THE LACK OF KNOWLEDGE around photovoltaic (PV) rapid shutdown safety features is concerning but not surprising. Firefighter training for the energy hazards that affect our profession has always been scarce. We have energy safety mechanisms and energy indicators all around us—we need only look.
Rapid shutdown is a PV electrical hazard safety feature that was developed for the fire service. It has been in the National Electric Code (NEC) for more than 10 years. To say that the incubation period for recognizing and using this safety feature is “over” is a gross misinterpretation of what is really going on in the fire service.
If this is the first time you’re reading about rapid shutdown, you are not alone. If you want to be prepared to address the challenges of PV, it is important for you to understand the history of how we got here.
The Rocky Road of Code Development
In 2007, the growth of PV on residential structures began to get the attention of the fire service. Rooftops throughout Los Angeles were getting crowded with PV systems, making ventilation operations unusually challenging and potentially dangerous, given the energy hazards. There was a desire to create a five-foot setback requirement around all edges of PV arrays, similar to the rooftop mechanical equipment boundary requirements.
The motivator behind this code change was the need for safe access to conduct ventilation operations, not necessarily a concern over electrical shock. This raised the concern of the PV industry, as most residential rooftops did not offer enough free space on individual pitches to allow PV arrays that met the growing electrical demands. The California State Fire Marshals Office had a draft document addressing fire service access to roofs with PV installations. Fire Marshal Robert J. Davidson, along with the National Association of State Fire Marshals, submitted a proposal to add firefighter safety requirements to the International Fire Code (IFC). A similar proposal was submitted to the National Fire Protection Association (NFPA) 1, Fire Code. At the initial hearing, the IFC Committee turned the proposal down and asked for some improvements.
As a result, the California State Fire Marshal’s Office created a PV task force to refine the proposed language for development of a public comment. This resulted in guidance that would be submitted as a proposal for the 2012 IFC. This would be the first time the IFC (and NFPA 1) would address PV systems and the fire service that drove the submission.
Addressing Electrical Energy Hazards of PV Systems
The roof ventilation access concerns that would be amended with the creation of a code-defining array of boundary language did much to improve our rooftop operations. Unfortunately, absent was any protection from a much larger concern: the energy hazards created by the now rapidly growing rooftop PV array systems.
In 2011, Captain Matthew Paiss, of the San Jose (CA) Fire Department, was the International Association of Fire Fighters (IAFF) representative on the NEC code-making panel. He drafted language that would address this shock hazard in an automatic approach. This proposal, titled “Rapid Shutdown,” required that the voltage in the array be reduced to that of a single PV module (80Vdc max), once the electrical utilities were secure.
New products on the market could accomplish this, specifically microinverters and direct current (DC) optimizers, commonly referred to as module-level power electronics (MLPE). Unfortunately, the proposal received strong opposition from string inverter manufacturers, and ultimately the language that was accepted for the 2014 NEC only reduced the voltage outside the array to 80V. The electrical hazards within the array could remain at full voltage. The PV industry’s concern was twofold:
- There were not a wide number of products available to meet this requirement.
- There would be a potential for a false sense of security should the device fail in an unsafe manner.
In the next NEC cycle, 2017, the IAFF pushed back. The organization once again sought the ability to reduce the voltage hazards within the entire system. As a result, the 2017 NEC finally required that the array be reduced to 80Vdc maximum. While this was the intent of the fire service, the impact of now having differing levels of electrical voltage present created significant confusion as to the potential energy hazards on any random PV system.
There were obvious questions that our fire crews would ask: When was it installed? What version of the NEC was in place when it was installed? And so, in response to this potential misunderstanding, multiple graphical labels were required to notify the fire service of this. Unfortunately, this only added to the confusion.
Even then, the lack of a robust and well-developed energy hazard training curriculum for the fire service was palpable. Here we had this energy safety feature, which had been championed by members of the fire service, and most of us had no idea it even existed. Then—and now—the problem is not that the codes change and we, the fire service, struggle to keep pace. That’s a complete copout. The problem is that the fire service has never prioritized energy hazard training. Compounding this issue is the fact that every time a new technology emerges, we get nervous and hand it off to our hazmat companies. This, in turn, creates even more turbulence for our crews, because it’s energy. It’s not hazmat.
The evolution of PV safety involves both rooftop ventilation access concerns as well as shock hazards. However, for this article, we will focus on the electrical energy hazard protections offered by rapid shutdown.
