on or off for parts of a circuit, have been integrated into circuits for nearly two hundred years. The first example of a relay dates back to the mid-nineteenth century, when Joseph Henry used a small electric signal to activate an. Protective relays and devices have been developed over 100 years ago to provide “last line” of defense for the electrical systems. Long term cost reduction (TCO) for trainings and maintenance by reduce variety of relays A fast and selective arc fault mitigation for air-insulated LV & MV switchgear and Relion protection and control relays and sensor. Battery energy storage systems (BESSs) that make electricity from solar, wind, and other renewable sources available on demand need comprehensive circuit protection. Littelfuse offers solutions with industrial power fuses, arc flash relays, ground fault protection, and surge protective devices. DERs encompass a wide range of decentralized energy sources, such as solar photovoltaic (PV) systems, wind turbines, microgrids. Today's energy systems require robust protection mechanisms for energy storage, combining traditional engineering principles with advanced data analytics and business intelligence.