UL 37.41: An Alternative to Module-Level Shutdown

    Firefighter entering a house with solar panels in an emergency situation

    Who are the NFPA and UL?

    The majority of US solar installers and electricians are familiar with The National Electric Code (NEC). It is published every three years by the ​​National Fire Protection Association (NFPA). The NEC handbook aims to prevent electrical fires by fostering clarity and safety to national electrical standards. Another safety standard for the solar industry is Underwriters Laboratories Inc. (UL), which has more than 1,000 safety standards including standards for PV-related products. UL’s standards are in place to ensure public safety, reduce costs, and improve the quality of products and services. Both NFPA’s NEC and UL’s standards are in place for safety reasons, to ensure the protection of home and business owners, first responders, and improve the U.S. solar market.    


    What are module-level shutdowns and UL 37.41’s Standards? 

    The NEC 2017 publication is important for the U.S. solar industry because of its updates to the rapid shutdown requirements. A rapid shutdown is when all conductors within an array’s one-foot boundary have to be reduced to 80 V or less within 30 seconds upon initiation. This requirement was added to the NEC handbook to protect first responders from potential shock hazards transmitted from the solar array while entering a building in an emergency situation. NEC’s 2017 module-level shutdown requirements were rapidly implemented after it was published, and remain in effect today in many states. 

    UL took the 2017 NEC update seriously and started working on a UL Standard for fire safety, this standard is 37.41. According to UL’s Website, “UL 37.41 is intended to establish and expand requirements for the evaluation of a module rapid-shutdown that can keep firefighters out of hazardous current paths while responding to emergency situations in homes and buildings with PV systems”. This UL Standard was later added to the NEC 2020 publication in section 690.12(B)(2). This offered an alternative for meeting the original NEC rapid shutdown requirements.    


    UL 37.41 as an alternative to module-level shutdown and what this means for contractors

    UL 37.41 is an expansion of the NEC’s module-level shutdown requirements. The addition now allows installers to use UL 37.41-compliant hazard control systems, which are UL-listed to meet rapid shutdown requirements. NEC’s hazard controls systems list added UL 37.41 compliant systems which expanded the options of solar equipment that can be used to meet the rapid shut down requirements. This 2020 NEC update was adopted in a few states across the U.S. when first published, and is continuing to become accepted in additional states. 


    UL 37.41 was created to be an alternative to module-level rapid shutdown requirements and became an extension of it during the 2020 NEC publication. Multiple states are continuing to adopt the 2020 NEC. The rapid shutdown requirements are in place to ensure the safety of first responders and all that will come in contact with the PV system to make these already-safe solar systems even safer. With the inclusion of UL 37.41, contractors can present more product options to home and business owners looking to add solar to ensure the array meets safety requirements.

    4 months 1 week ago
    Written by
    Ally Sharrock
    Support topic
    NEC code
    Ground mount
    Wiring and circuits
    Support keywords
    rapid shutdown
    fire code permitting
    Electric Code
    fire code
    solar array