Nothing you are about to read will result in revolutionary changes to how you approach and operate in proximity to these energy hazards. Regardless of what code cycle was in place when that PV system was installed, even if it is a legacy system with no rapid shutdown safety features, your emergency processes should not change. Your emergency process should always include the following steps:
- Be proactive in identifying PV energy systems.
- Check the meter during your initial 360. This can be a quick way to identify PV systems, as labeling for inverters and disconnects may be colocated with the meter.
- Open disconnects to isolate the energy hazard behind the inverter. This will have the added benefit of activating rapid shutdown if it is present.
- With fire conditions present, shutting down these systems should, ideally, be completed as early into the incident as possible. Shutdown should happen prior to accessing the roof, and it should be compulsory prior to engaging in overhaul operations.
Learning the Language
To fully understand rapid shutdown, you need to be familiar with the vernacular specific to this subject. It’s a language of its own, made up of components, devices, and concepts that are foreign to most responders. But your comfort and competence with the PV safety features of a modern structure will be critical to establishing your fluidity with the semantics of this energy source.
Module Level Power Electronics
In an effort to increase efficiency for a PV system and meet the safety expectations of rapid shutdown, many manufacturers have turned to module level power electronics (MLPE), small devices placed behind the modules on the roof. The ratio is often one device per module. Microinverters and DC power optimizers are the principal components used in MLPE. These two components serve different functions, so let’s better understand how they work.
DC Power Optimizers
The optimizer is designed to condition the DC power by regulating the voltage and aids in accounting for voltage disparities between each individual panel. These voltage variances can result from inconsistent sunlight caused by shading and rooftops with different facing slopes (photo 1).
1. DC optimizers are small devices that are often located on the underside of a single module/panel. They are designed to account for voltage disparities between each panel of an array system. Shading and differing roof facings can result in voltage disparities between panels, which can cause the entire array to become less efficient. DC optimizers correct this situation. (Photos by Chris G. Greene unless otherwise noted.)
The conditioned DC power is then directed to a central inverter for conversion to alternating current (AC) energy. A concern with using DC optimizers is that the increase in electronics for an array may lead to multiple failure points as well as add to the installation cost of the system. And, based on where they are located, maintenance of these components is difficult and time consuming. The simple fact is that an entire series of panels may need to be removed just to service one optimizer. On the other hand, the loss of one optimizer is not going to drop the entire system. It will simply result in a reduction of energy output.
Microinverters
These devices are used to convert DC to AC at the module level. PV panels convert light energy into DC electrical energy, which must be converted into AC energy for use in homes, buildings, and other structures. Unlike a traditional string inverter, which captures the DC energy from every panel in a PV array at one specific location, microinverters address this conversion at each individual panel. As you can imagine, this results in additional electronics for these PV systems. This raises the concern for more potential points of failure.
For example, a residential rooftop using MLPE may require the installation of 30 microinverters, one for each panel. On the other hand, this same system using a traditional string inverter would require only one inverter. But it’s not that simple. There are strengths and weaknesses to both string inverters and microinverters.
String Inverters
String inverter performance is tied directly to that of the lowest-producing panel in the string. So if you have a 30-panel system and five of these panels are heavily shaded, the entire string’s output is reduced. However, installing DC power optimizers at each panel can aid in correcting these production variables between individual panels. String inverter systems may be best suited for less complicated environments, like large commercial rooftops and ground mounted locations.
String inverters are located in a way that provides for easy access to the inverter box. It would be unusual to find the inverter underneath a panel or array. And there will be but one or two serving an entire residential rooftop system. This makes troubleshooting and maintaining the inverter relatively easy. With that said, the downside is that if the inverter fails, the entire system can shut down (photos 2A & 2B). Furthermore, the cost of using a central inverter design can be less expensive than a system using microinverters.
2A. & 2B. This 75-module PV system is comprised of two separate arrays, each controlled by a separate string inverter. Unfortunately for this system, when one of the inverters failed it took half of the system with it. The large energy loss caused by the string inverter failure may have been lessened by the use of microinverters for each individual panel. (Photos by Roy Blackford.)
The Evolution of Rapid Shutdown
Today’s rapid shutdown features are well developed and easy to identify. Fire crews that invest time in understanding PV challenges have a clear lane of travel that leads to actionable information that can increase their safety via rapid shutdown. But it didn’t start this way. Like most firefighter safety systems, improvements are developed incrementally over time. It’s a kind of evolution, and the path is well traveled for code-development committees. Every three years, opportunities to review, revisit, and strengthen these safety features arise. Here is a quick glimpse at how rapid shutdown has progressed through each code cycle.
