UL 3741: Photovoltaic Hazard Control and the 1000V System

This is part 2 of Sean White’s explanation of UL 3741, also known as photovoltaic hazard control and otherwise known as rapid shutdown. The UL 3741 listing came out in 2020 and it takes time for manufacturers to come up with and list products and systems. 

Surprisingly, a few months ago SMA came out with a system along with Sollega and IronRidge where they can have 1000 volts on a commercial system after the initiation of rapid shutdown.

Before we dive into this exciting (and perhaps controversial) new system, let’s take a look at the other methods to comply with rapid shutdown.

We’ve got NEC 690.12 Rapid Shutdown. It talks about controlled conductors controlled limits outside of the array boundary. What we’re interested in is inside of the array boundary, because that’s this new, exciting thing of the three different ways of doing it. 

Let’s look up 690.12B(2)(1) in the 2020 NEC (the difference between the 2020 NEC and the 2023 NEC in the code image posted here, is that the third method for inside the array boundary has been removed).

690.12(B)(2) Inside the Array Boundary

The PV system shall comply with one of the following:

1. A PV hazard control system listed for the purpose shall be installed in accordance with the instructions included with the listing or field labeling. Where a hazard control system requires initiation to transition to a controlled state, the rapid shutdown initiation device required in 690.12(C) shall perform this initiation.
   Informational Note: A listed or field-labeled hazard PV control system is comprised of either an individual piece of equipment that fulfills the necessary functions or multiple pieces of equipment coordinated to perform the functions as described in the installation instructions to reduce the risk of electric shock hazard within a damaged PV array for fire fighters. See UL 3741, Photovoltaic Hazard Control.
2. Controlled conductors located inside the boundary shall be limited to not more than 80 volts within 30 seconds of rapid shutdown initiation. Voltage shall be measured between any two conductors and between any conductor and ground.
3. PV arrays shall have no exposed wiring methods or conductive parts and be installed more than 2.5 m (8 ft) from exposed grounded conductive parts or ground.

It says, “690.12B(2) the PV system shall comply with one of the following” and there’s 1, 2, and 3. I’ll just tell you right ahead at a time, 2 is what we have been calling module level rapid shutdown. That’s the 80 volts within 30 seconds.

Then we have #3 method, which is pretty rare. We’ve called it the BIPV or building integrated photovoltaic method. That method requires no exposed wiring methods or conductive parts. That means you couldn’t do this with metal module frames. This method was removed in the 2023 NEC, which is not to say that we cannot still do this method, it just means that if we did, it would have to be part of a UL 3741 listed system, so the manufacturer would have to go and get it listed.

Now, there is a new way of doing things with this new “1000V after shutoff” listed system that includes an SMA inverter. When we are doing the 690.12 B(2)(1)/UL 3741 way, it states in the NEC, “a PV hazard control system listed for the purpose shall be installed in accordance with the instructions included with the listing or field labeling. Where a hazard control system requires initiation to transition to a controlled state, the rapid shutdown initiation device required in 690.12(C) shall perform this initiation.” 

Essentially, what we’re looking at is this hazard control system that’s listed. They changed the 2017 NEC the rapid shutdown wording, and we’re calling it a PV hazard control system (PVHCS) and take note that this is a system listing, so it requires the rack and inverter, among other things to be installed according to instructions.

UL 3741 just came out at the end of 2020, and you can go buy the standard for somewhere between $402-998, but you can also view it at the UL website for free. 

The SMA Sollega/ Ironridge UL 3741 listed system reminds me a lot of the 2014 NEC type of rapid shutdown where we didn’t have to shut down inside of the array. That’s what SMA, along with Ironridge and Sollega did, figured out a different way to do rapid shutdown without module level electronics.

Let’s talk about the SMA product. Here you can see a press release for their rapid shutdown without module level electronics. SMA got together with Sollega and IronRidge and they had this listed as a system. This system has to have particular equipment installed with it, more than just the inverter and the racking system. It also has some required wire management equipment, down to a particular brand of cable ties.

This is an interactive inverter and the array includes the rack. If the conductors coming off of this inverter are within a foot of the array, it’s within the array boundary. So as long as we’re a foot from this rack or that PV module, then we’re good to go with this type of system. If you have two arrays 2 feet apart from each other, then you would always be within a foot of an array, when you are in between them, so you would be within the array boundary.

That means we’ll see some interactive inverters up on the roof. When you turn off the AC side, all the AC wires go down to zero volts really quick, almost instantaneously. Then inside of the array, the voltage goes up to open circuit voltage from your maximum power voltage, because that’s what happens with a PV array – the voltage goes up when you turn it off. On a cold day, you potentially have up to 1000 volts inside the array after the initiation of rapid shutdown!

Also, this is a ballasted system, so it’s made for a low slope (flat) rooftop. It’s important to note that you can’t just do this with any ballasted system, even if it worked better, unless it’s listed to this UL 3741 standard.

When testing for hazard control/rapid shutdown, some of the things that they did is they verified that the firefighter was safe with up to 1000 volts inside of the array. A lot of this has to do with wire management. We can have the full voltage inside of the array. They’re claiming that it’s safe and they’ve tested it on wet firefighters. Perhaps you want to volunteer? We’ll spray water all over you in firefighter clothes. Ha!

In the IronRidge Manual, there’s a one foot boundary around the array and the inverter can be considered within the array boundary. Then we can see these must be some of the required HellermannTyton brand cable management clips and wire ties going around some PV wire and the BX IronRidge system. There is also the required RayTray, which is a plastic raceway. Of course, plastic does not conduct electricity. It’s a raceway that we can use with this particular UL 3741 IronRidge system. 

You know from the NEC that one to two family dwellings can go up to 600 volts and other buildings can go up to 1000 volts, while ground mounts, which don’t need rapid shutdown, can go up to 1500 volts. 

Remember: things can be listed as a PV hazard control system that are module level electronics or mid circuit interrupters. It’s not a new thing. It’s this new 1000V system that some may say shockingly surprising. But we know that firefighters have Chuck Norris blood and 1000V is no big deal for them (trying to make a joke and a point).

What we’re going to be seeing with people using this type of a system, which so far only works for low-slope roofs, is just an inverter at the array. You turn off the inverter on the ac side, maybe even at the main service disconnect to initiate rapid shutdown. As long as that inverter is within the array boundary (within 1 foot of the array, which includes the rack) and those conductors coming off the inverter are within the array boundary, that is considered your rapid shutdown switch. You’re not going to need an extra rapid shutdown switch unless you wanted to put your inverter farther away from the array or possibly if you were going to have some sort of energy storage system. Then you might need an extra rapid shutdown switch, because you want your system to be on when the grid goes down, if you have battery backup. But if you don’t have energy storage, turning off power to the building would shut this system off and initiate rapid shutdown as the inverter anti-islands.

That’s a new 1000V system under UL 3741! This system is controversial and new, so expect to hear some heated discussions in the future.