Complying with UL 3741: Are rooftop solar projects adequately protecting firefighters?

By Tom Kenning
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According to the Solar Energy Industries Association and Wood Mackenzie, nearly 4.2 million US homes are equipped with rooftop solar. Image: US Department of Energy.

Prominent fire safety experts, solar professionals and veteran firefighters have serious concerns about certain new PV hazard control solutions designed to comply with UL 3741, the most recent fire safety regulation to come into effect.

The UL 3741 Standard for Photovoltaic Hazard Control, introduced in 2020, represents significant progress in advancing solar system standards to better protect firefighters and first responders. But now, four years since UL 3741 was introduced, the interpretation of and compliance to these standards has been varied across the solar industry, which raises serious ongoing safety concerns.

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Specifically, the proliferation of non-DC-optimised systems without integrated rapid shutdown is putting firefighters and first responders at risk, according to industry commentators. If these technically compliant yet sub-optimal PV solutions proliferate, firefighters will continue to be at risk and they could lose trust in the solar industry.

The danger is not that solar systems themselves cause fires, but the risks faced by firefighters and first responders who encounter blazing buildings with rooftop solar systems, which are not equipped with the optimal safety measures to project these people.

Rooftop fire prevalence in the US

Once a rare sight, rooftop solar systems across the US are now the norm. According to data from the Solar Energy Industries Association (SEIA) and Wood Mackenzie, of the 85 million eligible homes in the US, nearly 4.2 million are already equipped with rooftop solar, and that number will continue to grow.  Even in states with lower rooftop PV penetration, such as Michigan, firefighters are increasingly responding to fires in locations with rooftop solar installations.

“It’s just by virtue of the amount of solar that has been installed,” says Anthony Granato, president and owner of Energy Response Solutions LLC and a 30-year veteran of the fire service. “In the past 15-20 years, in the early stages, it was few and far between. Now you’re starting to see jurisdictions that have hundreds, if not thousands of installations, whether it’s residential, ground-mounts, or a lot of commercial buildings.”

“My main focus is protecting the lives and safety of firefighters. We’re always trying to improve standards and codes to be ahead of technology, but we end up chasing technology. We need better solar systems that make rooftops safer for firefighters. That was the intent with UL 3741, but what we’re seeing is that there are loopholes to this important regulation.”

“A lot of questions about what to do”

Discussions about the need to reduce hazards for firefighters operating around solar PV arrays started 15 years ago, says Bill Brooks, a member of the UL Technical Committee and one of the architects of the UL 3741 standard in North America. A few years on, the fire service had been involved with a variety of rooftop fires, some of which were caused by PV, while others were fires atop structures where solar systems were located.

“There were a lot of questions about what to do,” says Brooks. “And there was – probably in most cases – unsafe practices around the PV arrays that put firefighters at risk. So, this was an opportunity to talk about how to improve safety for firefighters and what were reasonable means.”

“Firefighters are looking for a simple way of deactivating the system and the key aspect is with solar it generates power directly at the panel, so you can’t turn it off, other than to cover it, and that’s mostly impractical for anything of any size.”

Thus began a process of trying to implement requirements into the National Electrical Code (NEC), which began in 2014 by focusing on protecting conductors connected to the array. There was also introduction of the term ‘rapid shutdown’ in the NEC, which received some criticism, and the UL 3741 standard uses the term ‘PV hazard control’ as a more accurate take on what has been introduced.

The second option was reducing the voltage within the array to no more than 80 volts during a shutdown. The voltage is what provides the shock hazard to firefighters if they’re in contact with the equipment, and reducing voltage levels to 80 volts often involves segmenting the array, typically down to the module level, to improve safety.

An integrated third option came in where wires were kept eight feet away from grounded metal and were enclosed and protected to prevent contact with people.

“This third option, which was mainly designed for building-integrated rooftop systems, was intended to inform what the listing process would become in the future,” says Brooks. “So, UL 3741 is the outgrowth of that whole process. It’s a very complicated standard to implement and it looks like there’s some issues in how it was implemented.”

Right now, work is being done to fine-tune the standard by making it clear what the pass criterium are to ensure the same level of consistency across the board and avoid errors from the different certifying agencies.

Legislative loopholes

Brooks notes that even before the implementation of the 2017 fire safety codes, there were already products available that were compliant with these regulations.

“Many installers were using options like power optimizers or micro inverters, but at least half of the rooftop industry, if not more, were not using systems that had lower voltages,” he says.

Companies did start to shift towards MLPE (Module-Level Power Electronics) systems. Others started manufacturing isolator switches to be installed on every single module, but that brought additional cost and additional effort in the process.

UL 3741 did demonstrate methods that would require fewer switches, but this would put a heavier burden on the installers, who may not have been experienced in this approach.

