Solar power has gained popularity in recent years, reflecting a wider and ongoing shift away from traditional forms of energy toward renewable energy sources. Photovoltaic (PV) arrays capable of powering entire households are now more affordable than ever, making solar one of the most accessible forms of renewable energy for many people.

Modern solar systems are very safe, but – as with anything involving electricity – there are inherent risks that need to be mitigated to make them suitable for commercial and residential use. One part of this risk mitigation involves rapid shutdown devices.

Solar rapid shutdown devices are used to dramatically reduce the potentially dangerous residual energy often found in string inverter PV systems. Firstly, this keeps firefighters and other first responders safe in the event of a fire, where there could be a risk of electrocution. Secondly, it can protect components of a solar system that may otherwise be damaged during a fire.

In this article, we take a look at how rapid shutdown devices work, some of the regulations surrounding them, and when they are necessary in a solar system.

WHAT IS RAPID SHUTDOWN?

The term rapid shutdown refers to the process of mitigating risks by quickly reducing the DC voltage in a solar system. Rapid shutdown devices are pieces of equipment integrated into solar systems and specially designed to make rapid shutdown possible.

Solar panels convert the sun’s energy into usable electricity, so some parts of the system involve high voltages that can be harmful to human health. Under normal circumstances, this is safely contained by the system and kept away from people via insulated wiring, inverters, fuses, and so on. However, if a solar system is damaged by a fire, there may be a risk of electrocution to anybody near certain parts of the system, such as firefighters on the roof of a building.

If you have solar panels on your roof, hopefully you’ll never even need to make use of any rapid shutdown capabilities. However, if you are affected by a fire, then a rapid shutdown feature is extremely important in helping first responders safely put the fire out.

HOW DO RAPID SHUTDOWN DEVICES WORK?

If a fire breaks out in or around a solar power system, any rapid shutdown devices should be activated as a priority. Before anybody approaches the system, they can either cut the AC power to the system or activate the external initiator.

Rapid shutdown devices are constantly fed a signal called a “permission to operate” signal. When the AC power is cut or the external initiator is activated, this signal will no longer reach the rapid shutdown device. This causes the device to cut the serial connection of the DC-side PV modules (collections of solar cells linked together in one structure). As a result, the DC voltage will be reduced to safe levels within 30 seconds.

WHEN ARE RAPID SHUTDOWN DEVICES REQUIRED?

PV systems are very safe, but they can potentially be a fire hazard if damaged, when not installed correctly, or in the case of faulty equipment such as a malfunctioning junction box or inverter.

In string inverter systems in particular, voltages can be very high. In string systems, many solar cells are connected in a series, multiplying the voltage in the system up to as much as 1,500 V.

If there is a fire, simply turning off a string inverter solar system does not remove the risk. Potentially dangerous levels of DC voltage can remain, posing a risk to anybody in the vicinity, such as first responders fighting the fire from a rooftop. This is where rapid shutdown devices come into play.

Rapid shutdown devices can be integrated into solar systems to mitigate risk. In the event of a fire, the rapid shutdown device can be activated by firefighters, quickly reducing the dangerous levels of electricity and reducing the voltage to a harmless level.

This allows first responders to safely do their work without the risk of electrocution. Within 30 seconds, the devices lower the DC voltage to a safe level, so there’s no delay for first responders trying to put out the fire. Firefighters can safely put out the fire, prevent it from spreading, and protect you and your property.

Because rapid shutdown devices are so effective at improving safety, there are often recommendations and/or regulations in place to ensure they are used in residential and commercial solar systems, although this depends on the location. In the US, for example, several elements of the National Electrical Code (NEC) refer to the use of rapid shutdown devices, but different states enforce this in different ways.

Other countries around the world follow their own distinct sets of recommendations and regulations, with many having no regulations at all. However, as solar becomes a more prominent form of energy, this is liable to change at any time. The important thing to remember is, wherever you are in the world, make sure you understand the relevant rapid shutdown rules before you install a solar system.

WHERE DOES THE NEC SOLAR CODE APPLY?

In the US, the NEC includes a number of recommendations about RSDs. However, these recommendations are not federal law, meaning different states, cities, and municipalities around the US can choose whether or not to follow them.

Image – https://www.nfpa.org/NEC/NEC-adoption-and-use/NEC-adoption-maps

The NEC guidelines have been updated several times, with four versions released between 2008 and 2020. Some states do not impose a requirement to follow the NEC rapid shutdown guidelines, and others follow different versions of them. Due to these differences, it’s important that anyone buying or installing solar systems understands the rules in their specific region.

DO ALL INVERTERS HAVE RAPID SHUTDOWN CAPABILITIES?

