UL 3741 Compliant: How This C&I Rooftop PV System Complies with NEC 690 Without MLPE or Switches

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You probably forgot, but there are three ways to meet National Electrical Code (NEC) 690.12 B[2] — the infamous rule concerning the rapid shutdown of the system within the limits of the photovoltaic generator.

  • Listed n° 3 under 690.12 B[2] is building integrated PV (not relevant for typical PV module installations).
    Number 2 in the list shuts down the system voltage to less than 80V in less than 30 seconds within the array limits, resulting in the use of module-level power electronics (MLPE), i.e. microinverters, optimizers or stand-alone RSS devices.
  • Number 1 on the list uses a UL 3741 Certified PV Hazard Control System (PVHCS).

Going with option #2 and using MLPE for voltage drop 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 n was only published in December 2020.

Anyway, I was skeptical UL 3741 would be a choice. I have written as much on solarbuildermag.com. My reasoning was based on conversations with various manufacturers who noted that certification (which is expensive and time consuming, btw) would still require some device (like mid-circuit switches) in the network to keep the voltage low. If UL 3741 wasn’t going to reduce parts count or simplify systems, then “what’s the point?” I am used to it.

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

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

Indeed, the first rack and inverter pairing to attempt to pass UL testing together — without MLPEs or switches — is Sollega and SMA America. The information I had before was not necessarily incorrect, but Sollega was willing to commit the resources to undergo testing and proof of concept with SMA, even though no one knew exactly what that safe voltage threshold would be.

“And we found that 1,000 volts at the grid boundary was possible,” says Blair Reynolds, product manager at SMA America. “This was not necessarily our expectation at the start of the project, but it is a result that allows us to use the SMA CORE1 inverter itself as a means of controlling conductors leaving the grid boundary – without necessarily needing other switches in the system.”

The Sollega SMA+ system is officially UL3741 PVHCS certified. In this mea culpa, we’ll look at the UL 3741 testing process, how this new system passed, why it’s quite possibly the safest commercial and industrial (C&I) PV design ever, and how point this could impact your solar installation C&I strategy.

This article originally appeared in issue 1 of Solar Builder magazine. Get your free subscription, paper or digital (or both) here.

The path back to the channel

UL 3741 evaluates specific, defined abnormal conditions and fault tolerances related to anticipated firefighter operations that exceed the criteria of existing product safety standards. The test takes an FMEA [Failure Mode & Effects Analysis] approach, using a risk matrix against which to assess the outcomes of each potential scenario that could occur on a rooftop involving someone falling (or hacking) into the PV generator.

“The tests aimed to answer, ‘what if a first responder comes across a solar panel?’ under a specific set of conditions, best-to-worst case scenario,” Rothschild says. “We had to test in those scenarios to determine the likelihood of them being electrified, down to whether they have a specific tool in their tool belt or specific clothes, or if there are wet conditions and they fall into a panel and grab another panel in another row to potentially create a circuit, and so on.

NEC 690.12 defines safety thresholds based on voltage levels, but these are fairly arbitrary numbers because high voltage in a network is not inherently dangerous. UL 3741 takes this into account in its hazard analysis:

  • Limiting the voltage is one way to reduce the risk of electric shock.
  • Other protections can reliably reduce the risk of electric shock.
  • The risk analysis recognizes several levels of protection to protect firefighters from hazardous currents.
  • The rating is based on a defined set of interactions and conditions with firefighters.

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 immediately detect ground faults.
  • Transformerless PV string inverters isolate the PV array from the ground reference after the AC power is disconnected.

“For someone to be in danger, they would not only have to come into contact with live current, but also be grounded,” Reynolds explains. “They have to put themselves in this circuit, between the live current and the path to ground. Touching a live conductor is not in itself a safety risk; it is part of the current path that creates the danger.

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

“Reliable cable management is what made this technique work,” says Reynolds. “Ensuring cables are unlikely to be damaged greatly reduces the likelihood of a ground fault, which greatly reduces the likelihood of a firefighter coming into contact with live current. These multiple failures would have to occur for a firefighter to even be subject to an electrical safety problem.

A gain of inches

Sollega’s unique shelving ticked a few other crucial PVHCS boxes:

  • Sollega’s system is made of glass-reinforced nylon 6 (Class A fire rated), which reduces the risk of conduction. “Being non-conductive, as we move into higher rooftop voltages, will play a bigger role in rooftop installations,” Rothschild says.
  • Its integrated cable management ensures that there is always an “insulating air gap” between the conductors and the metal parts of the photovoltaic generator.
  • Sollega’s shelving extends approximately 18 inches beyond the edge of the panel on the north and south rows, allowing wiggle room when placing a string inverter inside the “boundary of the matrix”.

On this last point, let’s return to the NEC definition of a switchboard: “A mechanically integrated assembly of modules or panels with supporting structure and foundation, tracker and other components as required, to form a unit of production of direct or alternating current”.

So the 1 foot network boundary perimeter starts where the Sollega racking system ends, which is probably about 8 inches beyond a conventional racking system.

Add it all up on this FMEA risk matrix, and SMA+Sollega has passed UL 3741 up to 1000V in the grid boundary, with no need for mid-circuit switches to reduce voltages in the grid boundary in the event emergency.

Sunny Tripower CORE1 Projector:

SMA CORE1

The Sunny Tripower CORE1 offers intelligent IV curve diagnostics, advanced string monitoring and SMA Smart Connected. The world’s first stand-alone PV inverter for commercial rooftops, carports, existing ground mount and backfeed solar projects, the Sunny Tripower CORE1 helps reduce logistics, labor and service costs . No additional support required for installation on the roof. 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 with no MLPEs or intermediate circuit interrupters of any kind means potentially saving thousands of dollars from a large flat roof job (about 7 and 9 cents per watt savings by removing module-level switches). Reducing the number of connectors in the matrix by a factor of three also has a benefit in terms of reliability and security.

The challenge that remains after all the I’s are dotted on the deployment of the PVHCS will be convincing the Authorities Having Jurisdiction (AHJ) that what’s old is new and compliant again. But if AHJs really care about the 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 jumper, avoid connector mismatch, or any issues with more connectors and points of failure – it’s a better solution overall,” says Rothschild.

And now that SMA has proven it, you’d think other C&I string inverter manufacturers will try to follow, giving the installer more choice.

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


Chris Crowell is the publisher of Solar Builder.

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Tags: commercial and industrial, NEC 2020, quick stop, SMA America, Sollega
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