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Module-Level Rapid Shutdown: New Requirements for Fire Safety

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Working in the solar industry, you’ve likely heard about rapid shutdown requirements. But what are they, do they apply to you, and do you know how to comply?

Several states including California adopted newer rapid shutdown requirements starting in 2020, and many others had already adopted them prior to that point, so it’s important to understand what these requirements mean for your business.

In this article, we tackle this topic and what you need to know to design safe and legal PV systems in areas with these requirements. Specifically, we look at the requirements for module-level rapid shutdown in the U.S.

What Is Rapid Shutdown?

With the increased popularity of solar PV—especially on homes—the National Fire Protection Association (NFPA) wanted to find a way to ensure firefighters could be safer when responding to fires on buildings with solar PV.

In response, the NFPA, which publishes the National Electric Code (NEC)—standards for electrical wiring that are adopted by states and municipalities in the U.S., introduced rapid shutdown.

Rapid shutdown requirements aim to provide a simple method for firefighters to de-energize the DC conductors in a solar system and ensure safe conditions on a roof if there’s a fire, explained Edward Harner, Chief Operating Officer of Green Solar Technologies.

In the 2017 NEC, NEC Section 690.12 was updated to reflect new rapid shutdown rules. The update calls for module-level rapid shutdown instead of the array-level rapid shutdown required in the NEC 2014 code. States and municipalities adopt different versions of the NEC over time, but as of January 1, 2020, the NFPA reports that 31 states have adopted the 2017 NEC.

To meet these module-level regulations, smart modules, microinverters, or other module-level power electronics are needed.

Protecting Firefighters From Live Wires

“On a typical string inverter system, even after the inverter is switched off, the DC conductors remain live as long as the sun is shining. To protect our firefighters from hacking any live wires while ventilating the roof during a fire, rapid shutdown was introduced to kill any power in the system conductors,” Harner said.

Rapid shutdown devices are designed to lower the voltage in the DC system conductors to 30 volts within 10 seconds after the inverter is disconnected, he said.

For standard string inverter systems, the NEC requirements also call for a rapid shutdown device for every section of the conduit run that is more than one foot from the array, he said.

MLPE Needed

That means that all buildings subject to the NEC 2014 or 2017 codes will need module-level power electronics (MLPE) such as microinverters and optimizers to enable the rapid shutdown, said Sean White, the author of several books about solar energy and the 2014 Interstate Renewable Energy Council Trainer of the Year.

One of the drawbacks of requiring module-level rapid shutdown—as opposed to array-level shutdown—can be the additional cost, said Harner.

Considerations for Systems with MLPE

Another consideration to be aware of is the maintenance required for MLPE. “You need things to be replaced more often,” said White, who has solar PV on his roof at home. “I have 53 microinverters on my roof. There’s almost always one I need to replace. The microinverters have a monitoring system and tell me when they need to be replaced,” he said.

In addition, the rapid shutdown requirements limit a designer’s product choices.

Before the rapid shutdown requirement, designers could wire modules and put control devices outside the arrays, which was easy to do. The only way to meet the new requirement is to install an electronic device at each module, and there are limited options for achieving this goal.

One way to avoid installing electronic devices at each module is to use smart modules, such as those from SolarEdge or Enphase, which have rapid shutdown equipment built into their systems. While solar systems with these kinds of components don’t require additional gear, most others do—with a few exceptions, White noted.

Wildfires Boost Need for Rapid Shutdown

The need for rapid shutdown has increased because wildfires are being sparked more often by climate change, especially in California and Australia.

Without the rapid shutdown requirements, firefighting in the age of wildfires and climate change would be more complex—and dangerous to firefighters who are already grappling with the challenges of bigger and more frequent fires, said Harner.

Supporting Firefighter Safety

“There have been many reports in the news about firefighters needing to change their tactics at the last minute when, upon arrival to the scene, they find solar on a building,” he said.

Firefighters are safer due to rapid shutdown requirements. More and more, they’re also safer because they’re taking precautionary steps such as maintaining a database of buildings with solar. And fire officials are providing special training on fighting fires in buildings with solar.

For example, the city of Portland, Oregon is providing training to firefighters about how to respond to fires in buildings that have microgrids consisting of solar and storage.

“Structures with solar only add more complexity to the challenge of firefighting,” said Harner. “We want our firefighters to be safe, and we want to also promote and accelerate the adoption of solar technology.”