Module-Level Rapid Shutdown for Commercial Applications: Page 6 of 6
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In addition to testing equipment such as firefighter gloves and boots for their insulating properties, the research engineers also sought to define safe working distances between water hoses and live electrical equipment. These tests indicate, for example, that firefighters can eliminate hose stream shock hazard by working at a distance of 15 feet from a 600 Vdc power source or 20 feet from a 1,000 Vdc source. Alternately, firefighters can reduce the measured current to below the level of perceptibility by changing the hose stream from a solid stream to a 10° cone pattern. Other tests confirmed that tarps are not reliably effective as a means of de-energizing a PV array, that light striking a PV array from a fire or a fire truck is sufficient to pose an electrical hazard, and that cutting into PV modules or source circuits is a bad idea. This is all very practical information for firefighter training purposes.
Perhaps the most important UL fire test results are those showing that damaged PV arrays are inherently hazardous. For these experiments, researchers installed test arrays on a wood truss roof, ignited a fuel load inside the structure and then let the fires burn uncontrolled until the roof collapsed. The post-fire analyses revealed that while some portions of the arrays were completely destroyed and produced no power, other significantly damaged areas still produced partial or even full power. Based on these findings, the report concludes, among other things: “Severely damaged PV arrays are capable of producing hazardous conditions ranging from perception [of current] to electrocution. Damage to the array can create new and unexpected circuit paths.”
Unless follow-up fire research shows otherwise, it would be irresponsible to bet any lives on the premise that the presence of MLPE would change these findings in any meaningful way. According to UL standards, the safe voltage level in wet conditions is 30 V. In the absence of cell string–level disconnects, most PV modules are capable of putting out more than 30 V under normal operating conditions. Since module-level or cell string–level rapid shutdown does not change the inherent properties of PV cells, it is prudent for emergency responders to assume that a fire-damaged array presents a shock hazard due to the potential for inadvertent and unexpected circuit paths.
Engineering evaluation. The authors of DNV GL’s 2015 advisory, “Rooftop PV Systems and Firefighter Safety,” start by reviewing relevant literature, such as UL’s 2011 fire research findings and a joint PV and fire industry study conducted in Germany. Interestingly, the outcome of the joint industry analysis in Germany, the country with the largest number of rooftop PV installations in the world, was a set of firefighting guidelines that emphasize safe boundaries and tactics. Because module-level technologies and standards are not sufficiently established and have yet to prove their reliability, the German report advises against a MLPE mandate.
After a literature review, DNV GL researchers conducted firefighter interviews and surveys to explore the “procedures, issues and decisions that firefighters face when carrying out operations at a building that has rooftop PV.” The interviews indicate that firefighters welcome the improved setback and pathway requirements in the 2015 fire codes, but still see some room for improvement. With a building-specific approach to pathway layout, for example, AHJs could intentionally align access pathways with the best trench-cut locations for firefighters.
The surveys, meanwhile, indicate that firefighters are indeed very concerned about the inability to eliminate or significantly reduce shock hazard in the PV array. In the short term, they need to be able to identify energized versus de-energized components. On the face of it, this sounds like a collective vote in favor of module-level rapid shutdown. However, the vast majority of interviewees—and all of those in leadership roles—indicated that they would never directly engage with or remove damaged modules for roof ventilation.
According to the report: “Respondents expressed the desire for rapid-shutdown functions to work under damaged conditions, but none expected that they would. All would treat damaged arrays as energized.” The authors later conclude: “The ability to further de-energize circuits within the array is seen as a key to reducing the risk of accidental shock, but not as a rationale for intentional interaction. The real value of enhanced electrical protection is in its impact on decision making, enabling firefighters to carry out and improve operations more confidently.”
Researchers at DNV GL used an engineering evaluation methodology, known as a failure mode and effects analysis, to estimate the risks associated with different electrical hazard mitigation approaches. This methodology accounts for circumstances such as the severity of impact and the likelihood of occurrence and detection. The researchers then characterized the risks associated with different applications and scenarios (residential, normal operation; residential, single fault; commercial, normal operation; and commercial, single fault) in the context of different product topologies or design decisions. These probability-weighted results indicated that there are multiple acceptable risk mitigation options, including module-level shutdown with an 80 V limit, as well as “combinations of 1- and 2-pole string level disconnection, access-limited conductors [and] mechanically protected conductors.” All of these approaches “scored similarly as effective means to reduce the shock hazard within arrays.”
Rapid shutdown array standard. The DNV GL advisory largely supports SEIA’s contention that a prudent approach would be to develop a product standard for PV Equipment Safe for Proximity Firefighting. According to their public comments, some members of CMP-4 believe that rapid-shutdown PV array and PV equipment safe for proximity firefighting can mean the same thing. The safety standard is a work in progress, they suggest, and a rose by any other name would smell the same. In this case, however, the meaning of these words could be a matter of grave import.
To the uninitiated firefighter, equipment safe for proximity firefighting signals: “You can get close to this equipment, but not too close. Please do not touch.” In contrast, module-level rapid shutdown sends a misleading message: “I am now off.” While a touch-safe PV array is undoubtedly the long-term goal, we do not yet have a product safety standard that can render a damaged PV array safe for firefighters. Granted, we can make the roof safer with the touch of a button, but that does not mean the power is off.
Not surprisingly, UL’s representative on CMP-4, Timothy Zgonena, is going into the standards development process with his eyes wide open. Regarding his affirmative vote for the compromise NEC 2017 rapid-shutdown language, Zgonena comments: “UL understands the desired intention of the 80 V limit to reduce shock hazards. Unfortunately, 80 V can be a lethal electric shock hazard in this application. Further, it would be most appropriate to use a listed system consistent with the concept of 690.4(B) to limit the voltage, rather than some assemblage of equipment not specifically listed as a system. UL firmly believes that PV rapid-shutdown equipment specifically listed for this intended purpose is the best solution. We have made good progress since the first revision of 690.12 for the 2017 NEC. UL understands and supports the development of a science-based solution as the basis for the upcoming standard.”
David Brearley / SolarPro / Ashland, OR / solarprofessional.com
Backstrom, Robert, and David Dini, “Firefighter Safety and Photovoltaic Installations Research Project,” UL Report, November 2011
DNV GL, “Rooftop PV Systems and Firefighter Safety,” DNV GL Renewables Advisory, October 2015