This article applies to Solis commercial high voltage hybrid inverters paired with Projoy PEFS string-level rapid shutdown (SLRSD) receivers. If the inverter is going to be mounted within the 1 foot PV array boundary, then no type of rapid shutdown is required.

Applicable Solis Models:

• S6-EH3P30K03-LV-YD-H-US
• S6-EH3P30K03-NV-YD-H-US
• S6-EH3P40K04-NV-YD-H-US
• S6-EH3P50K04-NV-YD-H-US
• S6-EH3P60K04-NV-YD-H-US
• S6-EH3P75K10-LV-YD-H-US
• S6-EH3P100K10-NV-YD-H-US
• S6-EH3P125K10-NV-YD-H-US
• S6-EH3P150K10-NV-YD-H-US

The purpose of this article is to explain how the UL 3741 PV Hazard Control System pathway lets Solis commercial PV systems substitute string-level rapid shutdown (SLRSD) equipment for module-level rapid shutdown (MLRSD). It defines the certification distinctions between PVRSE, PVRSS, and PVHCS, lists the approved equipment pairings for the Solis commercial hybrid inverter series sold in North America, and provides installation instructions drawn from the Solis and Projoy manuals. Throughout this document, "S6-EH3P" is shorthand for the full inverter series designation.

The example systems shown in this article are much smaller than typical commercial systems. This is to simplify the diagrams since they are only for explaining concepts and keeping them simple makes them easier to read and interpret.

Single Line Diagram (SLD) example is attached to this article. Please scroll to the bottom of this article and then click on the file to download it. 

Common acronyms used in this article

1. What is UL 3741 and how does it enable string-level rapid shutdown (SLRSD)?

1.1 The standard

UL 3741, Standard for Photovoltaic Hazard Control, was first published by Underwriters Laboratories on December 8, 2020 and was updated in October 2025. It is referenced in the 2020 and 2023 editions of the National Electrical Code (NEC) at Section 690.12(B)(2)(1) as one of the accepted pathways for satisfying rapid shutdown requirements inside the PV array boundary.

The standard evaluates the design of PV Hazard Control Systems (PVHCS) which includes the combination of PV modules, wiring, combiners, inverters, racking, and other PV array-mounted equipment, such as rapid shutdown receivers, to determine whether the assembled PV array poses a shock hazard to firefighters in an emergency. Certification is granted at the system level by a Nationally Recognized Testing Laboratory (NRTL) once the entire assembly has been shown to keep firefighters out of hazardous current paths.

1.2 How UL 3741 changes the rules inside the PV boundary

NEC 690.12(B) defines the PV array boundary as one foot from the PV array in all directions. Prior to UL 3741, the code inside that PV boundary required controlled conductors to be reduced to 80V or less within 30 seconds of rapid shutdown initiation, which in practice forced the use of module-level rapid shutdown (MLRSD) equipment on every module.

Module-Level Rapid Shutdown (The Old Way)

                    Zoomed-in view of the bottom-left corner of the roof

Under NEC 690.12(B)(2)(1), a listed UL 3741 PV Hazard Control System is now an accepted alternative. The rationale is that if the racking manufacturer, working with the module manufacturer, inverter manufacturer, and NRTL, can prove that a firefighter falling onto or through the PV array is not exposed to a shock hazard, the 80V internal limit no longer applies. DC voltage inside the PV boundary can remain at up to 1000 V, depending on the specific PVHCS certificate. The specific voltage value is not what matters, the point is that once UL 3741 is used, 80V is no longer the requirement for within the PV array boundary. Each PVHCS listing defines its own internal voltage limit based on the NRTL evaluation of that specific PV array, racking, and inverter combination.

The rule outside the PV boundary is unchanged. Controlled conductors located more than one foot from the PV array, or more than three feet from the point of entry inside a building, must still be reduced to 30V or less within 30 seconds of rapid shutdown initiation. This is why string-level rapid shutdown is required instead of no rapid shutdown. Your options are to either install the inverter within the 1 foot PV array boundary and use no rapid shutdown, or outside of the PV array boundary with string-level rapid shutdown. 

