The safety conversation involving rooftop solar has largely been influenced by the NFPA and others responsible for developing nationally recognized codes and standards. First responders are worried about dangerous current running through a solar array when responding to a building fire. The 2014 edition of the National Electrical Code (NEC) requires rapid shutdown for roof-mounted solar PV (essentially a way for firefighters or others to quickly control and shut down an entire PV system’s circuits at a single point), while NEC 2017 takes the code one step further, requiring shutdown to occur at the module level.
The 2017 edition will take some time to go into effect and become adopted by states, but the 2014 code is largely accepted nation-wide and guides the current fire safety rules within solar.
Marvin Hamon, principal with Hamon Engineering, said many solar installers don’t see the need for rapid shutdown, but firefighters and insurance companies largely support a way to turn off the electrical charge of a PV array.
“The 2014 NEC rapid shutdown requirements were always intended as a starting point, and the 2017 revision was to expand on them,” Hamon said. “Module-level isolation is about as far as the NEC can take rapid shutdown since anything else requires changes in the products themselves.”
NEC 2014 and rapid shutdown
Firefighters want to know how to shut down a solar system to prevent shock when responding to an emergency. DC disconnects help to stop the flow of electricity, but they don’t limit dangerous voltages. Firefighters often have a false sense of security when using just DC disconnects because there still may be electricity flowing through a part of the system. That’s why NEC 2014 introduced rapid shutdown as a solution.
NEC 2014’s rapid shutdown requirements (NEC 690.12) provide some instruction for installers. They say first responders should be able to turn off a PV system’s voltages using a clearly labeled switch or disconnect. How and where these devices are installed is up to an installer’s interpretation of the NEC.
“The 2014 NEC was non-specific about several points in rapid shutdown, such as how the rapid shutdown would be initiated, how the array was to be isolated, etc.,” Hamon said. “It indicated what had to happen, when and at what point, but the rest was left up to the AHJ and installer to decide how to implement.”
When setting up a solar array, installers have to incorporate PV circuit conductors within 5 ft of entering a building or within 10 ft of the array. During rapid shutdown, solar arrays have 10 seconds to limit voltages to no more than 30 V (considered touch-safe in wet locations). Most of these requirements are easily met by inverters and optimizers currently on the market. If not, rapid-shutdown controllers and/or combiner boxes are often where first responders can find shutdown devices.
NEC 2017 and module-level shutdown
While PV shutdown requirements in NEC 2014 were essentially vague, proposed revisions to NEC 2017 are much more detailed—expanding from less than 150 words to more than 1,100. Module-level shutdown becomes law of the land with a new section enforcing an 80-V limit within the solar array boundary, essentially mandating the use of module-level power electronics (MLPEs). Many firefighters are advocates of the new revisions. Even those who don’t completely understand its technicalities think “module-level shutdown” sounds a lot safer than “rapid shutdown.”
Module-level shutdown can be accomplished using microinverters, DC optimizers, smart modules, AC modules and other MLPEs, which have all been on the market for some time. MLPE manufacturers obviously support the new code because their products inherently meet it. Though string inverters are not built for module-level shutdown, manufacturers are coming out with innovative solutions to meet the code such as incorporating optimizers. Other in-array shutdown devices may also be coming to market.
“Module-level shutdown requires module-level electronics and remote control,” Hamon said. “While the overall safety will be improved, it would be hard to overlook that the additional components might reduce the reliability of the system.”
However, many MLPE manufacturers argue that their products actually provide more system uptime than string inverters, though they come at an initially higher cost.
The effects of NEC 2017 have yet to be seen, and with a louder outcry from the anti-module-level-shutdown crowd, it will be interesting to see how AHJs handle future solar installations.