Different types of inverter systems have distinct rapid shutdown capabilities. Some systems are inherently capable of rapid shutdowns, whereas with others it is necessary to install additional equipment.

There are multiple different types of solar systems so, on the surface, it can be difficult to know if a given system has rapid shutdown capabilities. The best way to find out is to ask your supplier or installer about rapid shutdown features on their different solar systems.

For most people, microinverter-based systems are likely to be the safest option. String systems often don’t have rapid shutdown capabilities and, as discussed earlier, they can carry dangerous levels of voltage even after being turned off. Microinverter systems are designed to include rapid shutdown capabilities as standard, and they are built in a way that naturally limits the level of voltage in any given section to a safer level.

Do you already have solar installed on your property and want to know if it supports rapid shutdown? Well, to complicate matters, some string systems do have rapid shutdown capabilities, but only if equipped with a rapid shutdown device or an optimizer. If you’re not sure about your own system, it’s best to check with your installer or supplier.

In the two following sections, we take a look at some of the major differences between string inverter and microinverter-based systems and how their rapid shutdown capabilities can differ.

STRING INVERTERS AND RAPID SHUTDOWN

String systems do not have inherent rapid shutdown capabilities. In order to integrate rapid shutdown capabilities and comply with relevant regulations (depending on location), these systems need to be fitted with rapid shutdown devices. There are now some examples of string inverter rapid shutdown systems available for out-of-the-box use, using integrated rapid shutdown devices, but this is not necessarily the standard set-up.

In string systems, a single module may only carry a relatively low voltage of 40-50 V, which is not dangerous. However, when many modules are linked in a series, the voltage in the system may reach 1,000 V, or even as high as 1,500 V. This level of voltage has the potential to cause serious injury or even death. Normally, this would not be an issue. Firstly, all flowing electricity is contained within the system with various safeguards in place to protect humans and prevent overheating or fires. Additionally, solar systems are usually installed well out of the way of regular human contact (such as on a rooftop), so it’s unlikely that anyone other than an installer will go anywhere near it.

However, in case of an unforeseen event like a fire, the system can easily become damaged, rendering the normal safeguards ineffective. It’s also likely that people – usually firefighters – will be in much closer contact with dangerous parts of the system, possibly exposing them to the high voltages. Such high voltages can also be a fire hazard themselves if the system is damaged, for example if a wire becomes loose via improper installation, an earthquake, or other forms of damage.

Furthermore, string systems can retain dangerous levels of residual voltage even after the system has been switched off. To deal with this danger, first responders generally have to wait for the panels to completely burn out, often significantly delaying firefighting attempts. 

HOW RAPID SHUTDOWN WORKS ON MICROINVERTERS

As we’ve seen, different solar systems have varying rapid shutdown functionality. String inverter-based systems, for example, may not have any rapid shutdown features.

Fortunately, there are some systems that have inherent built-in rapid shutdown capabilities, known as microinverter systems. In these systems, components called microinverters are installed alongside your solar panels. The number of microinverters varies depending on the system, with some systems using one microinverter per panel and other involving multiple panels connected to a single microinverter.

Regardless of the precise set-up, the concept is largely the same, with the microinverter’s function being to convert incoming DC power into usable AC power.

When specialized PV modules are connected independently to microinverters, the voltage across each module is limited to 40 V, dramatically reducing the risk to humans as well as the risk of fire. Systems using microinverters in this way do not retain dangerous levels of residual voltage after being switched off, making them safe for first responders almost immediately.

Because of this in-built rapid shutdown functionality, microinverter systems may seem like the obvious choice for everyone. However, fitting a string inverter system with rapid shutdown devices can be a more cost-effective option in many cases, so it could be worth considering either system. Additionally, many households already have string inverter systems, so replacing this with a microinverter system would generally not be necessary, and installing rapid shutdown devices would be a preferable option.

Whatever system you have or are interested in purchasing, we strongly recommend ensuring it has rapid shutdown capabilities in order to reduce the risk of fires and keep your family, first responders, and your property safe in the unlikely event of a fire.

SAFER SOLAR WITH HOYMILES

Solar panels are a reliable, cost-effective, and clean way to fulfill our energy needs, and their popularity is only growing as the world moves away from fossil fuels. To ensure you can enjoy the benefits of solar power as safely and sustainably as possible, and in compliance with local regulations, it’s important to have integrated rapid shutdown technology.

Hoymiles microinverters feature integrated rapid shutdown capabilities, or your existing string inverter system can be easily fitted with our rapid shutdown devices, giving you peace of mind that your solar system is operating safely. Our microinverters also come with built-in monitoring functionality accessed via our innovative S-Miles Cloud dashboard to help you keep track of the performance and safety of your system from anywhere.