Ever since the 2017 National Electrical Code (NEC) was published, rooftop solar contractors have faced potentially time-intensive and costly safety requirements for module-level rapid shutdown. The code requires installers to utilize module-level power electronics capable of lowering each module’s voltage to 80 V or less within 30 seconds of rapid shutdown initiation to protect first responders. On smaller residential systems, rapid shutdown devices might add only a few extra minutes of install time and minor additional cost. But on huge commercial rooftops, they create a much larger challenge.

The 2017 code did include other ways besides using MLPE to satisfy this safety requirement, but they weren’t as easily attainable.

“Everybody is doing MLPE because that, up to recently, was the only way to meet the requirement, and right now it’s probably still the easiest way to meet the requirement primarily just because of equipment availability, AHJ acceptance [and] firefighter acceptance,” said Ryan Mayfield, founder of solar design firm Mayfield Renewables.

That won’t be the case for much longer. With manufacturing and testing finally in place, installers have a new option to satisfy fire safety requirements on solar rooftops — PV Hazard Control systems.

This option, listed as UL 3741, has been included in the code since 2020, but only now are products getting tested and certified to the standard.

To achieve UL 3741 certification, products must pass a series of tests designed to simulate situations firefighters may encounter on a solar rooftop. The testing analyzes what happens when first responders fall on damaged solar panels while wearing typical protective gear. There’s no prescribed combination of solar products to satisfy this new rating; the merits of each submitted product or product combination are scrutinized during testing.

The goal of the new standard is to make rooftop arrays safer for firefighters in an emergency. Some in the industry say 3741 certification is much more rigorous and scientific than rapid shutdown certification.

“I don’t know if I’ll get anybody ever on record from the code-making panels, but I have heard over and over again that 80 V was semi- pulled out of the air,” Mayfield said. “[3741] is based on a collective science and a collective agreement from the firefighter community, the electrical community.”

So far, at least one system by SMA and Sollega and one by SolarEdge have passed UL 3741 testing. The SMA collaboration uses the company’s Sunny Tripower CORE1 inverters paired with Sollega’s FastRack 510 mounting system — made of polymer material.

“[SMA] thought we were a good candidate given that our racking is polymer and non-conductive, and we worked together to achieve it,” said Elie Rothschild, sales manager at racking manufacturer Sollega. “What we’ve shown through the UL 3741 certification is that there’s a high degree of safety to prevent the firefighters from interacting with the array in any way that would cause them to become part of the current path and thus be subject to a shock hazard.”

In the SMA + Sollega system, wires are routed underneath both the modules and the non-conductive racking, which UL believed to be adequate to guard firefighters from a potential shock hazard when designed to specific requirements — including using nylon zip ties with a necessary air gap to ensure wires do not come in contact with the aluminum module frames.

“This is going to require that we are installing it to a specific way and we’re following the manufacturer’s requirements so that we are meeting that standard,” Mayfield said. “That’s somewhat of the leap that we need to make sure that the industry hears and accepts and takes on if they’re going to use this method.”

Other UL 3741-certified systems are far less reliant on design for compliance. Systems like SolarEdge’s, which also passes rapid shutdown certification, use MLPE to lower the per-panel voltage enough to adequately protect first responders. The additional UL certification proves that this optimizer-based system does keep first responders safe in case of emergency.

“We don’t require any special restrictions or limitations, such as wire management, racking, inverter placement, special training, quality of work [or] ongoing array inspections to maintain compliance or other means to maintain limited access to dangerous high-voltage components,” said Jason Bobruk, director of code compliance, and Magnus Asbo, senior director of technical marketing at SolarEdge, in an email. “We reliably de-energize the entire system and remove the dangerous hazards altogether.”

All SolarEdge three-phase inverters paired with optimizers are listed as UL 3741-compliant when installed with any racking and wire management configuration.

Other manufacturers working on UL 3741 certification

Most rooftop racking systems are made of metal. With the non-conductive nature of Sollega’s solution playing a large part in its hazard control system certification, it remains to be seen if metal racking systems will achieve UL 3741 certification.

“If you have a metal racking system, this is where it’s going to get a lot harder, because as soon as you use the metal supports to hold the conductors, they’re going to be in close proximity to the metal, which is going to be potentially energized,” Mayfield said.

Solar Power World reached out to almost a dozen inverter manufacturers to see if they plan to submit products for UL 3741 testing. Yaskawa Solectria is looking into certification with both IronRidge and Sollega, while Ginlong Solis is in talks with Sollega as well. Chint Power Systems is planning to get all its 1,000-V_DC inverter models UL 3741 listed. Fronius, APsystems, Tigo Energy and Growatt said they do not have plans in the works, and GoodWe did not respond by press time.

What happens to rapid shutdown?

While the new standard is a welcome addition for installers, there’s still a place for rapid shutdown systems in the mix. It will take some time for building departments to get comfortable with UL 3741 systems, and the current design constraints necessary for some products to meet the standard mean not all solar layouts will allow it.

In Sollega and SMA’s collaboration, for example, inverters must be mounted within 1 ft of the array to comply with its UL 3741 certification.

“Certainly, there will be systems that are difficult to design within those constraints. We think having both solutions available is the right approach,” said Brett Henning, commercial product manager at SMA America.

Tigo Energy went one step further, saying the rapid shutdown device manufacturer sees no reason to change what’s working.

“Right now, Tigo has no plans to create solutions for the UL 3741 standard. Module-level power electronics remain the best solution for the marketplace because it has the flexibility of providing rapid shutdown and/or module-level monitoring and/or optimization,” said James (JD) Dillon, chief marketing officer at Tigo. “We will certainly react to the needs of installers but have yet to hear a strong demand signal from the marketplace for this alternate standard.”

When the time does come that most installers are looking for 3741-certified systems, Mayfield said there’s still a role for the module-level rapid shutdown makers. Instead of selling rapid shutdown devices on a one-per-module basis, manufacturers could certify MLPE that shut down up to four modules in series under the new PV Hazard Control listing, cutting down on cost, install time and potential future O&M hassles.

Installers can expect to see even more options for fire safety compliance on rooftops in the near future.

“I think it’s great that we’re finally seeing manufacturers stepping up in order to provide hopefully what will be considered safer systems,” Mayfield said. “I think that’s going to help drive innovation within the industry, drive costs down and, hopefully, with the goal being putting up a safer system in the end because we can test to a standard.”

Updated on August 18 to clarify the design requirements for SMA + Sollega systems and to add that Chint Power Systems plans to get its inverters UL 3741 listed.