The NEC 2014 introduced Rapid Shut Down requirements in the new Article 690.12 provision (entitled "Rapid Shutdown of PV systems on Buildings"). The goal of this provision is to increase the safety of firefighters and first responders that may come into contact with live PV conductors during a building fire or emergency.
While a grid-tied inverter will automatically shut down during a power outage, the PV source circuit (conductors from the PV array to the central inverter) would still be energized and pose a hazard to a firefighter chopping through a wall or roof.
To address this, Article 690.12 in the NEC 2014 states: "First responders must contend with elements of a PV system that remain energized after the service disconnect is opened. This rapid-shutdown requirement provides a zone outside of which the potential for shock hazard has been mitigated. Conductors more than 5 feet inside a building or more than 10 feet from an array will be limited to a maximum of 30 V and 240 VA within 10 seconds of shutdown.â€
Enphase microinverters and SolarEdge DC optimizer/inverter systems meet this Rapid Shutdown requirement without any additional equipment. To make other string inverter systems compliant, MidNite Solar produces a line of Rapid Shutdown equipment. This includes components to disconnect battery-based equipment like the Inverter and AC generator.