Solar panel rapid shutdown requirements: a guide for consumers

National Electrical Code regulations require that most solar arrays installed in the United States must be able to be quickly turned off by first responders. Here's how that affects the design of your system.

Photo of an on-off power switch

Solar panels are extremely safe. According to data from Germany, only 120 fires out of 1.4 million solar installations have been traced back to solar equipment.

Compared to the 44,000 electrical fires that occur in the United States every year, the risk of fire caused by solar equipment is very low.

However, a greater hazard is the risk of live wires to firefighters who might need to work near solar equipment during an emergency. That’s what rapid shutdown requirements are for, and they’re the law in most states in the US.

What risk does solar equipment pose to firefighters?

If a building catches on fire, firefighting crews might need to access the roof. If solar equipment is installed there, the wiring can pose a hazard.

This is because the wires for even a small solar array might be carrying several kilowatts of electricity at hundreds of volts. This is dangerous whether those wires are carrying DC power (as is the case with a string inverter) or AC power (if microinverters are installed).

Every solar array has a power cutoff switch that is typically located near the electric meter or panel. However, that switch only disconnects the solar array from the grid. By itself, it doesn’t disable the solar panels themselves, which will continue to generate electricity as long as the sun is shining.

Because of this, firefighters need a way to disable a solar array right at the panels themselves.

What are module-level rapid shutdown requirements?

Rapid shutdown means that firefighters need a way from the ground to flip a switch and kill the power at the wires that surround a solar array.

The National Electrical Code (NEC) is an electrical code that is adopted by states or municipalities. Since the 2014, the NEC has specified rapid shutdown requirements.

However, a big change was made by NEC in 2017. Section 690.12 of the NEC 2017 code specifies that a rapid shutdown feature must be able to drop the voltage in wires within 1 foot of an array to 80 volts within 30 seconds.

One foot is a small boundary, so in practice this means that a rapid shutdown feature must be able to control every panel in the array. That’s why the NEC 2017 code is called a module-level rapid shutdown requirement. (Module in industry lingo means a solar panel.)

For a firefighter who might need to swing a metal axe on a roof with solar panels, this is an important safety change.

How do you comply with module-level shutdown requirements?

The most straightforward way to satisfy the module-level shutdown requirement is to use microinverters or power optimizers.

These are inverter components that are mounted behind the panels and do some or all of the work in converting DC power from the panels into useful AC power.

Because they’re mounted right at the panels, microinverters and power optimizers - as known as module-level power electronics (MLPE) - are able to satisfy the stringent 1-foot distance requirement in NEC 2017.

More than 80% of residential solar installations now use MLPEs. This is in part because they have big advantages when it comes to dealing with shade, but they also satisfy the rapid shutdown requirement.

How is rapid shutdown activated?

When microinverters or power optimizers detect that grid electricity has been disconnected, they will turn off automatically.

This means that if you throw the disconnect switch on the array, rapid disconnect will be activated for all the panels in the array. Within 30 seconds, any high voltage electricity in the system will have dissipated.

Photo of solar array disconnect switch
Example of a solar array disconnect switch.

By the way, this rapid shutdown feature also means that your solar array will automatically turn off in a blackout, even if the sun is shining. If you want to continue to use your solar panels when the grid is down, you’ll need a battery system.

Where is the shutdown switch located?

The shutdown switch will be located near your electric meter or panel.

The latest NEC 2020 code added a new requirement:

For one- and two-family dwelling units, all service conductors shall terminate in disconnecting means having a short-circuit current rating equal to or greater than the available fault current, installed in a readily accessible outdoor location.

This means that if your state or municipality has adopted NEC 2020, your switch must be located on an outside wall, making it easier for firefighters to access it.

However, as of 2022, only 24 states have adopted NEC 2020. Some major solar markets, including California, only need to comply with NEC 2017 at the moment. You can expect this to change in the future.

What about string inverters?

Even though inverters and power optimizers have advantages when it comes to shading, if you have a sunny roof without any obstructions, a string inverter can be an excellent and lower-cost option.

However, string inverters don’t have module-level electronics, which means that there isn’t an easy way for them to comply with NEC 2017.

To comply, string inverter companies have release module-level devices that can shut down a panel. For example, SMA has the SunSpec rapid shutdown device.

The SunSpec device retails for about $40. That’s less than the cheapest Enphase microinverter which is currently selling for about $80. However, that added cost erodes the main advantage of a string inverter, which probably explains why they’re losing market share to microinverters and power optimizers.

How many states must meet module-level rapid shutdown requirements?

At the time of writing, 42 states have adopted either NEC 2017 or NEC 2020, which means that module-level shutdown requirements apply.

For the latest status, check the website of the National Fire Protection Association.

#Inverters #Safety

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