PV Hazard Control systems can cut costs and save time for rooftop installers

An example of the UL 3741 certified system from Sollega and SMA.

Since the publication of the National Electrical Code (NEC) in 2017, rooftop solar companies have faced potentially time-consuming and costly safety requirements for module-level rapid shutdown. The code requires installers to use module-level power electronics capable of reducing the voltage of each module to 80 V or less within 30 seconds of the quick shutdown to protect first responders. On smaller residential systems, quick shutdown devices can add only a few minutes of extra installation time and small additional costs. But on huge commercial rooftops, they create a much greater challenge.

The 2017 code included other ways to meet this security requirement in addition to using MLPE, but they weren’t as easily feasible.

“Everyone is doing MLPE because until recently that was the only way to meet the requirement, and right now it’s probably still the easiest way to meet the requirement, mainly because of equipment availability, AHJ adoption [and] acceptance by firefighters,” said Ryan Mayfield, founder of Mayfield Renewables, a solar energy company.

That won’t be the case for long. Now that production and testing is finally complete, installers have a new option to meet fire safety requirements on solar roofs: PV Hazard Control systems.

This option, listed as UL3741has been included in the code since 2020, but only now are products being tested and certified according to the standard.

To achieve UL 3741 certification, products must pass a battery of tests designed to simulate situations firefighters may encounter on a sunroof. The test analyzes what happens when first responders fall on damaged solar panels while wearing typical protective clothing. There is no prescribed combination of solar products to meet this new assessment; the merits of each product or product combination submitted will be scrutinized during testing.

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

“I don’t know if I’ll ever get any of the code-making panels, but I’ve heard over and over that 80 V was taken halfway out of the sky,” Mayfield said. “[3741] is based on a collective science and a collective agreement of the firefighting community, the electrical community.”

So far, at least one system from SMA and Sollega and one from SolarEdge has passed the UL 3741 test. The SMA collaboration uses the company’s Sunny Tripower CORE1 inverters in conjunction with Sollega’s FastRack 510 mounting system – made of polymer material.

“[SMA] We thought we were a good candidate as our racks are polymeric and non-conductive, and we worked together to achieve this,” said Elie Rothschild, sales manager at shelving manufacturer Sollega. “What we have shown through the UL 3741 certification is that there is a high degree of security to prevent the firefighters from communicating with the array in a way that would make them part of the current path and thus subject to are at risk of shock.”

In the SMA + Sollega system, wires are routed under both the modules and the non-conductive racks, which UL says are sufficient to protect firefighters from a potential shock hazard when designed to specific requirements – including the use of nylon cable ties with the necessary air gap to ensure that the wires do not come into contact with the aluminum module frames.

“This requires us to install it in a specific way and we follow the manufacturer’s requirements so we meet that standard,” Mayfield said. “That’s kind of the leap forward we need to make sure the industry hears and accepts and takes on as they start using this method.”

SolarEdge’s optimizer-based systems are UL 3741 certified.

Other UL 3741 certified systems rely much less on design for compliance. Systems like SolarEdge’s, which also pass quick shutdown certification, use MLPE to lower the voltage per panel enough to adequately protect first responders. The additional UL certification proves that this optimization-based system keeps first responders safe in the event of an emergency.

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

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

Other manufacturers are working towards UL 3741 certification

Most roof rack systems are made of metal. As the non-conductive nature of Sollega’s solution plays a major role in the certification of the risk management system, it remains to be seen whether metal shelving systems will achieve UL 3741 certification.

“If you have a metal rack system, it gets a lot harder here because once you use the metal supports to hold the conductors, they get close to the metal, which is going to potentially get energy,” Mayfield said.

Solar Energy World contacted nearly a dozen inverter manufacturers to see if they plan to submit products for UL 3741 testing. Yaskawa Solectria is investigating certification with both IronRidge and Sollega, while Ginlong Solis is also in talks with Sollega. Fronius, APsystems, Tigo Energy and Growatt said they have no plans in the works, and CPS America and GoodWe did not respond in time.

What happens to a quick shutdown?

While the new standard is a welcome addition for installers, there is still room for quick shutdown systems in the mix. Building departments will take some time to become familiar with UL 3741 systems, and the current design constraints required for some products to meet the standard mean that not all solar installations allow it.

For example, in the collaboration of Sollega and SMA, systems must be designed with contiguous stringing – including no partial strings or sub-arrays – and inverters must be mounted within a certain distance from the array to meet 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 took it a step further, saying the manufacturer of quick shutdown devices sees no reason to change what works.

“At this time, Tigo has no plans to create solutions for the UL 3741 standard. Module-level power electronics remains the best solution for the market as it offers the flexibility to provide rapid shutdown and/or module-level monitoring and/or optimization,” said James (JD) Dillon, chief marketing officer at Tigo. “We will certainly respond to installers’ needs, but have not yet heard a strong demand signal from the market for this alternative standard.”

When the time comes when most installers are looking for 3741-certified systems, Mayfield said there’s still a role for the makers of module-level quick shutdowns. Instead of selling quick shutdown devices by the module, manufacturers could certify MLPE that shuts down up to four modules in series under the new PV Hazard Control list, reducing costs, installation time and potential future O&M problems.

Installers can expect even more options for roof fire compliance in the near future.

“I’m thrilled that we’re finally seeing manufacturers step up to hopefully deliver what are considered safer systems,” Mayfield said. “I think this will help drive innovation within the industry, reduce costs and, hopefully, ultimately aim to build a more secure system because we can test to a standard.”

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