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Rapid Shutdown For Commercial-Scale Systems

The first task is to identify whether rapid shutdown is required for your commercial solar installation and to what extent. 




This is determined by 1) which version of the National Electric Code (NEC) your utility/authority having jurisdiction (AHJ) enforces, and 2) where the system is installed. 

  1. The NEC published by the National Fire Protection Association (NFPA) receives major updates every three years and is in effect across the United States, but local and state bodies dictate and enforce different versions of the code. 


You can find out which version of the NEC code is being enforced in your state by using the NEC adoption map below:


NEC® in Effect 2/1/2020


NEC® Update Process In Progress 2/1/2020

Where is the National Electrical Code® in effect?


Currently, rapid shutdown is required under 2014-2020 NEC, encompassing some 43 states total. Keep in mind that your local utility/AHJ will have the final say when interpreting and enforcing the code, so your project-level requirements may be stricter than what is enforced at the state level.


  1. There are three common types of commercial solar systems: 

    1. PV systems installed in/on buildings, including flat and sloped rooftops

    2. PV systems installed on the ground, such as fixed-tilt systems or ballasted ground mounts

    3. PV systems installed on uninhabited constructs, such as shade structures and carports.


Rapid shutdown is not required for ground-mounted systems, and most of the time it’s not required for shade structures and carports, either. However, there have been some cases where the utility/AHJ will require it for barns and sheds even if they are uninhabitable. 


We strongly recommend checking with your local utility/AHJ regarding their interpretation/enforcement of the NEC in your region.


For the remainder of this article, we will assume that the installation takes place on an inhabitable building, in a state where rapid shutdown is required (i.e. 2014 NEC or newer). 


If you are using microinverters or module-level optimizers from a reputable manufacturer (Enphase, APsystems, SolarEdge, Tigo, etc.), your system is most likely compliant with rapid shutdown requirements, though there are a couple of exceptions that we will discuss later in this article. 


By default, the following scenarios will assume we are using a central/string inverter without MLPE devices.


Under the 2014 NEC

Many solar professionals are familiar with this version of the code, which states:


“PV System circuits installed on or in buildings shall include a rapid shutdown function that controls specific conductors in accordance with 690.12(1) through (5) as follows:

  1. Requirements for controlled conductors shall apply only to PV system conductors of more than 1.5 m (5ft) in length inside a building or more than 3 m (10 ft) from a PV array.

  2. Controlled conductor shall be limited on not more than 30 volts and 240 volt-amperes within 10 seconds of rapid shutdown initiation.

  3. Voltage and power shall be measured between any two conductors and between any conductor and ground.

  4. The rapid shutdown initiation methods shall be labeled in accordance with 690.56 (B)

  5. Equipment that performs the rapid shutdown shall be listed and identified.”


In other words, if we draw a 10 ft. boundary around our PV subarray, any DC or AC conductors outside of this encapsulated area are subject to rapid shutdown requirements. 


Note that if a subarray’s 10 ft. boundary overlaps an adjacent subarray’s boundary, the conductor run between these two subarrays can be controlled by one rapid shutdown device. This allows for great flexibility since we can have up to 20 ft. between subarrays (10 ft. boundary from each).


To meet these requirements, it is common practice that the inverter is mounted on the roof within 10 ft. of the PV array, and the AC disconnect at ground level is used as the rapid shutdown initiating device.


In this scenario, when the AC disconnect switch is thrown, the inverter will shut off and de-energize both the DC and AC conductors to the specified levels. It’s important to ensure that any capacitors within the inverter are also disconnected from the DC and AC conductors, or there is a possibility that the conductors may exceed the 30V / 240VA threshold after 10 seconds.


If the inverter is instead mounted at ground level, the DC run(s) from the rooftop array(s) to the inverter need to be controlled via an external rapid shutdown device. Rapid shutdown boxes, such as the Fronius RSB Quattro, tend to be proprietary to the inverter manufacturer and rely on hardware/software compatibility to function properly. These devices are commonly used to fulfill the 2014 NEC rapid shutdown requirements.


