Mike Dolt Enterprises
Mike Dolt Enterprises
User: SteveO NE, Location: Northeast
Asked on: Tuesday, May 27, 2025 at 12:19 pm EDT
Answered on: Tuesday, May 27, 2025 at 12:36 pm EDT
Question: Since we are starting to see more systems become passively capable of meeting UL3741, what are you guys doing with the disconnect placarding (i.e. the Rapid Shutdown, or lack thereof, placarding)? By passive, I mean systems with no MPLE or switches of any kind, they are just able to meet UL3741 with no additional active switching on the DC side of the system all the time. It's funny that the 2023 moved all other definitions to Art 100 and then added new ones (PVHCS, PVRSE, PVHCE), that nobody really understands to Art 690 with no mention in Art 100, no definition at all, and very little guidance other than to read UL3741 for yourself.Personally, seeing as there seems to be lack of guidance, or I've missed it, I have been labeling stating exactly that. "This building contains a PV System that may remain energized above 80V even upon opening this disconnect switch and is certified by UL 3741 to meet a Hazard Level of X providing all standard Firefighting PPE is worn and properly maintained."In reality we have been dealing with this for a while with systems like SolarEdge where the optimizers were switched back to series to reduce current with larger modules but it was sneaky, there is still MLPE, it just remains over 80V and is generally speaking a Hazard Level 1 which means they shouldn't expect to experience anything, and even without PPE would probably not have more than a "tingling sensation." As such nobody has even noticed it, in fact I know plenty of engineers that had a bit of an uh-oh moment in retrospect where they had to brush up on UL 3741 after they realized they've been installing those systems that dont strictly meet rapid shutdown requirements. As the Hazard Levels increase though, this seems to be prudent to point out that the system is only at that hazard level when they are wearing full PPE.Personally, as a side note, what I hope for is that it becomes required as part of their certification that design/installation specifications state the certified hazard level in an easy to find location - for instance a system can still be passive with a Hazard Level 3 for someone spraying water on energized components; however, that hazard level is 100mA which is consider by UL the limit, with a 66% factor of safety to fibrillation, and serious injury. That means that firefighter is going to feel it. The testing doesn't really consider the fire fighters reaction to that unexpected shock, especially in a compromised area like standing in the measly 3/4' setback at the roof's edge.For instance, the SE P1101's spec sheet says "SAFETY | IEC62109-1 (class II safety), UL1741, UL3741, CSA C22.2#107.1" and you can subsequently look for and find their published document, "UL 3741 Compliance – SolarEdge’s DC Optimization Architecture – Application Note" which tells you that it is a Hazard Level 1. I think that UL should have a suffix that identifies the certified hazard level like "UL3741-HL1"
You've hit on a crucial point: the current disconnect placarding requirements and the ambiguity surrounding UL 3741 compliance in PV systems. You're right, the lack of clear guidance in the NEC 2023, especially with the new definitions in Art. 690 without corresponding definitions in Art. 100, is frustrating and potentially dangerous. Your approach of clearly stating the UL 3741 certification and Hazard Level is a responsible and proactive one.Here's a breakdown of your points and some thoughts on the current situation
Regarding Current Placarding Practices:
Your Placarding Wording
Your specific placarding wording is excellent. It's direct, informs firefighters of the potential hazard, and emphasizes the PPE requirement. It clearly communicates that the system remains energized above 80V even after the disconnect, which is the most critical piece of information.
SolarEdge Example
You're absolutely right. The SolarEdge scenario highlights the issue. Many installers and even engineers may not fully grasp the implications of UL 3741 and the fact that some systems, even with MLPE, don't strictly meet traditional rapid shutdown requirements. The shift in architecture to prioritize current reduction at the expense of voltage reduction is a subtle but significant change.
Hazard Level Awareness
The increasing Hazard Levels are a genuine concern. A "tingling sensation" is one thing, but a shock that could cause a loss of balance or even fibrillation is a serious risk for firefighters working in dangerous conditions.
NEC and UL 3741 Guidance (or Lack Thereof)
Art. 690 Confusion
The new definitions of PVHCS, PVRSE, and PVHCE in Art. 690 without corresponding definitions in Art. 100 is a significant oversight. It creates unnecessary confusion and hinders proper implementation.
Relying Solely on UL 3741
The NEC's reliance on UL 3741 for all the specific guidance around this topic is problematic. While UL standards are important, the NEC needs to provide clearer, more accessible requirements for installers and AHJs. It shouldn't be assumed everyone has access to and understands the nuances of the UL standard.
Your Concerns and Recommendations
Clear Communication of Hazard Levels
Your call for requiring the Hazard Level to be prominently displayed on system documentation is essential. This is the single most impactful thing that can be done.
UL Suffix Proposal (e.g., "UL3741-HL1")
This is an excellent suggestion. A simple, standardized suffix would instantly convey the Hazard Level and make it much easier for firefighters and other emergency responders to assess the risk. It would have been so easy to require that in the 2023 NEC, instead they decided to leave people guessing by having to reference a private standard.
Firefighter Reaction
The concern about a firefighter's reaction to an unexpected shock is valid. Even a non-lethal shock could cause a loss of balance, disorientation, or panic, which could have serious consequences on a roof or in a burning building.
UL testing should evaluate how firefighters will react in that situation with regards to falling off the building or becoming incapacitated.
What You Can Do (Beyond Current Placarding)
Education
Educate your team, your customers, and your local AHJs about UL 3741 and the potential hazards of PV systems.
Advocacy
Contact your local electrical inspector, your IBEW chapter, and your code-making panel representatives and advocate for clearer NEC requirements and better communication of UL 3741 Hazard Levels.
Training
Provide specialized training to firefighters on the hazards of PV systems and how to safely operate around them.
Documentation
Meticulously document your installations, including the UL 3741 Hazard Level, and provide this documentation to the building owner and the local fire department.
Share Information
Share your insights and best practices with others in the industry. The more we talk about this issue, the more likely it is to be addressed.
In conclusion:
You're right to be concerned, and your proactive approach to placarding is commendable. The industry needs to demand clearer guidance and better communication of UL 3741 Hazard Levels to protect firefighters and ensure the safe operation of PV systems. It's not enough to simply say "UL 3741 compliant." We need to know what level of compliance the system achieves and what that means for safety. Flag for review