String-Level Rapid Shutdown (The New Way with UL 3741)

                   Zoomed-in view of the bottom-left corner of the roof

The string-level rapid shutdown (SLRSD) device is what satisfies the 30V or less outside the PV boundary rule under UL 3741. One SLRSD receiver is installed for every 5 PV strings ideally at the edge of the PV array. These devices connect to the PV strings and to the leads going back to the inverter. When the rapid shutdown initiation switch is engaged, the SLRSD receivers cease to receive AC power. The receivers then open within 30 seconds, isolating the home-run conductors and dropping their voltage to below 30V outside of the PV array boundary. 

This presents a substantial reduction in rooftop equipment compared to module-level RSD. A typical commercial system with 400 modules requires up to 400 individual MLRSD receivers under the traditional pathway. Under UL 3741 with SLRSD, the same system uses one SLRSD receiver for every 5 PV strings, often two or 4-6 devices total for the whole system. Solis commercial hybrids have a range of PV string inputs. The 30K models, which are the smallest, can connect to 6 PV strings maximum. The 75-150K models can connect to 20 PV strings maximum. For every 5 PV strings, there must be one SLRSD device. 

1.4 Projoy String-Level RSD Device JMS-PS1(B)

Projoy is a partner of Solis who is the OEM for the PEFS string-level RSD device. This device is UL 1741 CRD PVRSE-certified and UL 1741 CRD PVRSS-certified with Solis S6-EH3P models. To have a UL 3741 compliant system using Solis requires the installation of these SLRSD devices within the PV array unless the inverter itself is installed inside the 1 foot PV array boundary which negates the need for any string-level or module-level RSD equipment. The full datasheet for the Projoy PEFS can be downloaded by scrolling to the bottom of this article and clicking the link.

Front View

Bottom View

1.5 What value does string-level RSD and UL 3741 bring to the system owner?

Aside from being safer to firefighters, UL 3741 systems utilizing string-level rapid shutdown instead of module-level rapid shutdown can experience a cost savings as well. UL 3741 controlled-array certification with string-level RSD removes the need for a rapid-shutdown device at every module. Compared with module-level rapid shutdown (MLPE-based RSD, optimizers, or microinverters), this benefits both owner and installer.

Value to the System Owner

• Lower system cost. Fewer devices and less installation time means less total system cost for the owner of the system. 

• Far fewer failure points. For a 150 kW system with an 360-module array using module-level RSD it would have 360 active rooftop receivers in the array. String-level RSD replaces that with about 4 receivers for a 150 kW inverter. Module-level architectures have been shown to multiply potential points of failure by a factor of two to four over string-level designs.

• Less lost production. Most MLPE field failures are related to connector faults and overheating. If one or two devices fails, it may not be noticed for a while or may not be worth the service providers time to send out a tech to replace just one or two devices. If a string-level device fails, it will be quickly noticed and fixed.

Value to the Installer

• Lower system cost. Fewer devices and less installation time means less total system cost to charge the customer (owner) meaning more revenue compared to using module-level RSD.

• Faster installation and troubleshooting. Installing one device per string vs one device per PV module leads to quicker system builds and less complexity for the installer overall. The SLRSD receivers are typically installed near the edges of the PV array making it easier to access. 

• Simpler stocking and fewer callbacks. A lower rooftop device count means stocking fewer components. It also leads to fewer warranty truck rolls over the system life. 

2. PVRSE, PVRSS, and UL 3741 Certifications

The terms PVRSS, PVRSE, and PVHCS refer to related but distinct certifications. Understanding the differences is essential for specifying, selling, and installing a compliant system. Solis has UL 1741 CRD PVRSS with APS transmitter and Projoy string-level RSD receiver. Intertek approved Solis with Projoy to be added to the UL 3741 addendum of IronRidge and others.

2.1 Standard Definitions & Certified Equipment

3. How string-level rapid shutdown (SLRSD) is accomplished

3.1 How RSD works for the full system

An AC voltage source must be connected to the Projoy SLRSD receivers. Also, an external RSD switch must be connected to the Projoy receivers. The inverter does not need a transmitter internal or external since the Projoy device does not use PLC signal but relies on AC voltage to be present in order to keep the system out of RSD mode. Each Projoy receiver connects to between 1 and 5 PV strings. The receivers gets mounted within the 1 foot PV array boundary. The PV strings then connect the Projoy receivers to the inverter. Essentially, the receivers act as string-level switches disconnecting when they stop receiving AC power or the external RSD button is engaged.