2014 NEC: Rapid Shutdown Introduction into the NEC
This marked the beginning of the rapid shutdown safety features codification process. It was developed by a combined NFPA 70/NFPA 1 task group as a direct result of the PV wiring language that had been added to both fire codes for the 2012 editions. It provided a time stamp for legacy systems installed prior to 2014 and modern systems reflective of the code expectations articulated in Section 690.12, which mandated the installation of rapid shutdown. This section also defined the boundaries of influence for this application. It identified that all rooftop solar/ PV installations shall have a system that quickly reduces the voltages of electrical conductors that extend beyond a defined distance from the array to a safe level.
The principal changes included the following:
- Application is specifically for rooftop-mounted PV systems.
- Rapid shutdown shall reduce energy to a safe level. Safe level was designated as 30 volts, and this reduction shall be reached within 10 seconds of initiation of rapid shutdown.
- Boundaries of influence were defined as PV electrical conductors extending more than 10 feet from the perimeter of an array on a rooftop, and/or five feet inside of a building.
To meet these code expectations, most manufacturers turned to string inverters with rapid shutdown capabilities or MPLEs.
2017 NEC: Redefining Rapid Shutdown Voltages Limits and Boundaries
The most significant changes to the 2017 NEC with regard to rapid shutdown expectations were in the voltage variations that were now tied to boundaries that were redefined. They included the following:
- The array boundary was reduced from 10 feet to one foot.
- PV electrical hazards within the array boundary must be reduced to 80V or less within 30 seconds of rapid shutdown activations.
- PV electrical hazards outside of this one-foot array boundary must be reduced to 30 volts or less within 30 seconds of rapid shutdown activation. (See sidebar “Rapid Shutdown Boundaries (NEC 2017).”)
- Clearer definitions for locating and labeling rapid shutdown initiation devices were established, especially for one- and two-family dwellings. Specifically, initiation devices shall be located at readily accessible locations on the outside of the structure. The rapid shutdown initiation devices had to consist of at least one of the following:
- A PV system disconnect.
- A service disconnect.
- A readily accessible switch that clearly indicates that a system is “on” or “off.”
These changes were severe and represented a significant commitment to the safety of emergency personnel who may have to work in proximity to these systems. However, for the industry, it resulted in a narrowing of compliance options with regard to equipment and devices that could meet this expectation. Specifically, this began a decline in the use of the string inverters and gave rise to MLPE.
This resulted in more devices for the same system that had been installed just one year earlier, which, in turn, created a significant amount of unavoidable turbulence within the rooftop PV industry. And for some, this was coffin nails. For others, it represented a revolution that would tether them directly to the national fire service for perpetuity.
2020 NEC: The Birth of UL 3741 Rapid Shutdown-Compliant Systems
The most significant change to the 2020 NEC was the introduction of the concept of a UL-listed PV hazard control system (PVHCS), which offered more flexibility in addressing rapid shutdown code expectations by utilizing systems certified under the UL 3741 standard. Here is a list of substantive changes outlined in this code cycle:
- Introduction of the term PV hazard control system. This broadened the options for meeting the traditional rapid shutdown safety expectations to include UL 3741-compliant systems.
- PV system rapid shutdown initiation must be controlled by a single device/ switch. However, a system with multiple arrays would be allowed to share a service panel with up to six rapid shutdown switches.
- Updated labeling requirements related to the type and location of the rapid shutdown systems present (photos 3A and 3B).
3A. & 3B. This residential rooftop system is well Labeled. The meter is identified as “Net Meter,” which is another indicator of PV systems. There is a knife switch present for activating rapid shutdown. The absence of a string inverter box may indicate that the system uses microinverters. And, in this case, it does use microinverters, which are located on the undersides of each panel. (Photos by Hans Christiansen.)
UL 3741 overview: One of the intentions of UL 3741 was to help define the electrical hazards that a firefighter may encounter when interacting with a rooftop PV system. The 80V threshold for harm to a firefighter, identified in previous versions of the NEC, had never been satisfactorily validated via a scientific process.
