Many of the inverter and racking guides for UL3741 listings mention using string isolation devices for sub arrays outside of the array boundary. I have not been able to find a inverter or racking company that is aware of where to find this product on the market, nor are any SIDs included in the list of equipment under their approved equipment as part of the listed system. Would standard DC disconnects work here? Or is anyone aware of SIDs on the market that comply with UL3741?
UL 3741 String Isolation Devices
- Thread starter SunnyGirl
- Start date
jaggedben
Senior Member
- Location
- Northern California
- Occupation
- Solar and Energy Storage Installer
Does this documentation have more specific instructions? Because I find this odd. First, a subarray is still within the array boundary for rapid shutdown, unless it's not on a building. Second, UL3741 is for complying with rapid shutdown for firefighters and firefighters are not going to be opening isolating devices during operations.
PV isolating devices in the NEC are covered in 690.15(B) [2020 reference] and can be for example simply MC4 connectors or touch-safe fuse holders.
I'm not familiar with the content of UL3741 but I feel something is off here, unless the mention of isolating devices is just a suggestion or a mention of NEC requirements that are not directly related to being UL3741 compliant. Unless your documentation clearly says otherwise, I would not think the isolating devices actually have to be individually listed to UL3741. Like, I'd think they could just be NEC compliant isolating devices.
PV isolating devices in the NEC are covered in 690.15(B) [2020 reference] and can be for example simply MC4 connectors or touch-safe fuse holders.
I'm not familiar with the content of UL3741 but I feel something is off here, unless the mention of isolating devices is just a suggestion or a mention of NEC requirements that are not directly related to being UL3741 compliant. Unless your documentation clearly says otherwise, I would not think the isolating devices actually have to be individually listed to UL3741. Like, I'd think they could just be NEC compliant isolating devices.
BackCountry
Electrician
- Location
- Southern California
- Occupation
- Licensed Electrician and General Contractor
This is an example of the code being ahead of what’s being produced. UL3741 wasn’t really tried and tested until recently once manufacturers had the testing completed to comply.
The easiest method to comply with a sub array, as long as your sizing is small, is micro’s — problem solved. However, I doubt that you’d be asking if you’re using micros.
If you’re using SMA or CHINT, then I believe you’d be using a DC contactor. It’s not a manual throw by any means.
If you can share your layout I can provide more.
BandGap1.1eV
Member
- Location
- East Coast
You'd have to go back to the contactor based DC pass-through boxes we all used for 2017s version of rapid shutdown. Not sure if anyone makes them anymore.
Standard DC disconnects won't be adequate, they need to be able to shut down when rapid shutdown is enabled. Unfortunately, there are hardly any SIDs on the market, and even of those, the only ones you're allowed to use must be listed with whatever inverter and racking system you plan to use. That makes the options basically zero for SIDs. The only compliant method right now would be to use MLPEs for anything in the subarray, and depending how your inspector views it, that may require you to use MLPEs on your main array if the inverter doesn't isolate voltage between your main array and your subarray.
The situation around SIDs is likely to get much better over the next year. I started a website (ul3741.com) to track the progress of 3741 listings, after the news around Ironridge getting their flush mount racking certified. Since residential systems will start being able to use 3741, the demand for SIDs is definitely gonna go up, so the market will hopefully rise to the occasion and give us more options on that front.
Tesla has an isolation device but it only works on a string of 2-3 modules. If a string length is six then you might be able to use just two on the string ends. Not an optimal solution and only one racking manufacturer I know of has them as an option.
What I would like to see is a single SID that goes on the end of a string that can handle typical full string lengths, or a multi-SID box that several strings can pass through. After all, the idea is to have an isolation option with as few additional connectors as possible.
What I would like to see is a single SID that goes on the end of a string that can handle typical full string lengths, or a multi-SID box that several strings can pass through. After all, the idea is to have an isolation option with as few additional connectors as possible.
