Meet UL 3741: How this C&I rooftop PV system complies with NEC 690 with out MLPE or switches

By Chris Crowell April 18, 2022 View Profile

You’ve probably forgotten this, however there are 3 ways to fulfill National Electric Code (NEC) 690.12 B[2] — the notorious rule concerning fast system shutdown contained in the PV array boundary.

  • Listed No. 3 beneath 690.12 B[2] is building-integrated PV (not related for typical PV module installs).
    Listed No. 2 is shutting system voltage to lower than 80 V in lower than 30 seconds inside the array boundary, which interprets to utilizing module-level power electronics (MLPE), both microinverters, optimizers or standalone RSS gadgets.
  • Listed No. 1 is utilizing a UL 3741 licensed PV Hazard Control System (PVHCS).

Going with possibility No. 2 and utilizing MLPE to drop voltage has change into the default path to compliance for a lot of since NEC 2017. This is due partly to an enormous head begin: The customary for UL 3741 wasn’t really revealed till December 2020.

Regardless, I used to be skeptical UL 3741 could be a lot of a alternative. I wrote as a lot on My reasoning was primarily based on conversations with numerous producers who famous that certification (which is expensive and time-consuming, btw) would nonetheless require gadgets of some form (like mid-circuit interrupters) inside the array to maintain voltage low. If UL 3741 wasn’t going to cut back half counts or simplify methods, then “what was the point?” I mused.

Well, a system is debuting this yr that proves me incorrect. It is a UL 3741 PVHCS promising a easy string inverter design, with no MLPE or switches. Elie Rothschild, CEO of Sollega, a industrial racking producer, was the primary 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 mentioned, noting the Sollega system is fabricated from a polymer (glass bolstered nylon) and due to this fact non-conductive.

Indeed, the primary racking and inverter pairing to try and make it by way of UL testing collectively — with out MLPE or switches — is Sollega and SMA America. The data I had beforehand wasn’t essentially incorrect, however Sollega was prepared to dedicate the sources to bear testing and show the idea with SMA, although nobody knew precisely what that security voltage threshold could be.

“And we found that 1,000 volts within the array boundary was possible,” says Blair Reynolds, product supervisor 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 formally UL3741 PVHCS licensed. In this mea culpa, we’ll have a look at the UL 3741 testing course of, how this new system handed, how it is vitally probably the most secure industrial and industrial (C&I) PV design ever, and the way impactful that may very well be to your C&I solar set up technique.

This article initially appeared within the Q1 situation of Solar Builder journal. Get your free subscription, print or digital (or each) proper right here.

The path again to string

The UL 3741 customary evaluates for particular, outlined, irregular circumstances and fault tolerances associated to anticipated firefighter operations that exceed the standards of current product security requirements. The testing takes an FMEA [Failure Mode & Effects Analysis] method, utilizing a threat matrix on which to guage the outcomes of each potential situation that might happen on a roof involving an individual 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 security primarily based on voltage ranges, however these are pretty arbitrary numbers as a result of excessive voltage in an array shouldn’t be harmful in itself. UL 3741 takes this under consideration in its hazard evaluation:

  • Limiting voltage is one means to cut back electrical shock hazards.
  • Other safeguards can reliably scale back shock hazards.
  • Hazard evaluation acknowledges a number of ranges of safeguards as defending firefighters from hazardous currents.
  • Evaluation relies on an outlined set of firefighter interactions and circumstances.

Modern PV methods already begin with two inherent security options to cross such a battery of checks:

  • Transformerless inverters have day by day insulation resistance measurement and steady residual present detection that detect floor faults instantly.
  • Transformerless PV string inverters isolate the PV array from floor 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 secure interactions with firefighters, SMA realized that the wire administration method is de facto crucial space to give attention to with a view to design a easy and repeatable course of for making certain a secure set up.

“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 acquire of inches

Sollega’s distinctive racking checked a couple of different essential PVHCS packing containers:

  • Sollega’s system is glass bolstered nylon 6 (Class A hearth rated), so it reduces conductive threat. “Being non-conductive, as we get into higher voltages on the roof, will play a more important role in rooftop installations,” Rothschild says.
  • Its built-in wire administration helps guarantee that there’s at all times an “insulating air gap” between the conductors and metallic elements inside the PV array.
  • Sollega’s racking extends about 18 in. past the panel’s edge on the north and south rows, which permits for wiggle room when putting a string inverter contained in the “array boundary.”

On that final level, let’s return 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 begins the place Sollega’s racking system ends, which might be 8 or so inches past a standard racking system.

Add all of it up on that FMEA threat matrix, and SMA + Sollega handed UL 3741 as much as 1,000 V inside the array boundary, while not having mid-circuit interrupters to lower voltages inside the array boundary throughout an emergency.

Sunny Tripower CORE1 highlight:


The Sunny Tripower CORE1 provides an clever IV curve diagnostic, superior string monitoring and SMA Smart Connected. The world’s first free-standing PV inverter for industrial rooftops, carports, floor mount and repowering legacy solar tasks, the Sunny Tripower CORE1 permits logistical, labor and repair value reductions. No extra racking required for rooftop set up. Integrated SunSpec PLC sign for module-level fast shutdown compliance to 2017 NEC. ShadeFix optimization generates power similar to conventional DC optimizers

Why do you have to care?

A system that meets UL 3741 with out MLPE or mid-circuit interrupters of any form means probably eradicating 1000’s of {dollars} from a big flat-roof job (about 7 and 9 cents per watt value financial savings by eradicating module-level switches). There can be a reliability and security profit that comes with lowering the variety of connectors inside the array by an element of three.

The remaining problem after all the I’s are dotted on the PVHCS rollout might be convincing authorities having jurisdiction (AHJs) that what’s previous is new and compliant once more. But if AHJ’s are really involved with PV system and firefighter security, then they higher stand up to hurry as a result of UL 3741 PV Hazard Control Systems are examined nicely past typical PV security requirements.

“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 confirmed it, one would suppose different C&I string inverter producers will try to comply with, permitting for extra installer alternative.

“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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Tags: industrial and industrial, NEC 2020, fast shutdown, SMA America, Sollega

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