1. AC voltage reaches the Projoy receivers in the PV array.

2. Each Projoy SLRSD receiver gets installed on the home-run leads of between one and five PV strings within the PV array. Ideally, the receivers get installed at the edge of the array easily accessible to technicians.

3. Initiation of the external rapid shutdown switch turns off the Projoy receivers immediately. The home-run conductors between the PV array and the inverter drop below 30V. The DC voltage inside the PV boundary remains at whatever the full open-circuit voltages are; however, this is fine because the PV array as a whole is certified under UL 3741 to be safe to firefighters.

4. Disengaging the rapid shutdown switch restores power to the Projoy receivers. The PV string voltages return to normal.

Note: The Rapid Shutdown switch on the side of the inverter only disables the internal RSD transmitter. Since Projoy devices do not use PLC, disabling the internal RSD transmitter has no effect on the Projoy SLRSD receivers. This is why an external RSD switch must be installed and connected to the Projoy receivers. Turning off AC power is not enough either, since systems with backup (or off-grid) will rely on the SLRSD receivers to be powered even when the grid has failed.

3.3 System Overview (Array)

In this example, each Projoy SLRSD receiver is connected to four different PV strings. However, the number of PV strings that will connect depends on your design. The range is 1-5 strings at 30A max. Imp.  

4. Non-Contiguous Arrays 

A PV array becomes non-contiguous when there is a physical gap, or separation of more than 2 feet, between different parts of the array. While large commercial arrays consisting of 20 to 40 modules rarely encounter physical boundary issues, residential rooftops frequently require arrays to be split across different roof planes or physically separated to avoid obstacles.

When a PV array is physically separated, determining how to comply with rapid shutdown rules becomes critical. If an array is non-contiguous, installers must utilize one of the following three compliance strategies:

1. Create a Dedicated String (String-Level RSD) 

A separated array can be treated as its own independent string with its own dedicated string-level rapid shutdown device. However, to generate power, this separated array must meet the inverter's minimum input startup voltage.

• The minimum startup voltage for the commercial hybrids is 180V.

• Best Practice: We highly recommend designing the string to produce at least 216V (a 20% buffer) to account for voltage drops and varying environmental conditions.

• Keep in mind that a small sub-array of only 2-3 modules is typically insufficient to hit this voltage threshold; 4 or more modules are usually required depending on the specific module voltage.

• The smaller string must connected to a separate MPPT from the larger strings as the drawing below shows.

2. Use Module-Level Rapid Shutdown (MLRSD)

If the separated array is too small to meet the 120V recommended startup threshold and extending the racking material is not viable, the system must utilize module-level rapid shutdown (MLRSD) instead of string-level RSD.

• Critical Rule: You cannot mix module-level rapid shutdown devices and string-level rapid shutdown devices on the same inverter. If MLRSD is required for a small, non-contiguous sub-array, the entire system connected to that inverter must utilize MLPE and not SLRSD.

Mixing String-Level and Module-Level RSD Equipment in the Same System

If some areas of the roof have many obstructions and trying to design with string-level RSD is too difficult, it is possible to use module-level RSD on some inverters and string-level RSD on other inverters. As the diagram below shows, different inverters can have string-level or module-level RSD so long as they are not mixed on the same inverter. The Projoy receivers do not use PLC so there is no chance of cross-talk occurring. 

5. Approved Equipment

The following S6-EH3P three-phase high-voltage hybrid inverter models (480/240/208V) are approved for the UL 3741 pathway with Projoy PEFS or with no SLRSD if the inverter is installed within the PV array boundary. All of these hybrid models have a PV system voltage rating of 1000VDC. 

5.1 Solis S6-EH3P(30-60)K-NV-YD-H-US hybrid inverter models

5.2 Solis S6-EH3P(75-150)K-NV-YD-H-US hybrid inverter models

5.3 Projoy PEFS String-Level RSD receiver

The Projoy PEFS is a string-level rapid shutdown receiver that does not require PLC signal to operate. Instead, it relies on an AC connection that is broken when the RSD switch is engaged. When the SLRSD devices lose AC power, they open/disconnect. This device is only required for systems where the inverter is installed outside of the PV array boundary. If the inverter is installed inside the PV array boundary, no string or module-level equipment are required for UL 3741. Each Projoy PEFS device can connect to between 1 and 5 PV strings.

