Rapid shutdown devices (RSDs) are designed to protect firefighters responding to fires in buildings with rooftop PV arrays. Standard practice for firefighters is to disconnect the building from utility power to prevent electric shock from exposed, energized wires. However, rooftop PV systems complicate this safety protocol, as the system’s DC conductors remain energized whenever sunlight is on the array.
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"Rapid shutdown is an electrical safety requirement set for solar panel systems by the National Electrical Code (NEC). Simply put, it provides a way to quickly de-energize a rooftop solar panel system."
What are Rapid Shutdown Devices?
Rapid shutdown requirements were first introduced during the 2014 revision of the National Electrical Code (NEC) with the addition of article 690.12. Initially, this article required that all “controlled conductors” more than 10 feet from a PV array be reduced to less than 30 volts within ten seconds of rapid shutdown initiation via listed and identified rapid shutdown equipment.
These new requirements effectively left installers with two ways to achieve compliance. One option was the installation of DC relays that would disconnect all system DC conductors more than 10 feet from the array upon the actuation of a switch located on the exterior of the building. The other option was a system architecture based around module-level power electronics (MLPE). As the name implies, these are small electronic interfacing devices connected in series with either individual modules or pairs of modules and then in parallel with each other to form either AC or DC strings.
These vary from relatively simple devices whose sole function is to internally lower their output voltages to below 30 volts whenever they sense the disconnection of utility power, to more complex devices such as DC power optimizers and AC microinverters that serve many additional functions, including module-level power point tracking, production monitoring, remote troubleshooting, and more.
In practice, most opted for the adoption of MLPE since it did not require additional wire runs for the DC contactors and therefore seemed like a simpler installation. Additionally, power optimizers or microinverters came with the added benefits mentioned above. However, as will be discussed in the next section, MLPE’s apparent simplicity belied a hidden safety risk.
Why do RSDs Fail?
Even the simplest MLPE are extremely complex electronic devices compared to PV modules. Their internal circuitry contains dozens, if not hundreds, of components that can overheat and fail, especially considering the harsh conditions in which they operate, including extreme hot and cold temperatures, daily thermal cycling, routine exposure to rain, snow, and humidity, and intrusion by birds, squirrels, or insects. Furthermore, the failure of an MLPE can easily cascade into the failure of the module it controls or other MLPE on the string. Despite this, failure of the MLPE itself is not the most pressing danger associated with them.
PV systems that rely on MLPE to achieve rapid shutdown inevitably involve close to double or even triple the number of wire connections compared to a traditional DC string. This is because each MLPE must be connected to the module(s) it controls as well as to the DC or AC string of which it is a part. Inevitably, each additional connection point becomes another potential point of failure. This means that even a perfect installation will have effectively double the chance of a thermal failure occurring on the roof compared to a string without MLPE.
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Of course, in reality, no installation is ever perfect. According to most manufacturers’ instructions, installers are supposed to use connection devices and procedures that are typically highly specific to certain manufacturers or even certain models from the same manufacturer. In practice, the correct connectors are not always available, and they may be installed by inexperienced installers working against strict completion deadlines or simply trying to get off a sunny roof before the heat index rises above 100°F in the summer.
As a result, many installers are forced to improvise connections using incompatible connectors or field-fabricated jumpers. Although these connections may appear to function perfectly well at initial system startup, they will have a higher chance of overheating and degrading over time, slowly getting worse and potentially cascading beyond the original failure. If the system is not regularly inspected for these kinds of failures, as is the case with most small residential and even many medium-sized commercial systems, this creates a dangerous situation on these rooftops.
Understanding the NEC and Rapid Shutdown: Cycles of Change
Before considering potential solutions to the problem of overheating RSDs, it is worth noting how fluid the regulatory situation surrounding them is. Although PV systems have been installed on rooftops for decades, it was only in the 2014 edition of the NEC that rapid shutdown requirements were included. Each subsequent three-year NEC revision cycle has introduced significant changes to these requirements. Notably, the 2017 revision effectively reduced the “array boundary” (beyond which all conductors needed to be shut down) from 10 feet to 1 foot, further pushing the industry towards the adoption of MLPE-based solutions.
The complexity of keeping track of and complying with these changing requirements is further increased by the fact that the publication of a new edition of the NEC by the code-making panels is not synonymous or concurrent with its adoption into law across all or even most jurisdictions.
