The Unintended Safety Risks of the NEC
The National Electrical Code (NEC) has always had a strong impact on the solar industry and how solar systems are built. Examples are the Arc Fault Current Interruption (AFCI) requirement, which was introduced in the National Electrical Code of 2011, or 'rapid shutdown' in NEC 2014, or 'module level shutdown' in 2017. What are the drivers for these code changes and do they always have the intended outcomes?
As Californian AHJs are underway to adapt to the NEC 2017, the new requirements for Module Level Shutdown (NEC Art. 690.12 to be specific) are among the most discussed and most controversial topics in this version of the code. Obviously, the main driver for code changes is to increase safety of systems in general and safety for firefighters specifically. Safety is the number one priority of the NAFPA and the NEC Committee.
Considering the new code's rapid shutdown requirements, safety in general may not be improved but actually diminished. Today's market solutions actually increase risk instead of reducing it. That's an outcome which code authors certainly did not want and unfortunately did not foresee. Why is it that today's solutions increase fire and safety risks? There are two major areas of risk in the NEC 2017.
The first and most important area of risk is that all solutions available today are based on traditional Module Level Power Electronics (MLPE), which increases the number of connection points by a factor of 2-3 in a typical residential system. By doing so, the number of potential failures or human errors increases by the same factor - inherently, this significantly increases the risk of thermal events happening. Based on studies, the risks of solar PV are, if any, bad installation practices and connection points (e.g. loose connections, improper crimping, or connector mismatch).
The 2020 version of the NEC recognizes the need for a standard to define the compatibility of mating connectors from different manufacturers. Recent news reports of solar systems catching fire are related to connector issues. The 2017 NEC unfortunately never defined such a safe standard, and yet obligates installers to increase the number of connection points (and their risks) by a factor 2-3 times.
Another risk factor of MLPE based solutions is software switches. There are two ways to process a shutdown, one is via a hardware switch (such as a dedicated chip) or a software routine (running on a processor). Hardware switches are deemed safer, because the software could be locked in a position where the module cannot switch OFF. The only way to guarantee a software to be safety-reliable is to develop it according to SIL (“Safety Integrity Level”) rules as it is done in car and plane applications. This is, however, currently not the case with MLPE solutions.
Since optimizers or micro inverters have no DC disconnects per definition, they do not have to be certified to UL 98B as with DC disconnect. Therefore, the industry relies on "safety switches" whose reliability and safety is only based on manufacturers' claims - without any UL certification. It is a fact that switches react to continually elevated ambient temperatures with accelerated contact degradation - especially on the roof. Switches should be inspected and maintained once a year in order to discover any signs of overheating. This also includes modules with integrated MLPE. Unfortunately, these standards do not exist.
While the code does not specifically require module-level shutdown via MLPE, solar installers do not have other choices. The industry is simply not ready to offer safer solutions today. The code authors probably foresaw the challenge for the industry by not requiring UL 690.12 before 2019. However, going into 2020, there is still no other (safer) alternative available. Installers have no choice other than building systems that are more complex, more costly, and less safe.
The last paragraph of section 90.4 in the code allows AHJs to "permit the use of products, constructions, or materials that comply with the most recent previous edition of this Code", if solutions are not available when the code is adopted. Due to the lack of safe solutions available, this might be the right thing to do at this point.
Given the fact that safety is the number one concern, the solar industry cannot adopt flawed code requirements that escalate weak points of systems and that force installers to take on these risks. The only feasible way to fulfill the code requirement AND get to the intended outcome of more safety is for the solar industry to offer module-integrated solutions that do not increase the number of connection points and that are based on an open industry standard to ensure highest safety. The SunSpec Communication Signal for Rapid Shutdown can be this industry standard.
To summarize, the two main areas of risk in the NEC 2017 are the sharp increase in connection points and the lack of a safety standard for switches on the roof. Today’s solutions on the market do not mitigate these risks. Until safe solutions are available, Authority Having Jurisdictions (AHJs) should ask themselves if the adoption and execution of NEC 690.12 Rapid Shutdown of PV Systems on Buildings should limit designs to module-level solutions - and if that's really the right thing to do for safety.