6. Installation Instructions

S6-EH3P(30-60)K-NV-YD-H-US

S6-EH3P(75-150)K-NV-YD-H-US

The steps below combine the PV and rapid shutdown installation procedures from the Solis S6-EH3P user manuals and the Projoy PEFS installation manual. Always follow the manufacturer manuals for the complete procedure, torque specifications, and safety warnings; the summary below is not a substitute for those documents.

6.1 Pre-installation checks

1. Assuming the PV array and the inverter are fully installed 
2. Verify the PV array (all PV strings) are fully disconnected from the inverter before beginning.
3. Use a multimeter to confirm the PV string voltages at the inverter are 0 V
4. Confirm the polarities of each PV string are correct: positive to positive, negative to negative.
5. Confirm the inverter DC switch, battery breaker, AC-Backup breaker, and AC-Grid breaker are all in the OFF position.
6. Confirm the PV insulation resistance to ground is higher than 20 kΩ.
7. Confirm that the DC voltage of each PV string will not exceed the S6-EH3P maximum input voltage on the coldest expected day (calculated using module Voc, temperature coefficient, and worst-case local temperature).
8. Confirm the Isc and Imp of each string will not exceed the S6-EH3P maximum input current of 21 A per string (max short-circuit current 32.5 A per string).
9. Verify strings connected to the same MPPT are within ±10 % of each other in DC voltage.

6.2 Installing the Projoy receivers

1. Mount the Projoy receivers within the PV array boundary. They cannot be installed more than 1 foot from the edge of the PV array racking.
2. Each receiver can connect to between 1 and 5 PV strings. 
3. The MC4 connectors on the bottom of the receiver are split into two groups. 
   1. The connectors on the left are from the PV array going in to the receiver. 
   2. The connectors on the right are going out from the receiver to the Solis inverter.
4. Connect the PV string leads to the Projoy receiver "IN" ports on the left side.
5. Connect leads going back to the inverter to the "OUT" ports on the right side of the receiver.
6. Do this for all of the receivers in the PV array.

6.3 Wiring the Projoy PEFS

The Projoy PEFS has two other ports in addition to the PV string inputs. The left port is for the external rapid shutdown (RSD) initiation switch. The port on the right is for a single-phase (L-N) AC voltage source: 100-270VAC 300mA maximum. 

1. Connect a two-core cable to the RSD port of the Projoy receiver. 
2. Connect the other end to an external rapid shutdown initiation switch somewhere near the inverter(s). 
3. Connect a three-core cable to the AC port of the Projoy receiver. The core-rating should account for the AC voltage used at 300mA.
4. Connect the other end to an AC source (100-270VAC)

6.4 Connecting PV strings to the S6-EH3P(30-60)K-NV-YD-H-US 

The S6-EH3P series have a range of MPPTs from 3 to 10. Each MPPT has two string inputs, meaning the range is 6 PV strings to 20 PV strings. After the Projoy devices are installed, connect the PV leads from the outputs to the inverter DC input terminals as follows:

1. Strip 1/2 in of sheath from the end of each PV cable.
2. Pull up on the orange tab above the target PV terminal to open the terminal gate.
3. Insert the end of the PV cable into the open terminal.
4. Push the orange tab down to clamp the terminal gate onto the PV cable.
5. Give the PV cable a gentle tug to confirm the connection is tight. If the connection feels loose, repeat Steps 1 to 4 and push the cable deeper into the terminal before releasing the lever.

Caution: If DC conductors are accidentally reversed, or if the inverter is not operating correctly, do not turn off the DC switch. Doing so may cause a DC arc that damages the inverter or starts a fire. Instead, measure the DC string current with a DC clamp multimeter, wait for the current to drop below 0.5 A (either by waiting for reduced irradiance or by covering the modules), then open the DC switch and disconnect the strings.

6.5 Connecting PV strings to the S6-EH3P(75-150)K-NV-YD-H-US inverter

Please see the full user manual, which contains steps for making MC4 connectors which then connect to the inverter PV inputs. Pages 36 to 38 explain the full process in detail. 

Solis Manual_S6-EH3P(75-150)K10-NV-YD-H-US

7. Inverter Within the PV Array Boundary (No Rapid Shutdown Installed)

Under NEC 690.12, placing a string inverter directly within the 1-foot boundary of a solar array eliminates the need for module-level power electronics (MLPEs) to achieve rapid shutdown compliance. The code dictates that conductors leaving the 1-foot array boundary must be controlled to 30V or less within 30 seconds of initiation. 