Their isolation device works with voltage up to 600v. The PVHCS restrictions, for the MCI-1 with Tesla's racking or solar roof, requires there to be no greater than 165v after rapid shutdown is triggered. Ironridge's racking certification does not have that restriction, and as long as it's 600v or less within the array boundary, you can use an MCI on both sides of the string to isolate the wires that go outside the array boundary.
The reasons the restrictions are different is due to the different ways UL and Intertek handle the listing. UL gets real particular about keeping the voltage under 165v, while Intertek seems to think that there are other ways to comply with the standard. Tesla decided to go through UL as the certifying laboratory, while Ironridge (and most everyone else) went with Intertek. Wheeeeeeee confusionnnnn
Also, other SID's exist like that. MidNite has a SunSpec-activated RSD combiner that Certainteed uses for their shingle certification, and Delta has their own SID (that I assume is not SunSpec) that works with their inverters and are listed for use with GAF Energy shingles. The stuff is out there, but there hasn't been a lot of cross-pollination between listings yet.
Well, there is a reason that Tesla calls this a Mid Circuit Interupter. Because there usually needs to be more than one in a string. Here is a link to the application instructions. Outside of the Tesla solar roof tiles the instructions indicate no more than three modules per MCI. If there are no more than six modules in the string then an MCI on the string positive and negative conductors will work.
| Tesla
service.tesla.com
Much like the NEC and AHJs the UL Standards get interpreted by the NRTLs and the same product tested to the same standard by two different NRTLs can have different results. It's a problem the Standards TCs are supposed to be made aware of and revise the standard to make it more clear and less subject to interpretation. But in reality, this often does not happen. If you remember back in the day bi-polar inverters were all listed by ETL because the UL NRTL refused to list a bi-polar inverter that used strings over 600V as a 600V inverter.
I can't see the datasheet for generic PV arrays, because Tesla locks them behind a login, but is this it?
If so, that doesn't say anything about the capability of the device, just that 165v is the max they allow for the PVHCS approval. Also, according to their instructions for their racking and generic racking, if you'd have 6 modules in a string, you'd need one on the positive side, one in the middle of the string, and one on negative side, to keep internal array voltage at 165v.
Contrastingly, the Ironridge documentation shows 6 panels in a row, with MCI-1s at either end, which shows the capability of the MCI-1 is much higher than 165v: https://ul3741.com/wp-content/uploa..._Flush_Mount_UL3741_Installation_Addendum.pdf
Tesla really firewalls everything. I can see all the large battery information behind the login but none of the other Tesla equipment. You will need to talk to someone with access to the residential equipment behind the login.
Having 2 MCIs for 6 modules indicates to me that one set of contacts is only rated to break 3 modules worth of voltage. Double it and you need two sets of contacts in series, one on one end of the string and one on the other would do it as long as they opened together. It lists the string voltage rating as 600V, there is nothing in there that says the MCI is rated for 600V. My guess is the MCI is rated for 165V which with 3 modules allows a module Voc of 55V corrected for cold temperatures that will allow for most standard residential modules.
Tesla is not very open about the specs on the MCI because people are given a recipe to follow and are not expected to assemble their own system from components. So they tell you, mix this many of this specific module with this many MCIs and you are good and you don't get to know why.
Also, there are many 3741 listed arrays that allow the full string voltage inside the array. So a limit of 165V seems unnecessarily low if imposed as part of the listing. It's not impossible since reduced voltage might help pass the listing requirements, but in general, the goal of a racking manufacturer in getting the 3741 listing is to allow full string voltage inside the array.
Fwiw, I just checked with my guy at Ironridge who works with the 3741 process and he confirmed that yes, MCI-1s can handle up to 600v. They aren't limited to 165v because of technical reasons, that's just a limitation of their PVHCS process with the UL lab.
BandGap1.1eV
Member
- Location
- East Coast
Had no idea this site existed. Thanks very much. Now we just need manufacturers to start testing with their boxes to get a global 3741 listing!
Maybe someday Tesla will release the MCI datasheet and then we will all know.