Regardless of PVRSE vs. PVRSS listing type, some very reliable RSD technologies employ a dedicated control cable to carry the signal that keeps the PV circuit closed and functioning.
With the provisions of National Electrical Code (NEC) 2017 article 690.12 now being applied in over 30 states and growing (and which we re-examined this week), many suppliers and contractors are struggling to determine which Rapid Shutdown Devices (RSD) on the market are compliant and compatible with the modules and inverters they use.
Compliance
Questions about compliance should focus on whether the equipment has the proper listing under UL 1741 or 3741 and whether that listing requires or prohibits other devices being in the system as well.
It also depends on the compliance path chosen. Article 690.12 offers three options for rapid shutdown inside the array boundary:
- A rapid shutdown PV array listed to UL 3741,
- Controlled conductors with less than 80 VDC after shutdown, or
- Eliminating exposed conductors entirely.
Since the listed array required by option one is not yet widely available, and option three applies only to Building-Integrated PV (BIPV) systems, option two – using RSD, optimizers, or microinverters – is most commonly applied at present. For energy storage systems that aren’t AC coupled with a microinverter or optimizer-based grid-tie inverter, an independent RSD is typically necessary.
While some form of RSD is required and provides safety benefits for first responders, home-owners and end-users are unlikely to perceive any direct benefits, so it’s tempting to go for the lowest-cost product that will pass inspection. However, failing to do the homework on RSD could lead to several negative customer experiences that go beyond just failing an inspection.
It is also important that the solar and storage industry take RSD seriously to avoid trouble with regulators, fire departments, and insurance companies as more homes and businesses sport PV arrays on their rooftops.
Rapid Shutdown Showcase: A review of commercial solar RSD strategies and solutions
PVRSE vs. PVRSS
Compliant RSD will have either a Photovoltaic Rapid Shutdown Equipment (PVRSE) or a Photovoltaic Rapid Shutdown System (PVRSS) listing under UL 1741 and labeled appropriately. PVRSE listings consider the RSD components independently of other equipment in the system. They tend to be more flexible while the PVRSS listing normally includes the inverter and related components as a system. It is important to understand that the current-carrying conductors entering the inverter must be below 30 VDC within 30 seconds, and inverters having PVRSE or PVRSS on the label have been tested for this. Either label is typically acceptable to the AHJ so long as all of the RSD components – including the PV inverter or charge controller – are labeled as one or the other.
If in doubt, the listings can also be verified with the Nationally Recognized Testing Laboratory (NRTL) of record – UL and ETL for example, using their online tools.
Topology options and trade-offs
Regardless of PVRSE vs. PVRSS listing type, some very reliable RSD technologies employ a dedicated control cable to carry the signal that keeps the PV circuit closed and functioning.
While the control cable does require installation, it can typically be run alongside the PV cable to minimize additional cable management parts and labor. The control cable connects to a power supply, usually located in the rapid shutdown initiator (RSI). Because it’s completely independent of the inverter and other devices, this kind of RSD is least likely to experience nuisance trips or fail to respond when activated.
Some RSD approaches avoid the need for a control cable by using a wireless signal such as Wi-Fi or ZigBee. While this avoids conflicts with arc-fault protection, fail-safe requirements dictate that the PV circuit must open if the wireless signal is lost – so be sure the radio connection is robust for this route. This detail is particularly important for large arrays or mounting structures that can interfere with the signal.
Most RSD rely on a control signal, such as SunSpec RSD, that is carried on the DC power conductors of the array. While this avoids the need for a control cable or radio connection, the control signal may interfere with common arc-fault protection devices and cause nuisance tripping. The false arc-faults often generate service calls and can lead to the user or service technician disabling the RSD all together, which is, of course, a code infraction. If the RSD under consideration uses a power line carrier (PLC) signal, be sure that it carries an up-to-date PVRSS listing that includes the specific inverter or charge controller with which it’s installed.
Other considerations
Beyond nuisance trips, it’s also important to ensure that the RSD doesn’t prematurely fail as it will take the attached module – if not the entire string – out of commission until it is replaced.
Look for products from reputable companies, such as IMO, that have a track record for producing reliable, durable products and are stable enough to stand behind a 20-year warranty.
Be sure to understand what does and does not initiate a shutdown.
Most RSD for grid-tied systems will shut down the array during an outage, even if no one activates the switch. While this is arguably preferable for systems that can’t island, solar + storage systems with backup functionality can still use the array during an outage. In many cases, this can be addressed by powering the RSD from the hybrid inverter output or directly from the battery.
Also, consider what happens when the RSD is activated to shut down the array.
Does it work as advertised? Can it be reset without climbing on the roof? Is there an indicator that lets first responders now the array is safely shut down? Each RSD approaches the operation differently, and everyone involved must know what to expect.
Finally, be sure to ask peers about their experiences with the solutions you are considering. Trade shows, such as the NABCEP CE conference and local trade association events are great places to get unfiltered feedback about what works and what doesn’t. Suppliers are often also on hand, and the best ones aren’t afraid to field difficult questions or answer challenges on their claims.
Ultimately, installers are held responsible for the experience they deliver to their customers. It’s more than worth it to do the homework and ensure they’ve got the best products for the job.
For more information:
UL listed equipment lookup tool
Intertek (ETL) Listed equipment lookup tool
Co-authors
Joe Covington, North America Sales Manager, IMO Automation. Joe is responsible for strategy and market development as it pertains to IMO’s rapidly expanding footprint in North America.
Paul Dailey, Director of Product & Market Strategy, OutBack Power. Paul has spent his 20-year career developing, marketing and deploying distributed generation technologies, from micro-CHP to solar + storage.
Jeff Spies, President, Planet Plan Sets, a solar plan drafting company serving contractors in all 50 states. Jeff also serves as board member and secretary for NABCEP, Chairs the Codes/Standards committee for the California Solar and Storage Association (previously known as CALSEIA).
— Solar Builder magazine
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