Meet UL 3741: How this C&I rooftop PV system complies with NEC 690 without MLPE or switches

SMA and Sollega UL 3741

You’ve likely forgotten this, but there are three ways to meet National Electric Code (NEC) 690.12 B[2] — the infamous rule regarding rapid system shutdown inside the PV array boundary.

  • Listed No. 3 under 690.12 B[2] is building-integrated PV (not relevant for typical PV module installs).
    Listed No. 2 is shutting system voltage to less than 80 V in less than 30 seconds within the array boundary, which translates to using module-level power electronics (MLPE), either microinverters, optimizers or standalone RSS devices.
  • Listed No. 1 is using a UL 3741 certified PV Hazard Control System (PVHCS).

Going with option No. 2 and using MLPE to drop voltage has become the default path to compliance for many since NEC 2017. This is due in part to a big head start: The standard for UL 3741 wasn’t actually published until December 2020.

Regardless, I was skeptical UL 3741 would be much of a choice. I wrote as much on solarbuildermag.com. My reasoning was based on conversations with various manufacturers who noted that certification (which is costly and time-consuming, btw) would still require devices of some kind (like mid-circuit interrupters) within the array to keep voltage low. If UL 3741 wasn’t going to reduce part counts or simplify systems, then “what was the point?” I mused.

Well, a system is debuting this year that proves me wrong. It is a UL 3741 PVHCS promising a simple string inverter design, with no MLPE or switches. Elie Rothschild, CEO of Sollega, a commercial racking manufacturer, was the first to alert me.

“Given the nature of how these systems come together and are certified, the mounting system actually plays a crucial role in the overall safety of the PV array,” he said, noting the Sollega system is made of a polymer (glass reinforced nylon) and therefore non-conductive.

Indeed, the first racking and inverter pairing to attempt and make it through UL testing together — without MLPE or switches — is Sollega and SMA America. The information I had previously wasn’t necessarily incorrect, but Sollega was willing to dedicate the resources to undergo testing and prove the concept with SMA, even though no one knew exactly what that safety voltage threshold would be.

“And we found that 1,000 volts within the array boundary was possible,” says Blair Reynolds, product manager with SMA America. “That was not necessarily our expectation going into the project, but it is an outcome that allows us to use the SMA CORE1 inverter itself as the means for controlling conductors leaving the array boundary — without necessarily needing any other switches in the system.”

The SMA + Sollega system is officially UL3741 PVHCS certified. In this mea culpa, we’ll look at the UL 3741 testing process, how this new system passed, how it is very likely the safest commercial and industrial (C&I) PV design ever, and how impactful that could be for your C&I solar installation strategy.

This article originally appeared in the Q1 issue of Solar Builder magazine. Get your free subscription, print or digital (or both) right here.

The path back to string

The UL 3741 standard evaluates for specific, defined, abnormal conditions and fault tolerances related to anticipated firefighter operations that exceed the criteria of existing product safety standards. The testing takes an FMEA [Failure Mode & Effects Analysis] approach, using a risk matrix on which to evaluate the outcomes of every potential scenario that could occur on a roof involving a person falling (or hacking) into the PV array.

“The testing was focused on answering, ‘what if you have a first responder fall on a solar array?’ under a specific set of conditions, from best to worst case scenarios,” Rothschild says. “We needed to test in those scenarios to determine the probability of them getting electrified, down to if they have a specific tool in their tool belt or specific clothing, or if there are wet conditions and they fall into a panel and are grabbing another panel in another row to potentially create a circuit, and so on.”

NEC 690.12 defines thresholds of safety based on voltage levels, but those are fairly arbitrary numbers because high voltage in an array is not dangerous in itself. UL 3741 takes this into account in its hazard analysis:

  • Limiting voltage is one means to reduce electric shock hazards.
  • Other safeguards can reliably reduce shock hazards.
  • Hazard analysis recognizes multiple levels of safeguards as protecting firefighters from hazardous currents.
  • Evaluation is based on a defined set of firefighter interactions and conditions.

