What solar installers need to know about rapid shutdown

The 2017 update of the National Electric Code is officially in effect in 30 states, while most other states are in the process of adopting the new rules. Complying with the 2017 NEC requires solar installations on all buildings to have the capability of module-level rapid shutdown in case of emergency.

This rule pertains to any building-mounted PV but excludes solar carports, which are considered shelters but not buildings, said Matthew Paiss, technical advisor for battery materials and systems at Pacific Northwest National Laboratory (PNNL) and former electrical safety trainer and first responder.

There are three ways to satisfy the 2017 NEC, according to Ken Boyce, principal engineer director of energy and power technologies at UL:

1. Install a “listed” or field-labeled rapid shutdown system

The term “listed” comes from the original name of UL’s certification program developed a century ago. The National Electric Code uses the word “listed” but does not specifically say products and systems must be certified through UL — it instead talks about the overarching characteristics of organizations that would perform that certification service. Obviously, Boyce said UL thinks manufacturers should certify their products for rapid shutdown through UL, but other nationally recognized testing labs could work too.

“We’ve been extremely active in the development of these requirements with the industry and with other stakeholders like the first responder community to try to bring that level of confidence for first responders that have to interact with these systems as they carry out their critically important duties to protect society and people and property, and we want to make them as safe as possible,” Boyce said.

At Solar Power International 2019, many inverter and rapid shutdown device manufacturers displayed their SunSpec certifications. SunSpec developed important protocols for communication among different equipment in the rapid shutdown process but is not a listing body like UL. Seeing the SunSpec label alone does not tell installers a product is NEC 2017 certified for rapid shutdown.

The other part of the first compliance option, field-labeling, refers to a limited engineering evaluation that occurs after installation at a specific site. If the installation complies with NEC 2017 requirements, it can be field-labeled as a rapid shutdown system if the AHJ permits that.

2. Install a system capable of lowering its voltage to 80 V within 30 seconds

Microinverters are typically seen as inherently compliant with rapid shutdown because they convert high-voltage DC power to lower-voltage, safer AC power that can drop down to 80 V within 30 seconds. However, the 2017 NEC does not specifically talk about microinverters as a way to satisfy the rapid shutdown requirement, according to Boyce.

He said some microinverter designs have capacitators on the front end of the circuitry, so unless an inspector actually performs a voltage measurement, there’s no way to know it’s below the 80-V limit. UL has proactively performed these tests to verify that certain microinverters can meet the 80-V threshold, but testing after the inverters have been installed is a challenge.

3. Install a BIPV system with no metal components, no exposed conductors and not within 8 ft of grounded metal

Solar shingles, tiles or other building integrated PV (BIPV) systems can meet NEC 2017 requirements if they include no metal components or exposed conductors or they are not near any grounded metal. These systems are segregated from grounded and conductive parts.

The future of PV Hazard Control systems

There is a revised first option currently being finalized for the 2020 NEC that Boyce said will likely be the future of rapid shutdown requirements: Build an array that’s listed as a PV Hazard Control system, per UL 3741.

“There was some discussion around how rapid is rapid? What’s actually shut down? It’s not entirely shut down, so this term ‘PV Hazard Control’ emerged as maybe a clearer way to talk about what’s actually being accomplished in terms of providing that protection,” Boyce said.

Boyce said installing a PV Hazard Control system could be an acceptable way to satisfy NEC 2017 even before the 2020 version is formally adopted in a given jurisdiction, but it depends on the AHJ inspecting the system.

Currently, string inverters require “listed” add-ons to be rapid-shutdown compliant. A rapid shutdown transmitter or initiator is installed into the inverter either during manufacturing (like Fronius’s offering) or aftermarket. That transmitter is paired with a sensor that’s attached to the array. If the rapid shutdown switch or breaker is initiated at the inverter by a first responder, like in the case of a fire, the transmitter contacts the sensor at the array and stops the flow of energy.

With the future PV Hazard Control requirement, the rapid shutdown functionality will be integrated comprehensively in the solar system, rather than the current standard that allows different rapid shutdown-certified pieces and parts to be used.

“It is clearer in the 2020 NEC that to comply with the requirements, you need a PV Hazard Control System that’s listed for that purpose,” Boyce said. “So that means specifically designed and evaluated and tested and certified for the purpose of producing PV Hazard Control.”

Tigo Energy was founded in 2007 and is a pioneer of rapid shutdown. The company is also a leader in prioritizing system-level certification — Tigo rapid shutdown devices are UL-certified to work as a system with most major inverter manufacturers including SMA, Yaskawa Solectria, Ginlong Solis, ABB and Sungrow.

Even before UL’s 3741 standard becomes a requirement in 2020 and beyond, Tiffany Douglass, director of marketing for Tigo, encourages installers to only buy system-certified RSDs like Tigo’s.

If devices are using power-line communications to initiate rapid shutdown and do not have a “system” UL certification, the RSDs could suffer from false detection of arcs or no detection of actual arcs, which could result in a safety and fire risk, according to Douglass.

Why rapid shutdown is a necessary function

PNNL’s Paiss recently retired from his post as captain for the San Jose Fire Department after 23 years. He never dealt with a structure fire involving solar PV during his tenure, but he knows the importance of protecting first responders and homes as solar becomes more common — and systems installed in the past show signs of aging. These issues were front of mind when he worked on developing rapid shutdown requirements in the NEC.

“Our concern was multi-pronged. It wasn’t just the safety of the responders. When you reduce the voltage, you reduce the energy available to start fires. You don’t eliminate it, but you reduce it. So, we were concerned about the number of fires being started by faulty wire management, arc faults not being detected, and as these systems age and installation breaks down, there’s going to be a higher likelihood of additional faults,” Paiss said. “If the system had rapid shutdown [ability], if there was a fault that is detected, one of the requirements of the NEC is that it initiate and put itself into a safe state.”

How installers can build code-compliant systems

Paiss said it’s unlikely that inspectors will test to make sure rapid shutdown systems work. It’s easy for them to flip the switch or breaker to initiate rapid shutdown, but in order to check and see if the array is reduced to 80 V or less within 30 seconds, inspectors would have to get on the roof and put voltmeters into connectors to test it.

Boyce said this difficulty is why the 2020 NEC is moving toward the system-level rapid shutdown certification requirement. When UL 3741 goes into effect, an official sticker showing the system certification will be enough for the AHJ to know this system is up to code.

“I think we can drive more to having a system that was specifically designed and developed to accomplish this protective function,” Boyce said. “It was tested through this rigorous program and certified for that specific protective feature, and then make it as simple as looking for the UL certification mark.”

For now, Paiss said the important thing for contractors to do is provide the AHJ with the spec sheets stating the products are listed to UL 1741. Some manufacturers take artistic license on marketing new products that may not necessarily be officially listed, and AHJs should catch on to that easily, he said.

“If they provide them a cut sheet that says ‘tested to rapid shutdown’ but not actually saying it’s ‘listed,’ any AHJ worth their salt is going to bounce that right back to them and say, ‘This is not listed,'” Paiss said.

To be most confident of successful AHJ inspection, contractors should look for UL certification of the system. Contractors should also make sure they understand the requirements for the specific AHJ in the location where they’re installing.

“Understanding the requirements that are in place when the installation is being done in that particular jurisdiction is really a good starting point for making sure that you’ll be successful in having that installation approved by the regulatory authority in providing the highest level of safety,” Boyce said.

Updated on Jan. 14