Code Corner: Rapid Shutdown of PV Systems: Part 2

If you can install a string inverter within 10 feet of the PV array, as shown here, you do not need to install any remote switches to comply with 690.12.
MLPEs are well-suited for rapid shutdown. In this scenario, shutting off the AC service to the building shuts off the inverter and isolates DC conductors to the module level.

In HP164, “Code Corner” discussed the requirements of Section 690.12, “Rapid Shutdown of PV Systems on Buildings”—a new section of the National Electrical Code (NEC). This article covers options for meeting this requirement for common types of residential PV installations. The NEC doesn’t specify exact compliance methods—there is no single “rapid shutdown initiation device” (RSID) or strategy. In most cases, there is more than one way to meet this requirement and options will vary depending on the system type. Proper planning and consultation with the local authority having jurisdiction (AHJ) will help ensure that this requirement is effectively implemented.(See "Part 1" here.)

Section 690.12 does not specify the location of equipment used to meet the rapid shutdown requirement. However, the goal of rapid shutdown is to help firefighters and emergency responders protect themselves—so it makes sense to group any additional rapid shutdown switches with the main AC service disconnect, or wherever responders will go to control the utilities that supply the building. For stand-alone systems, locating the shutdown means in an easily visible, accessible, and well-labeled location meets the spirit of the requirement.

Batteryless Grid-Tied Systems

Meeting Section 690.12 requirements for batteryless, grid-tied systems that are interconnected on the load side of a service is fairly straightforward. The strategies and equipment used depend on the inverter and system topology. Supply-side connections are addressed later in this article.

String inverters. Fulfilling 690.12 requirements in systems with string inverters depends on the equipment layout. If the inverter is mounted on the roof within 10 feet of the array or on an exterior wall within 10 feet of the array; the AC connection to the inverter is de-energized when utility power is shut down (also shutting down the inverter, as is typical with these systems); and if DC capacitance in the inverter is not an issue (see below) then no additional device is required to control the PV system conductors. Per 690.56(C), a label (ideally at the main AC service disconnecting means) to alert emergency responders must state: “PHOTOVOLTAIC SYSTEM EQUIPPED WITH RAPID SHUTDOWN.”

Providing additional information is also helpful, such as: “Disconnecting AC service to the building also isolates energized PV system conductors to within 10 feet of the PV array.” Including a map that shows the array’s outline, its location on the roof, and the 10-foot perimeter is also helpful for emergency responders.

If the string inverter is located beyond 10 feet of the rooftop array or is connected to the array by DC circuits that run for more than 5 feet through a building, some means of automatic disconnection for the DC conductors, whether they be PV-source or -output circuits, must be provided. A rooftop-mounted, remotely controlled disconnect, as allowed in NEC Section 690.15(C) and 690.17(A), would suffice. This could be a contactor; shunt trip switch or breaker; or power-operated switch or relay. This normally open disconnect would be closed when utility power is present. If AC power to the building is shut off, the disconnect automatically opens and de-energizes the conductors from the disconnect to the inverter. Note that additional control wiring will need to be run to the disconnecting device; if the device is in a combiner box and also is locally operable, then it could also fulfill the 690.15(C) requirement for a disconnect at roof-mounted combiner boxes.

In some inverters, DC capacitors can energize DC circuit conductors after the inverter is off. Unless the inverter capacitors discharge to less than 30 V within 10 seconds, those same conductors that are controlled within 10 feet of the array (or 5 feet into the building) must also be isolated at the inverter to protect first responders from capacitive charge in the inverter. Contact the inverter manufacturer for details, as some types (such as transformerless) and brands of inverters are available that do not remain energized on the DC side after being shut off.

If there is DC capacitance beyond the allowances of 690.12, then a second switch, operating in conjunction with the rooftop disconnect, is required. A DC disconnect, wired in series to operate with the rooftop disconnect when the main AC disconnect is shut off to the building, and also manually operable to function as a DC equipment disconnect for the inverter, would be the ideal approach.

This disconnect, which isolates the DC side of the inverter, must be located at the inverter. If the inverter is inside the building, then remote operation of this disconnect is required. It must open when AC power is shut off to the building or a separate RSID must be installed. If so, group it with the main AC service disconnecting means, and clearly label both.

Comments (0)