Modern PV systems already start with two inherent safety features to pass such a battery of tests:

  • Transformerless inverters have daily insulation resistance measurement and continuous residual current detection that detect ground faults immediately.
  • Transformerless PV string inverters isolate the PV array from ground reference after disconnection from AC.

“For someone to be at a safety risk, they’d have to not only come into contact with live current but also a path to ground,” Reynolds says. “They have to put themselves in that circuit, in between live current and path to ground. Touching a live conductor on its own is not a safety risk; it’s becoming part of the current path that creates the danger.”

After conducting the empirical testing for safe interactions with firefighters, SMA realized that the wire management technique is really the most important area to focus on in order to design a simple and repeatable process for ensuring a safe installation.

“Reliable wire management is what enabled this technique to work,” Reynolds says. “Ensuring the cables are unlikely to be damaged greatly reduces the likelihood of a ground fault, which greatly reduces the likelihood a firefighter would come into contact with a live current. It would take these multiple failures to occur for a firefighter to even be subject to an electrical safety issue.”

A gain of inches

Sollega’s unique racking checked a few other crucial PVHCS boxes:

  • Sollega’s system is glass reinforced nylon 6 (Class A fire rated), so it reduces conductive risk. “Being non-conductive, as we get into higher voltages on the roof, will play a more important role in rooftop installations,” Rothschild says.
  • Its integrated wire management helps ensure that there is always an “insulating air gap” between the conductors and metallic parts within the PV array.
  • Sollega’s racking extends about 18 in. beyond the panel’s edge on the north and south rows, which allows for wiggle room when placing a string inverter inside the “array boundary.”

On that last point, let’s go back to the NEC definition of an array: “A mechanically integrated assembly of modules or panels with a support structure and foundation, tracker and other components as required, to form a dc or ac power producing unit.”

So, the 1-ft array boundary perimeter starts where Sollega’s racking system ends, which is probably 8 or so inches beyond a conventional racking system.

Add it all up on that FMEA risk matrix, and SMA + Sollega passed UL 3741 up to 1,000 V within the array boundary, without needing mid-circuit interrupters to decrease voltages within the array boundary during an emergency.

Sunny Tripower CORE1 spotlight:

SMA CORE1

The Sunny Tripower CORE1 offers an intelligent IV curve diagnostic, advanced string monitoring and SMA Smart Connected. The world’s first free-standing PV inverter for commercial rooftops, carports, ground mount and repowering legacy solar projects, the Sunny Tripower CORE1 enables logistical, labor and service cost reductions. No additional racking required for rooftop installation. Integrated SunSpec PLC signal for module-level rapid shutdown compliance to 2017 NEC. ShadeFix optimization generates power comparable to traditional DC optimizers

Why should you care?

A system that meets UL 3741 without MLPE or mid-circuit interrupters of any kind means potentially removing thousands of dollars from a large flat-roof job (about 7 and 9 cents per watt cost savings by removing module-level switches). There is also a reliability and safety benefit that comes with reducing the number of connectors within the array by a factor of three.

The remaining challenge after all of the I’s are dotted on the PVHCS rollout will be convincing authorities having jurisdiction (AHJs) that what’s old is new and compliant again. But if AHJ’s are truly concerned with PV system and firefighter safety, then they better get up to speed because UL 3741 PV Hazard Control Systems are tested well beyond typical PV safety standards.

“Not having to make jumpers, avoid incompatibility with connectors or any issues with more connectors and points of failure — it’s an all-around a better solution,” Rothschild says.

And now that SMA has proven it, one would think other C&I string inverter manufacturers will try and follow, allowing for more installer choice.

“The solar industry is long overdue for some alternative options to meet code compliance,” Reynolds says. “This is a validation that well-engineered, installed and maintained PV systems are fundamentally safe. The assumptions that led us to an electrical code that implies that 80 Vdc is what constitutes the threshold of safety are deeply flawed. UL 3741 now provides a much more reasonable path for code compliance by actually testing the equipment for safe interaction with firefighters.”


Chris Crowell is the editor of Solar Builder.

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  • Chris
    May 9, 2022 10:50:36

    Seems like a great solution!