By relocating the inverter from the ground to the roof, the high-voltage DC conductors remain entirely contained inside the array's footprint and never cross that boundary. The only conductors exiting the 1-foot zone are the AC output wires. When emergency responders throw the main AC disconnect, the inverter immediately ceases exporting power, instantly dropping the AC lines to zero volts and perfectly satisfying the outside-the-boundary rapid shutdown requirement.

8. Compatible UL 3741 Certified Racking Systems

9. Frequently Asked Questions (FAQ)

Q1: Do I still need module-level rapid shutdown if I use UL 3741?

A1: No. That is the point of UL 3741. The listed PV Hazard Control System replaces the 80V inside-the-PV-array-boundary limit that would otherwise force MLRSD. You still need equipment that controls the voltage on the home-run conductors outside the PV boundary and that is what the string-level RSD receiver does.

Q2: Where does the SLRSD device physically go?

A2: Directly under the PV array or at its edge, on the home-run leads that carry DC current back to the inverter. Choose a location that is accessible so the device can be serviced or replaced without lifting modules. One SLRSD can connect to between 1 and 5 PV strings. It can be mounted on the PV module frame or on the rail just next to the PV module but within the 1 foot PV array boundary.

Q3: Do I need to run separate control wiring to the SLRSD device?

A3: Yes. The Projoy receiver requires an AC voltage source (L-N) to stay closed. When the RSD initiation switch is engaged, the receiver shuts down (enters RSD mode).

Q4: Can I use a Projoy receiver with an APS or Tigo transmitter?

A4: No. Projoy receivers do not use PLC signal to turn on and off. No transmitter is required with Projoy, you just have to run separate AC power cables to the array for the RSD receivers.

Q5: What voltage can be inside the PV boundary under UL 3741?

A5: It depends on the specific PVHCS certificate, but typically up to 1000V is allowed on the roof in the US and 1500V on the roof in Canada. The important point is that once UL 3741 is used, 80V inside the PV array boundary is no longer the ceiling. Outside the PV boundary, the ≤ 30 V within 30 seconds rule still applies, however, and that is where Projoy SLRSD devices come in as they do control the DC voltage outside of the PV array boundary.

Q6: How is a rapid shutdown event initiated on a Solis S6-E32P?

A6: By engaging an external rapid shutdown initiator wired to the Projoy receivers. Doing this turns off power to the receivers and causes them to enter safety mode where the DC voltage between the PV array and the inverter reduces to a safe level.

Q7: What happens during a grid outage if the system is in backup mode?

A7: If the S6-EH3P is providing backup power from the battery and grid AC is lost, the Projoy rapid shutdown receivers continue to stay on as long as the power source for them is wired to the backup-side of the inverter. Only engaging the RSD initiation switch forces the system into rapid shutdown mode. If the goal is to have the system enter RSD mode every time AC power from the grid is lost, then the source power for the RSD receivers should be connected to the grid-side of the system instead of the backup-side.

Q8: Who is responsible for obtaining the UL 3741 certification?

A8: The racking manufacturer typically holds the UL 3741 PV Hazard Control System listing. The listing covers a specific combination of modules, racking, mounting hardware, inverter, and SLRSD receivers. Always consult the racking manufacturer's UL 3741 installation addendum before designing or installing the system. Whether receivers are required or not depends on whether the inverter(s) is/are to be installed within the 1 foot PV array boundary or if they will be installed outside of it. 

Q9: Is UL 3741 required, or is MLRSD still an option?

A9: MLRSD is still fully permitted under NEC 690.12(B)(2). UL 3741 is just an alternative that many installers prefer because it reduces the number of receivers, simplifies troubleshooting, and can lower equipment cost. Choose the pathway that best fits the project.

Q10: How do I check which racking systems are UL 3741 certified and approved with Solis?

A10: Refer to the current Solis USA and Canada Compatibility List (PE-CP-0005-R00) at solisinverters.com/us or contact Solis USA service at usservice@solisinverters.com. The compatibility list identifies each certified pairing and flags any that are still pending PVRSS or UL 3741 certification. 

Q11: If I plan to mount the inverters within the 1 foot PV array boundary, what do I do about non-contiguous arrays where the PV array is physically split up by more than 1 foot between them.

A10: In that case, you would need to deploy SLRSD devices for all non-contiguous arrays that would have to be designated their own strings.