“One of the cruxes of the problem with UL 3741 is that some of the products that come on the market seem to indicate to the installer that you don’t really have to change anything you’re doing,” adds Brooks. “You just use these products and you’re all good. I think that’s an oversimplification of the process and when you oversimplify something, you may remove some of the benefits or some of the hazard reductions that were intended in the process.”

This issue opened the gateway for solutions using traditional string inverters to perform rapid shutdown, creating an opportunity for developers to not engage with all components of hazard reduction.

“In such solutions, the array conductors need to be completely secured with wire guards and other means to avoid firefighter interaction,” says Brooks

“However, we are seeing UL 3741 solutions coming out to the market with very little protection. Wires are energized to 1000VDC and are being held away from grounded metal with plastic cable ties. This was not the intent of the standard as it was conceived and where we are seeing inconsistencies for tests and validation of these solutions.”

Greater responsibility for manufacturers

The most readily available option initially was MLPE, switches or isolation devices, says Jason Bobruk, director of code compliance at Solaredge. Once the standard came out, different options were offered, but at the cost of putting more responsibility on the manufacturer of that solution and ensuing uncertainty over who would be classed as the manufacturer.

Eventually the racking manufacturers have taken on the responsibility, and with one exception, the early commercial solutions all came from racking manufacturers.

“With that option comes great responsibility,” says Bobruk. “You have to be sure that you’re following all the nuances in the standard and making sure that, if you’re not going to use a voltage reduction, you really better have those conductors protected from the firefighters.”

The standard allows several approaches to do this, but some of those approaches are open to varied interpretations from both the manufacturer and the test labs, which has been an unanticipated and unintended consequence of the legislation. There are fears that a few players are calculating hazard probabilities and then ignoring the potential dangers of those where only a small probability of occurrence is found.

“There’s a lot of math, there’s a lot of statistics,” adds Bobruk. “You could in essence, calculate your way out of having to do anything.”

The commentators in this article are most aligned in their belief that the solution is to go for a more prescriptive approach with regard to legislation.

Reducing risk on the roof

Previous systems that had no voltage reduction were not protected, says Granato. This was exacerbated by rooftop solar systems usually involving electricity lines running inside attics and walls and along rooftops, where firefighters are often cutting holes, which makes it very difficult to extinguish all the hidden hotspots of a fire safely.

Moreover, different US states have adopted the codes at different times, resulting in varying levels of protection in the rapid shutdown systems across different areas. This can lead to confusion among firefighters.

Last year, in Cape Cod, Massachusetts, a firefighter stood inside a burned and damaged solar array on the rooftop of a church, while using a metal pry bar to pull a panel from the roof. Granato contacted the firefighter and found that he was unaware of the danger he was in and was merely following orders.

Granato believes the system without MLPE was installed before the UL 3741 standard came in and could have had a voltage of up to 1,000 volts. Thus, Granato is now travelling the US as one of just a handful of educators trying to spread awareness to the fire service of the dangers of damaged PV panels where firefighting is involved.

“Passion drives my purpose of educating the fire service to try and make sure everybody’s safe,” he adds.

Not all UL 3741 systems are created equal

While some products entering the market include voltage reduction, others have a system that remains at 1,000 volts, leading to various misconceptions.

“The main message is that not all UL 3741 systems are created equal,” says Bobruk. “Users and emergency responders need to really understand the nuances of those differences, and how to put the systems together appropriately so that they’re safer for emergency responders.”

The widely held perception that a system without MLPE is cheaper upfront is one such instance, because this ignores potentially higher costs over the lifetime of a system, he adds. Firstly, not having voltage reduction involves added complications, such as needing to find a qualified electrician to install it safely. There is also a misconception that rapid shutdown will not be required for this. Mechanical solutions and non-voltage control solutions also require ongoing inspection, maintenance, and sometimes even field testing just to remain compliant with the UL listing.

While these are still valid approaches, Bobruk highlights that despite assumptions, safety in later years can be affected, and the overall return on investment can be impacted, and these added layers need to be considered beyond the initial pricing. There also needs to be a lifetime of upkeep rather than just passing inspections on day one. For example, the wiring is connected by cables and zip ties that degrade over time.

“Sometimes the systems go without maintenance near the end of life and that’s where they might be the most dangerous,” adds Bobruk. “They haven’t been inspected, things haven’t been addressed that have failed over time, including optimisers. Power optimisers have the potential to fail, but at least you get electronic notification.”

Brooks agrees that one of the major concerns is that players were drawn towards some of the newer solutions on the market purely because of lower costs.

Additionally, he points out that insurance rates are heavily tied up in the various costs. The insurance industry keeps a close eye on events and if it sees the fire service is hesitant about working with some buildings because of the introduction of certain new equipment, then it responds by changing how its prices the insurance for a building.

The commentators hope that future amendments to the codes or industry-driven directives can mitigate the fears raised around use of non-DC-optimised UL 3741 solutions and help bring firefighters back on board. There are also hopes that solar developers, installers and owners can better understand the balance of initial upfront costs versus that of lifetime costs when choosing solutions in the code.

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