UL formed a standards technical panel (STP) and worked with industry stakeholders, Sandia National Laboratories, and the U.S. Department of Energy to capture data specific to this gap. This resulted in an empirical data support structure for process evaluationand validation of both systems and products.
The standards committee developed several criteria to demonstrate the shock hazards that firefighters may encounter. These included the following:
- Electrical resistance of both tools and the human body.
- Occupational Safety and Health Administration (OSHA)-required personal protective equipment.
- Potential current pathways through a firefighter.
The result of the project was scientifically validated and defined shock hazards that accounted for voltage, current, and resistance measurements. It provided a repeatable test for the industry to reference. To pass the test, manufacturers must prove that firefighters will not be “exposed” to shock hazards when they work in proximity to these systems. And therein lies the concern: If a system proves that it can protect and prevent exposure to shock hazards, absent specific voltage reduction expectations, that system can meet the UL 3741 standard.
Is a rapid shutdown-compliant system the same as a UL 3741 system? Not exactly. While both are intended to provide safeguards from electrical shock to the firefighters, they achieve this in different methods. UL 3741 is a specific standard that defines the requirement for a PVHCS.
It describes and defines a methodology to protect a firefighter from electrical shock exposure. Rapid shutdown is a general term for this safety expectation, which is intended to rapidly reduce the voltage of the array to a safe level for firefighters. UL 3741 is a standard set by UL, which outlines the requirements for a PVHCS, which may be used to achieve compliance with rapid shutdown code expectations.
Our perspective on UL 3741: What we appreciate about traditional rapid shutdown methods is that they are tied directly to voltage boundaries and limitations to safeguard our fire crews. On the other hand, UL 3741 rapid shutdown-compliant systems are defined as safe if they can meet the “validated concept of safety” as articulated in this standard, keeping in mind that UL 3741 does not include specific voltage limitations.
We are not suggesting that voltages, current, and resistance considerations were absent from the development process that birthed this standard. They were absolutely part of the development. But there was no compulsory voltage reduction requirement of this system when a firefighter activated the PV shutdown initiation device. This means that even if a firefighter shuts down power to the string inverter, the array can remain at full voltage. If the system can meet the safety expectations of the UL 3741 standard, it can be installed on a roof.
This has always felt conceptual rather than definitive. For example, today a UL 3741-compliant system would be allowed to operate at full voltage (600Vdc in residential and 1,000Vdc in commercial) by simply hiding the wiring under the array and, in some applications, this fully energized wiring could be held in place by plastic zip ties.
2023 NEC: Labeling and Clarification on Applicability of Rapid Shutdown
The changes to the 2023 NEC were minimal. They were, for the most part, confined to rapid shutdown labeling alternatives and better clarity on rooftop structures that were exempt from rapid shutdown requirements. These include the following:
- Labeling: The requirement for specific colors on “Buildings with Rapid Shutdown” labels was removed, which has allowed for more design flexibility if the text clearly contrasts the background.
- Exemptions: Standalone carports and shading trellises may be exempt from this requirement. Exemption determination is based on the likelihood of firefighters having to access the roof structure to perform a rescue.
Today, the fundamental safety and boundaries of influence for the rapid shutdown safety features of a PV system remain largely unchanged from that of the 2020 NEC.
Maintenance Frequency for Rapid Shutdown and UL 3741 Systems
Unlike building fire safety systems such as a fire alarm panel or sprinkler system, there is no codified functional test or even a basic maintenance requirement for PV systems—not even for the rapid shutdown firefighter safety features. However, this is frequently identified as a code gap and may be addressed in future versions of the NEC as it applies to these systems. Currently, the PV industry recommends annual inspections for UL 3741 systems and every five years for traditional rapid shutdown systems.
PV Industry Consternations
The PV industry’s frustrations are real. They boil down to two primary issues within the area of rapid shutdown safety requirements:
- Too many additional components. The industry equates this to an increase in failure points.
- Price. These systems are becoming more complicated, and what are we really getting for this?
It’s hard to quantify safety when the barometer is the absence of electrical shock injuries to a fire crew. The fact is the installation costs of some of these systems are rising. And, to date, we have yet to document a single death or even a severe injury to a fire fighter that can be traced back to a PV system. But that’s a good thing. We don’t need to add to the data field of injuries and deaths in our profession.
