Powering Down Carefully

Safety for solar photovoltaic (PV) systems has come a long way since the first installation requirements were inserted as Article 690 into the 1984 National Electrical Code (NEC). Between the 1984 NEC and the 2014 edition, there have been hundreds of proposals and comments to change the requirements in Article 690, and many of these suggested changes have been implemented into the NEC. A very significant proposal and accompanying comment for the 2014 NEC has resulted in a change that will help first responders, electricians and other electrical service personnel deal with the shutdown of a PV system.

New Section 690.12, “Rapid Shutdown of PV Systems on Buildings,” reflects this significant change. A clear understanding of this section should provide the knowledge to ensure safety from shock and fire for PV systems and conductors on or in a building.

In the 1984 NEC, an inverter or power-conditioning unit was used to convert the solar modules’ direct current (DC) output to alternating current (AC). The Code required the inverter or power conditioner to match the voltage level or waveform of the AC output to the voltage and wave form of the utility company’s power. Basic requirements were in 690-61 for disconnecting power originating from the power-­conditioning units to ensure that output from an interactive power-conditioning unit that was connected to utility power was automatically disconnected and could not be reconnected until the utility voltage was restored. In addition, Section 690.13 required a means to disconnect all current-carrying conductors of the PV power source from all other conductors in a building or structure.

Section 690–14 also required the PV system to comply with all of the disconnecting requirements in Part H of Article 230 for power-company-supplied services for premises wiring. All of the requirements located in Article 690 in the 1984 NEC and subsequent NEC editions were safety-related. First responders and others involved in disconnecting power to PV systems struggled to determine the necessary requirements to shut these systems down. The PV modules, panels and arrays were not shut down since solar cells develop voltage with the possibility of current, whenever there is sunlight.

Section 690.12 in the 2014 NEC states that PV systems installed in or on buildings must include a rapid-shutdown function to control specific conductors of the PV system. The control of these circuits applies to PV system conductors where 5 or more feet of the conductors are located inside the building, or where these PV conductors are more than 10 feet from the PV array and where the conductors are located on or inside the building. The controlled conductors are limited to not more than 30 volts (V) and 240 volt-amperes (VA) of power within 10 seconds of the rapid-shutdown initiation. The 10-second time period permits capacitors installed in the PV system to discharge to 0V. The voltage and power values must be measured between any two conductors and between any conductor and ground. The equipment that performs the rapid shutdown must be listed and identified.

For an example of these requirements, let’s assume the PV power source (string of modules) has a combined output that measures 30V, the maximum current for the string would be a total of 8A (30V × 8A = 240 VA) to match the requirement in 690.12(2). This power-source circuit (the definition in 690.2 states these circuits are from modules to the common connection point of the DC system) is connected to other power-source circuits at a combiner box. The output of the combiner box is called a PV-output circuit (see the definition in 690.2) and connects the combiner box to the inverter. A control device can be installed at the combiner box with the other end of the control circuit located at the inverter location or at the service-disconnecting means location.

In accordance with Section 690.56(C), Section 690.12(4) requires rapid-shutdown-initiation methods to be labeled with a reflective permanent plaque or directory if the building or structure has both utility service and a PV system. The label must state the following in capitalized text: “PHOTOVOLTAIC SYSTEM EQUIPPED WITH RAPID SHUTDOWN.” The letters must be at least 3/8-inch high in white letters on a red background.

Rapid-shutdown system methods and design are not covered in the NEC but are addressed in the PV product standards. There is more than one method to accomplish this rapid shutdown.

About the Author

Mark C. Ode

Fire/Life Safety, Residential and Code Contributor

Mark C. Ode is a lead engineering associate for Energy & Power Technologies at Underwriters Laboratories Inc. and can be reached at 919.949.2576 and Mark.C.Ode@ul.com.

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