SOLAR ENERGY
How to Ensure Safety Under UL 3741 Understanding the risks and limitations of solutions without DC optimization by Jason Bobruck, Bill Brooks, and Kleber Fachini
DC-optimized systems allow for two panels to be connected in series, meaning that, upon rapid shutdown initiation, the maximum voltage within the array is 125VDC
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he introduction of the UL 3741 Standard for Photovoltaic Hazard Control in 2020 was hailed as an important milestone in the solar industry’s continuing drive to improve solar system safety. Building on rapid shutdown best practice first introduced in NEC 2014, it was conceived to equip the solar industry with a broader array of rapid shutdown options in PV arrays, with the primary objective being to protect firefighters when dealing with fires in buildings with solar installations. In this respect, UL 3741 has achieved notable success. However, concerns are arising regarding the safety of some of the PV hazard control solutions that have entered the market since the regulation’s introduction.
To mitigate this increased risk, UL 3741 states that solar arrays need to be made safe and not allow any exposure to dangerous voltages during firefighter interaction. This means that every wire, module, or energized component in the array that can be accessed during firefighter operations must be protected from damage using robust wire protection measures i.e., sturdy guards and conduits, etc. Furthermore, regular inspections are required to ensure that all protective elements and wire management components must provide this level of protection throughout the project lifespan — in other words, whatever means of protection that is used in the beginning of the project will need to be maintained for up to 25 years. Here is where some of these alternative solutions start to lose traction: not due Beyond compliance: The need for holistic safety approaches in to any flaws in the UL 3741 standard itself, but how it is being interpreted by some PV installations manufacturers and certification agencies. Prior to the introduction of UL 3741, the solar industry relied on MLPE (Module Level The biggest concern is non-DC-optimized systems (without integrated rapid Power Electronics) technologies, including DC-optimization, to provide rapid shutdown. shutdown) in which exposed, accessible live wires with voltage levels up to 1000VDC DC-optimized systems allow for two panels to be connected in series, meaning that, upon are held in place by nothing more than plastic cable ties or plastic raceways. Despite rapid shutdown initiation, the maximum voltage within the array is 125VDC. As a result, clearly being not compliant with UL 3741, these solutions have somehow slipped these systems fall within Hazard Level 1 (or lower) inside the array boundary, eliminating through the certification process. From day one, those exposed live wires may pose the need for additional protective measures to comply with UL 3741. a significant risk to firefighters. And that risk will only increase as aging and normal UL 3741 opened the door to allow solutions that use few if any MLPE devices weathering, including sun exposure and wind, potentially cause the plastic ties and (including DC-optimization) to be code compliant. In these solutions, AC conductors raceways to deteriorate. Once this happens, more sections of conductors may become are de-energized using traditional string inverters. However, the DC array itself exposed, accelerating the shock hazard risk to firefighters. Furthermore, the likelihood maintains high voltage levels (1000VDC) even after a rapid shutdown is initiated, of arc-faults and ground faults developing into fires is increased as the array will be corresponding to Hazard Level 3. maintained at 1000VDC, even during array or inverter faults. Not only does this put firefighters at risk, but operations & maintenance (O&M) and asset managers should be For most utility-scale solar facilities, the concerned that with some non-DCrising cost of vegetation management is optimized solutions, field crews will the top budget item. have to work around 1000VDC instead of Now there’s a solution to this growing Hazard Level 1 voltage levels. The hazards problem. The Renubot is designed by of an accidental short or broken module solar O&M professionals specifically can be exacerbated by the 1000VDC for vegetation management on these voltage level present in the array.
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2023 NOVEMBER • DECEMBER /// www.nacleanenergy.com
Assessing the true cost of compliance
There are several reasons why companies might choose to rely on a non-DCoptimized system without integrated rapid shutdown to perform this crucial function. The first and most common is that they may not adequately consider the wear and
