2023 code changes to rapid shutdown requirements solar installers should know

The National Electrical Code (NEC) is a frequently changing set of rules published by the National Fire Protection Association (NFPA), also referred to as NFPA 70. The latest edition was published in August 2022, but four jurisdictions have already adopted it with 13 others in the process to adopt.

The solar industry should pay particular attention to changes in one part of the 2023 code — section 690.12. This section regarding rapid shutdown requirements has significantly changed over the past few NEC editions. The 2023 version included some minor but important changes as well.

2023 rapid shutdown changes

Compared to the 2020 code cycle, the 2023 changes in this section were minor. The focus was on clarifications and editorial changes to place rapid shutdown requirements in one location.

A second exception was added to clarify that parking shade structures, carports, solar trellises and similar structures are not required to comply with 690.12 since firefighters typically do not perform rooftop operations on these types of buildings.
An exception was added to allow PV system circuits originating from arrays not attached to buildings that are terminated on the exterior of buildings. These PV system circuits installed in accordance with 230.6 are not considered controlled conductors for the purpose of 690.12.
Option (3) for inside of the array boundary was deleted since these systems must be evaluated according to UL 3741.
Rapid shutdown marking requirements from 690.56(C) were moved to 690.12(D), including minor changes in the requirements for the marking.

Even though module-level options are allowed in the code, a number of factors make it clear that opting to satisfy UL 3741 instead is the way to go in the future.

Avoid high numbers of additional connection points and mismatching PV connectors.
Eliminate interference of rapid shutdown devices and arc fault detection.
Eliminate the reliability concerns due to the rapidly increasing number of electronic components in an extremely harsh environment.
A method of better wire management and wire protection that complies with UL 3741 to reduces the chance of faults and the likelihood of thermal events.

Issues with module-level rapid shutdown

Solar power electronics, such as inverters and combiner boxes, are well-established technologies that have been installed for over two decades, whereas module-level power electronics (MLPE) are still relatively young in the industry. MLPE require significantly more components and connection points within the PV system, which can create problems as all these individual components must be properly integrated to function correctly. Some examples of these problems would be the interference caused by MLPE and the ability of arc-fault detection to properly detect arcs, as well as installation errors such as mismatched or not fully mated connectors. As the number of components and connection points increase, these types of complications are more likely to occur.

Electrical noise created by additional components could be interpreted as an arc-fault and cause false or unwanted tripping. On the other hand, MLPE could also prevent arc-fault detection by filtering, masking or attenuating real arc events.

Figure 1 illustrates an example of a string with five modules and how the number of connection points are affected with and without module-level shutdown devices (MLSD).

Even a string with only five modules, which is short for a string inverter, would have more than twice (2.6-times) as many connection points compared to a module-integrated solution or a PV system utilizing UL 3741, the Standard for Safety PV Hazard Control. UL 3741 provides an option for PV hazard control without the use of MLPE.

Figure 2 illustrates the number of connection points in a PV system as a function of the number of modules with and without MLSD. The dramatic increase in connection points with MLSD is alarming. Improper installation practices, such as missing dust caps, leave PV connectors exposed and susceptible to dust and oxidization when the installation is not completed promptly.

Each additional electronic component installed in the harsh environment of a rooftop increases the risk of a failure.

With the assumption based on Warranty Week research that the constant annual failure rate of one component is 0.075% (750 parts-per-million (ppm)) and 4,000 components are used in one or multiple PV systems, three failures per year or 45-60 failures in a 15-20-year PV system lifetime can be expected. Table 1 lists examples of failure rates for different system sizes and multiple systems.

Even if the failure of one MLSD does not affect the entire string, it would be labor-intensive to remove the existing modules and the faulted MLSD and replace it. This could also create new problems caused by disconnecting, testing and reconnecting each system component during the troubleshooting process or the availability of compatible components 10 years from now.

The assumption of a constant annual failure rate is a simplification and neglects the early failures of components. When these are factored in, it increases the number of failures significantly.

History of section 690.12

It is important to know that section 690.12 is intended to reduce the shock hazard originating from PV system circuits on buildings for firefighters. This does not include installers during installation and maintenance!

Section 690.12 was introduced for the first time in the 2014 edition and was modified multiple times. The 2014 edition had only requirements for outside of the array boundary, but this term was not introduced in this edition.

In the 2017 edition, additional requirements for inside of the array boundary were introduced. This edition included three methods to comply with the inside-of-the-array-boundary requirements, however the only method available at the time was the use of MLPE. Both requirements inside and outside of the array boundary must be fulfilled.

The direct linkage to UL 3741, Standard for Safety Photovoltaic Hazard Control, was added in the 2020 edition. UL 3741 was published on December 8, 2020, and provides methods to analyze and reduce the hazard for firefighters operating inside of the array boundary.

UL 3741 provides two methods, or a combination of both, to reduce the hazard for firefighters. One method is to limit voltages inside of the array boundary to a level that would not create a hazardous current flow through the firefighter’s body. The other is to reduce the risk of a fault due to firefighter interaction and/or to reduce the risk of any secondary faults causing current flow through the firefighter’s body.

Additionally, UL 3741 makes PV arrays safer due to improved and detailed installation instructions, defined materials and methods and performed risk analyses. These methods and reducing the number of components and connection points contribute to a safer and more reliable system.