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Rapid Shutdown of PV Systems Using String Inverters, Micro Inverters, or Power Optimizers
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One of the more notable changes to the 2014 National Electrical Code regarding solar installations is article 690.12 entitled “Rapid Shutdown of PV Systems on Buildings” that focuses on the rapid shutdown of roof and ground mounted photovoltaic systems. The purpose of this additional regulation is to reduce the electrical hazards for first responders/firefighters from the PV circuits in a system array. Although the addition of a rooftop or ground mount disconnect switch can isolate a PV array interrupting current to a centralized inverter, it does not provide voltage isolation. There can still be voltages greater than 300 VDC on the PV side of the switch as well as high voltages due to inverter capacitors and parallel PV array circuits from the inverter on the other side of the switch.
This 2014 NEC regulation may seem to be ‘old news’ but news that may not be understood by people thinking about the addition of a solar PV system to their home or business. Since this requirement impacts PV system installations, it is important for those considering the addition of a solar PV system to their home or business to understand which circuits require shutdown, the maximum allowable voltage on circuits shutdown, and the timeframe required to accomplish the shutdown. This ‘blog’ discusses a utility-interactive PV system where a PV array is roof mounted, the PV source circuits are run in metal conduits, and the inverter is mounted in a garage, basement, or side of the house depending upon the region of the country. This discussion does not include the additional topic of battery storage considerations in regard to the new regulations.
NEC 2014 Article 690.12 Requirements for Rapid Shutdown
The primary objectives of article 690.12 are to shut down the photovoltaic system conductors within 10 seconds in order to protect first responders from elements of a PV system that remain energized after the AC service has been shut off. Basically, the requirements apply to:
- Conductors of more than 5 feet from an array inside a building or more than 10 feet from an array outside a building
- Limiting conductors to no more than 30 volts and 240 volt-amps within 10 seconds of shut down
- Providing marking; informing firefighters with information as to how to perform a shut down
So why the additional requirement of article 690.12? The National Fire Protection Association established a task group to address proposals for the 2014 National Electrical Code that is normally updated every three years. The 2014 regulation was added to prevent first responders and firefighters from potentially dangerous shock hazards while trying to extinguish a fire while cutting though walls or roofs and possibly making contact with Direct Current conductors. The new article requires that “conductors more than five feet inside a building or more than ten feet from a collector array will be limited to a maximum of 30 volts and 240 volt-amps within 10 seconds of shutdown”. In other words, circuits must have a remotely activated switch within 5 feet of entering an attic and another shutdown switch within 5 feet of the inverter.
String inverters don’t inherently meet the NEC 2014 Article 690.12 standards. Even though a rooftop switch disconnect can shut off a PV system, opening a disconnect switch may not reduce shock hazard on many PV systems. Danger may still exist in that there are typically power sources on both sides of a disconnect switch. During daylight hours of operation, three hundred volts (300Volts DC) plus exists on the PV array side of the switch. On the other side of the switch, capacitors and parallel array circuits, connected to a string inverter, can keep that part of the circuit energized long after the disconnect switch is opened. Adding Rapid Shutdown capabilities for a string inverter system to meet 2014NEC additional regulations would therefore require replacing the rooftop array junction box with a ‘rapid shutdown’ box near the electrical service entrance of the structure, and running conduit and a control wire between the controller and the junction box. The inverter must be disconnected at two different points because of the current from the PV array itself as well as from the capacitors within the inverter. The cost of such an additional capability varies, depending on the installation.
Microinverters and Power Optimizers
A system using micro-inverters or power optimizers (Module Level Power Electronics or MLPE’s), is most likely already conforming to the 2014 NEC Article 690.12 because most systems will be de-energized as the power from the grid is interrupted.
Microinverters convert low voltage DC to utility grid-compliant AC voltage, and power optimizers provide low voltage DC to a centralized inverter for conversion to grid-compliant AC voltage. When the utility grid is available, AC power is then exported to the electric service for use by loads onsite or exported to the grid for others to use. When the grid fails, or the PV system AC circuits are disconnected from the utility service by an AC circuit breaker, utility meter removal, or other AC disconnect, these MLPE’s stop producing AC power and are no longer able to provide AC voltage or current into the inverter output circuits or to the grid thus meeting Article 690.12 requirements.
Systems with power optimizers achieve rapid shutdown compliance when AC power to the system inverter is disconnected. Upon loss of an AC signal the inverter sends a signal to the optimizers that automatically can reduce their voltage output to one volt. In this scenario, a string of 18 panels with optimizers would be reduced to eighteen volts as compared to its typical 350 volts.
Proposed 2017 NEC Changes for Rapid Shutdown : Possible Over-Regulation
Safety is always a priority for any PV installation and as of August 1, 2016, only the states of AZ, IL, IN, KS, MO, MS, NV, PA, and TN were not in the process of adopting the 2014 NEC regulations for Rapid Shutdown. While the currently implemented changes have provided additional safety for first responders and fire fighters, there are additional proposals for the upcoming 2017 NEC standards that may drastically impact how future systems are designed, installed, and cost, no matter whether string inverters, microinverters, or power optimizers are used.
The 2017 NEC proposed revision to article 690.12 would include reducing the 10-foot termination requirement of DC conductors within the PV array to a 1-foot boundary. It would also add a requirement to include an in-array 80 volt limitation inside the one-foot boundary while still maintaining the current 30 volt conductor limitation, ultimately requiring that individual panels be shut off remotely from a central switch. The rapid shutdown systems existing under the 2014 NEC requirements would not meet this new requirement as the string voltages between the panels and the rapid shutdown box could still exist up to 600 volts DC. Essentially, this new proposal mandates that all rooftop PV systems have some form of in-array or module-level shutdown devices.
Although it is not clear how a new in-array 80 volt limitation would increase firefighter and first responder safety, the additional regulation could actually decrease the safety for installers. If the solar industry is forced to include a more aggressive module-level shutdown, where is the concern for the increased risk in time spent for installers when they are swapping out failed or worn-out module-level devices? You must consider that service personnel will be exposed to a higher potential for falls – the number one cause of workplace deaths in construction. This proposed 2017 NEC additional regulation to article 690.12 will cause an increase to PV system complexity, installer and maintenance risk, cost, and reliability issues, which, will offer little, if any, performance benefit and which will, in turn, affect the future potential of rooftop PV installations. Careful considerations must be given to such changes. It appears that the NEC is attempting to fix a problem that perhaps does not exist.
The Essential Differences of String Inverters, Micorinverters, and Power Optimizers, also written by Russell provides basic information relative to this post.
Russell H. Plante is the author of Solar Energy: Photovoltaics and Domestic Hot Water – A Technical & Economic Guide for Project Planners, Builders, and Property Owners
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- Discusses cost and operating expenses relative to investment and return on capital which will be beneficial to project planners, installers, energy managers, builders and property owners
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Appreciation is given to Nick Sampson, Nate Bowie, and Geoff Sparrow of ReVision Energy for their comments and contribution to this article.
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