In practice, passing legislation to adopt the new code into law takes time, and individual states typically adopt the new standards on radically different timelines. As of March 2024, the 2023 NEC was in effect in eight states; the 2020 NEC was in effect in 27 states; the 2017 NEC was in effect in nine states; and the 2008 NEC was in effect in two states, with the remaining four states leaving the decision to individual counties or municipalities.
Furthermore, many state or local governments may adopt various NEC “amendments” or develop their own unique interpretations, leaving solar contractors operating across different jurisdictions to face a patchwork of requirements.
While none of this relieves solar contractors of their obligation to take the potential risks of overheating RSDs seriously, it does mean that their approach, above all, requires humility, flexibility, and a consistent willingness to reevaluate their work before hidden problems catastrophically emerge.
Potential Solutions and Ongoing Challenges
Since one of the primary ways that RSDs fail is at the connection points, installers need to continuously re-emphasize quality training of field installers on connection procedures and conduct thorough inspections of connections during commissioning to ensure they are made in accordance with manufacturers’ instructions.
Realistically, though, it will be difficult for any installer to ensure that 100% of connections are made perfectly. Even if they could, the sheer number of additional connections and components involved means the probability of failure can never be reduced to zero. As a result, the strategy of any installer or O&M contractor operating rooftop PV systems using RSDs or MLPE needs to include a plan to proactively inspect both old and new systems for signs of overheating devices or connections and to repair or replace them before they catastrophically fail.
While this may be more readily apparent to operators of commercial rooftop systems, who are more accustomed to planning and budgeting for O&M, it is arguably no less important for owners of smaller residential rooftop systems, which are more likely to be installed on roofs made of flammable asphalt shingles.
Globally, firefighters participate in an average of 2 solar-related fires per week. Even when the system is turned off, the solar panel still outputs power at high voltage. That’s why every solar PV roof should add a module-level rapid shutdown device to keep firefighters and homeowners safe.
Although there are different ways that an RSD or its connectors can fail, all of them ultimately produce thermal events as degraded electrical components overheat and degrade further. This sets off a vicious cycle that can ultimately result in dangerous electrical arcing if the heat builds up to a point where electrical insulating materials start to melt. While this highlights the potential dangers posed by overheating RSDs, it also underscores the potential role of thermography (especially drone-based thermography) in detecting and addressing them.
How Drone Thermography Identifies Overheating RSDs
RSDs are typically installed in direct contact with the underside of the PV module. This means that as they begin overheating, due to either their internal components or their connectors failing, a heat transfer effect occurs through the surface glass of the module. As a result, spotting an overheating RSD is possible when performing an aerial IR scan.
This approach is more favorable when considering how time-consuming it is to perform a visual or handheld thermographic inspection of every single RSD unit, as large-scale commercial rooftops could contain hundreds or even thousands of them. Additionally, technicians often face difficulties accessing every RSD due to pitched rooftops, little to no walking space between rows, or wind deflectors blocking the view of the underside of modules.
In comparison, a drone can perform a low-level flyby of dozens of modules within seconds, allowing technicians to remain on the ground and watch a live feed for any hotspots or other thermal anomalies that could indicate an overheating RSD (as well as many other common problems with PV modules). Technicians can then flag those specific modules for further visual inspection and repair or replace them as necessary before a dangerous electrical event occurs.
Getting Started with Rooftop Fire Prevention and RSDs
Preventing rooftop fires is a critical concern for operators and maintainers of rooftop PV systems using RSDs. The Drone Life specializes in detecting overheating RSDs through advanced drone-based thermal imaging, ensuring the integrity and safety of your systems. With the ever-present risks and complexities associated with solar systems, it is essential to leverage the expertise of experienced drone service providers.
At The Drone Life, we offer personalized consulting and customized aerial service solutions tailored to your specific needs. Our team of professionals is dedicated to helping you mitigate fire risks and maintain the safety of your rooftop PV systems. Schedule a free consultation with one of our drone experts today to discuss how our services can help you get started on a proactive fire prevention strategy.
What's included in YOUR GUIDE...
Techniques for Exposing Warrantiable Items on Your PV Sites.
A Proven Roadmap for Successfully Implementing Drones in Solar.
Strategies for Identifying DC Losses and Avoiding Downtime.
