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The rapid shutdown requirements for photovoltaic (PV) systems, introduced in the codebook aim to enhance firefighter safety by reducing the voltage of PV conductors in case of emergencies. Initially, module-level power electronics (MLPEs) were the primary solution, but the introduction of UL3741 in 2020 offered an alternative. UL3741 defines hazard control measures and provides compliance pathways, allowing some systems to bypass the need for MLPEs. The standard, developed through empirical testing, addresses firefighter safety by defining shock hazards based on voltage, current, and resistance data
The rapid shutdown requirement for PV systems was first introduced in the 2014 to address safety concerns, particularly for firefighters interacting with PV systems during emergencies. The code focuses on controlling conductors within the array boundary to minimize shock hazards. The most common method of achieving this has been through installing MLPEs, which reduce conductor voltage within the boundary to less than 80 V within 30 seconds of shutdown initiation.
The next revision of the codebook provided a delayed provision to allow systems “listed or field labeled as a rapid shutdown PV array” to meet the requirements, anticipating the creation of new solutions. This led to the development of UL3741, a standard introduced in 2020 for Photovoltaic Hazard Control, offering an alternative compliance method to using MLPEs. The introduction of UL3741 gave the PV industry a broader set of options for meeting rapid shutdown requirements.
UL3741 aims to mitigate shock hazards that firefighters might face when interacting with PV systems. The standard was developed through the collaboration of industry stakeholders, Sandia National Laboratories, and the US Department of Energy, focusing on defining the risks firefighters encounter during PV system shutdowns. By assessing various conditions, such as the electrical resistance of firefighter safety gear, body resistance, and current pathways, UL3741 establishes a test method to ensure that PV systems do not expose firefighters to dangerous shock hazards.
The application of UL3741 within PV systems is evolving fast, with different products currently available that meet the standard. Some MLPE manufacturers and racking system manufacturers have designed products that comply with UL3741, allowing series string configurations without exceeding the 80 V threshold. These systems provide additional options for PV designers and installers, especially for rooftop installations, where MLPEs have traditionally been used to meet these safety requirements.
UL3741 introduces the term “PV Hazard Control System (PVHCS),” which is referenced in the 2023 version of the codebook as well as a method to control shock hazards within the array boundary. PVHCSs are specifically designed to protect firefighters and meet the safety requirements by controlling the voltage of conductors within the array.
A key challenge in applying UL3741 is understanding the critical components required for compliance. Installation requirements are defined by the manufacturer and may include specific accessories, wire management solutions, and racking systems. Manufacturers must list all the critical components that were part of the system when it was tested and certified to meet UL3741. Ensuring that these components are used correctly is essential to maintaining compliance with the standard.
Certification agencies such as UL and Intertek provide testing results and certificates to manufacturers, detailing the components necessary for compliance. These certificates offer valuable guidance for installers, outlining the specific materials and installation methods required for the system to meet UL3741 standards.
UL3741 defines various hazard levels to assess firefighter exposure to shock hazards. Hazard Level 1, for example, limits direct current exposure to prevent strong involuntary muscular reactions. If a system meets this level, no additional protection is required. However, if a system presents a higher hazard level, additional protective measures, such as specific wire guarding and racking systems, must be implemented to comply with the standard.
Manufacturers can test their systems against these hazard levels, and some have developed solutions that comply with UL3741 without the need for MLPEs. This flexibility allows manufacturers to choose different pathways to meet the standard, depending on the system design and installation requirements.
UL3741 provides an innovative solution to the challenges posed by rapid shutdown requirements for PV systems. By focusing on firefighter safety and offering alternative pathways to compliance, UL3741 allows PV designers and installers to explore new system configurations that were previously restricted by code. As the industry continues to adapt to this standard, education and patience will be essential for gaining acceptance among jurisdictions and code officials. Overall, the introduction of UL3741 represents a significant step forward in ensuring the safety of PV systems while providing more flexibility and cost-efficiency by omitting MLPE products in projects.
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