Rapid Shutdown for PV Systems: Page 6 of 7

Understanding NEC 690.12

Residential Applications

Most residential PV systems use microinverters or ac modules, dc-to-dc converters or string inverters. You can deploy any of these options in a manner that complies with NEC 690.12. In addition, one residential string inverter scenario does not require additional equipment to meet 690.12.

Microinverters and ac modules. PV systems installed using microinverters or ac modules inherently comply with 690.12. Loss of ac power immediately de-energizes all PV system circuits outside the array area(s); only circuits internal to the module and conductors between modules and external microinverters remain energized. At present, Enphase Energy leads the microinverter market, which includes vendors such as ABB (Power-One), APS America, Darfon, ReneSola and SMA America. While ac modules currently have a smaller market share than microinverters, they are available from LG Electronics (Mono X ACe module) and SolarBridge’s TrueAC module vendor partners.

For load-side–connected microinverter or ac module systems without battery backup, the simplest way to initiate rapid shutdown is to turn off utility-supplied power to the building. This not only is cost effective, but also coordinates well with standard emergency response practices. First responders can initiate rapid shutdown by opening the main service disconnect or opening the PV system disconnecting means. In the former case, you could install the plaque or directory used to fulfill NEC 690.56(C) at the main service. Additional wording could direct emergency personnel to open the service disconnect to initiate rapid shutdown.

DC-to-dc converters. Because dc-to-dc converters can control voltage at the array, these systems are well suited to accomplish rapid shutdown. However, they are compliant only if they meet the 10-second time limit, which is largely a result of inverter selection. For example, SolarEdge offers complete power conversion systems combining module-level dc-to-dc converters with proprietary transformerless string inverters that a nationally recognized testing laboratory (NRTL) has verified can provide rapid shutdown. Not only is rapid shutdown available in all SolarEdge inverters, but installers can also upgrade existing installations to add this functionality. Other vendors, such as Tigo Energy, supply dc-to-dc converter solutions that are inverter agnostic. If you select one of these solutions, you must verify compliance with NEC 690.12.

For example, Trina Solar’s Trinasmart modules integrate dc-to-dc converters from Tigo Energy into the module junction box. Trinasmart modules can therefore control voltage within the array so that only circuits internal to the module remain energized in the event of an emergency. If you install Trinasmart modules with an inverter that meets the 30-volt and 10-second time limits, then the installed system is 690.12 compliant. However, if you install Trinasmart modules with an inverter that does not bleed down under 30 volts or otherwise isolate the capacitor bank within 10 seconds, then the installation is not compliant—unless you also install an external contactor or similar device to control the voltage source at the inverter.

String inverter systems. You need additional hardware to bring conventional string inverter systems into compliance with NEC 690.12. At a minimum you must install a contactor combiner or similar device in a manner that controls dc circuits in accordance with 690.12(1)—within 10 feet of the array or within 5 feet of entering a building—along with control circuits to actuate the contactor. Further, if the inverter’s capacitor-bank voltage does not drop below 30 volts within 10 seconds, you must add a contactor or switch to isolate the capacitor bank.

Emergency responders can initiate rapid shutdown in string inverter systems manually using a dc disconnect or a rapid-shutdown controller; alternatively, the systems can shut down automatically upon loss of ac power. If a dc disconnecting means at the inverter initiates rapid shutdown, this switch must also activate a switch or contactor within 10 feet of the array or 5 feet of entering the building. Where loss of ac power remotely activates both of these switches, the service disconnecting means can serve as the rapid-shutdown initiator for the PV system if labeled per 690.56(C). Where a dc disconnect initiates rapid shutdown, you should locate this device and the inverter it controls near the service equipment, and you must label it per 690.56(C).

The relative cost and complexity required to facilitate rapid shutdown in string inverter systems varies based on inverter selection. For example, Bentek has developed a rapid-shutdown system, consisting of a ground-level Rapid Shutdown Controller and a roof-mounted 2- or 3-string Rapid Shutdown Module that works with transformer-isolated or transformerless inverters. It is relatively cost effective to use this rapid-shutdown system with an inverter that meets the 30-volt and 10-second limits, such as ABB (Power-One) UNO series inverters or SMA America TL-US series inverters. However, costs increase if the system requires two Rapid Shutdown Modules, one at the roof-mounted array and another at a ground-level inverter. MidNite Solar and SolarBOS also provide rapid-shutdown solutions for residential string inverter applications. SMA America is working on a proprietary rapid-shutdown system that is compatible with its TL-US series inverters equipped with Secure Power Supply outlets.

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