Under the 2017 NEC (with 1/1/2019 update)

Several changes have been made since the 2014 NEC, which are discussed in our articles. Here is the code excerpt that pertains to us (pay special attention to 690.12(B)(2)(2)).


“PV system circuits installed on or in buildings shall include a rapid shutdown function to reduce shock hazard for emergency responders in accordance with 690.12(A) through (D).

  1. Controlled Conductors. Requirements for controlled conductors shall apply to PV circuits supplied by the PV system.

  2. Controlled Limits. The use of the term, array boundary, in this section is defined as 305 mm (1 ft) from the array in all directions. Controlled conductors outside the array boundary shall comply with 690.12(B)(1) and inside the array, boundary shall comply with 690.12(B)(2).

  1. Outside the Array Boundary. Controlled conductors located outside the boundary or more than 1 m (3 ft) from the point of entry inside a building shall be limited to not more than 30 volts within 30 seconds of rapid shutdown initiation. Voltage shall be measured between any two conductors and between any conductor and ground.

  2. Inside the Array Boundary. The PV system shall comply with one of the following:

    1. The PV array shall be listed or field labeled as a rapid shutdown PV array. Such a PV array shall be installed and used in accordance with the instructions included with the rapid shutdown PV array listing and labeling or field labeling. Note that listing/field-labeling as a rapid shutdown PV array, AKA a “Hazard Control” system, requires the unreleased UL 3741 standard, and is of no use to us as of today.

    2. Controlled conductors located inside the boundary or not more than 1 m (3 ft) from the point of penetration of the surface of the building shall be limited to not more than 80 volts within 30 seconds of rapid shutdown initiation. Voltage shall be measured between any two conductors and between any conductor and ground. This is crucial!

    3. PV arrays with no exposed wiring methods, no exposed conductive parts, and installed more than 2.5 m (8 ft) from exposed grounded conductive parts or ground shall not be required to comply with 690.12(B)(2). This applies to building-integrated PV systems which we are not.

The requirement of 690.12(B)(2) shall become effective January 1, 2019...


Essentially, at any point within the strings/within the array boundary, the voltage cannot exceed 80V after the 30-second mark. This requirement extends to the devices within the array (even between/underneath modules/MLPE devices). Under this version of the NEC, upon rapid shutdown initiation, we cannot have any more 600V or 1000V strings on the roof anywhere. 


Keep in mind that the 2017 NEC is in effect in 31 states, and this section of the code will most likely apply to your commercial-scale system!


So what options are available? 

Currently, there are a handful of ways to deal with this requirement, with more solutions in the works:

  1. Microinverters (Renvu recommends: Enphase, APsystems), or

  2. Optimizers* (Renvu recommends: Tigo TS4-O, SolarEdge), or

  3. Dedicated module-level RSD devices (Renvu recommends: Tigo TS4-F, APsmart)

*Some optimizers rely on a single input for series-connected modules – this may or may not be code-compliant!


Under the 2020 NEC

Some updates have been enacted concerning rapid shutdown. Fortunately, the only state using the 2020 NEC as of the writing of this article is Massachusetts, and the core requirements for rapid shutdown remain unchanged. 


Here’s a summary of the minor updates:

  • Section 690.13 requires the disconnecting means to be lockable to limit accessibility. This is to prevent an unqualified person from prematurely re-energizing the system after it’s been shut down.

  • Labeling requirements concerning rapid shutdown have relaxed slightly. Please consult the full 2020 NEC code for more information (including but not limited to subsections 690.31, 690.52, 690.53, and 690.56).


Conclusion

If you are in search of 2017 NEC-compliant inverter solutions for your commercial-scale project, look no further! We’re proud to offer a host of solutions from industry-leading manufacturers. 




Sources:

https://www.purepower.com/blog/nec-690-12-rapid-shutdown-of-pv-systems-on-buildings

https://www.purepower.com/blog/2017-nec-690.12-rapid-shutdown-important-changes

https://iaeimagazine.org/magazine/2019/2019september/pv-installations-for-the-2020-nec/

https://solar-distribution-us.baywa-re.com/solar-r-e-view-magazine/commercial-projects/new-rapid-shutdown-requirements-may-be-challenging-for-commercial-projects/