As far as the concerns over a more complicated system go, I (Greene) think about my first vehicle. In 1985, I turned 16. My father gave me a 1966 Ford F100 pickup truck—a 352 engine and a three-on-the-tree manual transmission. It had arm-strong steering and nonpowered drum brakes. I loved that truck. And I thoroughly destroyed it within two years. But when I consider the safety features of a modern vehicle and compare them to that old truck, it’s a wonder that I survived my adolescence behind the wheel of that time capsule.
From a safety perspective, it’s not even close. And yes, today’s vehicle safety systems are more complicated and have resulted in higher costs, but this has also led to a safer and more reliable vehicle. A PV system is understandably simpler than a modern vehicle, but the layering of safety enhancing features to ensure the ongoing welfare of our fire crews is part of the evolution for any responsible industry (photo 4).
4. This is typical signage indicating the presence of rapid shutdown safety features. Unfortunately, these labels are often painted over. Fire crews must know what they are looking for. The obscured language on the label says “PHOTOVOLTAIC SYSTEM EQUIPPED WITH RAPID SHUTDOWN.”
The simple truth is that we have more PV systems on our rooftops than ever, and our fire crews are having reflective encounters. These numbers are only going to increase. To ensure our safety, we must continue to develop and support these protective features, and to date, rapid shutdown is our best bet.
Rapid Shutdown and the Firefighter Knowledge Gap: Why It Persists
Barriers to understanding rapid shutdown may, in part, have to do with inconsistent code adoption cycles for the fire service. In short, where you work may determine which version of the NEC is in effect. For example, if you reside in an area that is still using the 2008 NEC, the safety features offered by rapid shutdown would not be required, but systems may still offer this level of safety. This isn’t to say that states that haven’t adhered to a timely code adoption process are not addressing safety. It simply means that you may have difficulties discerning this.
Tribal reservations are also unique in this regard. They are sovereign nations, and advancing code language can be different between each nation. Therefore, harmonizing the NEC with tribal codes can present significant challenges.
With that in mind, recent events may offer more consistent solutions to this dilemma. On October 19, 2024, at the International Code Council (ICC) Board of Directors meeting in Long Beach, California, the Board approved the establishment of the Native American Code Officials (NACO) as the first Code Council chapter dedicated specifically to Native code officials. Construction requirements and approaches differ significantly among the 674 federally recognized tribes and over the 100 state-recognized tribes across the United States. (Compliments to the ICC and the NACO for all their efforts.)
So that’s it? Our lack of knowledge on this subject is simply a product of geography? Are you kidding me? We need to be honest about this subject. It’s an energy hazard. And, historically, the national fire service has not made energy hazard training a priority. This must be rectified.
Where Do We Go from Here?
With all the expectations of a modern fire crew these days, it’s no wonder that we are all feeling like our plates are full. A word to the wise: The energy hazard landscape for our profession is evolving at the speed of light. And your crews are already dealing with this challenge. Now they can continue to rely on anecdotal stories of success or failures, or our leadership can be proactive and embrace an anticipatory ethos that seeks out real energy hazard training for the fire service.
Authors’ note: Thanks to Matthew Paiss, Robert J. Davidson, Chris Towski, and Pete Jackson for their help with this article.
CHRIS G. GREENE is a captain (ret.) with the Seattle (WA) Fire Department and a national speaker on energy response hazards. He is the creator of Seattle Fire’s Energy Response Team and assisted in designing its “Energy One” response apparatus. He is a contributing author to Fire Engineering for energy emergencies and creator of the “Lithium-Ion Revolution” teaching platform. He was the 2017 Seattle Fire Officer of the Year and keynote speaker at the Washington State Energy Hazards and Lithium-Ion Battery Symposium. Greene is a technical panel member for UL-FSRI’s Safety of Batteries and Electric Vehicles. He also represents the IAFF on the following NFPA standards committees: NFPA 30A, 850, and 12 and the NFPA 800 “proposed” Battery Safety Standards Committee.
TONY GRANATO is a 31-year veteran of the fire service. Prior to his retirement, he spent most of his career with the Manchester (CT) Fire Rescue EMS Department. He is president/owner of Energy Response Solutions LLC. In addition to serving as a Connecticut fire instructor with more than 14 years of teaching experience, he has codeveloped and taught three PV safety programs to firefighters nationwide since 2013. He is an active member of UL 1741 and 3741 standard technical panels; a licensed Connecticut E2 journeyman electrician since 1989; and the primary representative for the IAFF on NFPA 70 (code-making panel 4), which primarily deals with article 690, solar PV systems.
