Module-Level Rapid Shutdown for Commercial Applications: Page 4 of 6

“Representatives of the fire service tell us that they need to quickly ventilate a structure that is on fire, and as a 10-year veteran of the fire service and chief of a rural fire department, I can tell you that they are right. However, the position that vertical ventilation is the best and only way to ventilate, and that the fire service therefore needs full and unlimited access to the roof, does not align with modern fire science. Studies performed by the NIST [National Institute of Standards and Technology] and others have shown that positive-pressure ventilation is more effective than traditional vertical ventilation. According to this research, vertical rooftop ventilation can no longer be considered the gold standard for effective fireground operations.

“Section 690.12 should ensure that a local source of electricity (the PV array) can be easily disconnected from a building electrical system in the event of a fire, and 690.12(B)(1) achieves that goal. Requiring conductors within the array zone to be controlled to 80 V does not provide a touch-safe environment, and there are thousands of legacy PV installations currently installed that are not controlled within the array zone; therefore, 690.12(B)(2) creates a false sense of security for the fire service. Fire operations should not be in the array zone when there are better options.”
—Phil Undercuffler, director of strategic platforms, OutBack Power

The Compromise Solution

As a result of this pushback, CMP-4 developed a second revision of NEC 2017 690.12 that provides three compliance options inside the array boundary, each of which offers a unique set of challenges. The first option is to use a listed rapid-shutdown PV array, which assumes the existence of an as-yet-unwritten UL product safety standard. The second option is to limit the potential of controlled conductors to not more than 80 V. This option assumes that UL fire testing will show that divergent product classes (such as microinverters, ac modules, dc-to-dc converters and smart modules) provide an as-yet-unproven level of shock hazard mitigation under abnormal operating conditions—most important, after a fire has compromised and damaged the PV modules and associated solid-state devices. The third option is to deploy PV arrays with no exposed wires or conductive parts at least 8 feet away from exposed grounded conductive parts, which seems to belong in a product safety standard rather than in the NEC.

While the requirements for what should happen inside the array boundary are contentious, areas of common ground do exist. For example, both fire service and solar industry representatives on CMP-4 seem to agree that the compromise solution is not ideal. A common concern from stakeholders on both sides is whether the three compliance options allowed inside the array boundary provide an equivalent level of safety. Given that no one seems particularly happy with 690.12(B)(2) as written, it seems fortunate that this subsection will likely come with a delayed enforcement date of January 1, 2019. This delay will give UL and its standards technical panel members time to develop a rapid-shutdown PV array product safety standard that meets NEC 2017.

Ultimately, 14 of 17 eligible voters on CMP-4 voted in the affirmative, which suggests that the formally adopted 2017 rapid-shutdown language, due out in October, will adhere closely to the second revision, for better or worse. Bill Brooks, a solar industry representative on CMP-4, voted in favor of the second revision. However, he concedes: “The new version of 690.12 is a significant step in PV system safety that will be difficult for the PV industry to master in the first several years of enforcement.”

Brooks continues, explaining his affirmative vote: “While [compliant] products are commonly available and used in the residential market, the more difficult market is the commercial PV market. In the commercial market, margins are even tighter, and costs and reliability have to be carefully managed. Once these new standards and products become mature, the PV industry and all those whom it affects will have safer and better PV systems. Much work is necessary between now and then.”


Commercialized MLPE are competitive in residential applications. GTM Research data show that in 2014, when US states began adopting rapid-shutdown requirements, the combined market share for module-level solutions—including microinverters, dc optimizers, ac modules and smart modules—already accounted for more than half of the total residential product mix. Since that time, the market share for string inverters has eroded—presumably because more rapid-shutdown markets come on line each year—from 48% in 2014 to 40% in 2015. GTM Research estimates that the residential market share for string inverters could fall to 30% in 2017, which may prove optimistic given that California is now poised to adopt rapid shutdown.

Article Discussion

Related Articles