Rapid Shutdown for PV Systems: Page 2 of 7
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Controlling PV System Circuits
Firefighters are accustomed to encountering roof-mounted electrical equipment, particularly on low-slope commercial roofs. However, PV systems are unlike conventional ac electrical equipment in terms of size, electrical characteristics and prevalence on residential roofs. As rooftop PV systems have become increasingly common in the built environment—especially in California, Hawaii, Massachusetts, New Jersey and other major US solar markets—demand has increased for firefighter training resources, such as CAL FIRE’s Fire Operations for Photovoltaic Emergencies (see Resources), to educate first responders about PV power systems.
One of the first questions asked at firefighter trainings is: “How can we shut the PV system down so that we won’t get shocked when responding to an emergency?” Unfortunately, there is no simple answer to this question, given the way most PV systems are designed and deployed today. Using rooftop dc disconnects to control PV power circuits is inherently problematic. The rapid-shutdown requirements for PV power systems on buildings address this problem.
Problem with dc disconnects. One of the first activities that firefighters perform when responding to a fire in a structure is to control the energy utilities. Often this involves closing a gas shut-off valve and opening an ac service disconnect. Many fire departments have sought to address the shock hazard associated with PV system circuits in an analogous manner by requiring rooftop dc disconnects in an attempt to eliminate that source of power. However, dc PV power circuits and conventional ac circuits are not analogous, and the differences can be life threatening. Though rooftop dc disconnects are important to the process of shutting down and isolating a PV array, they generally stop the flow of current only and do not provide voltage isolation as they would in a utility-supplied ac power system.
Requiring a rooftop dc disconnect seems like a simple and logical approach to shutting off a PV system. However, opening this disconnect does little, if anything, to reduce shock hazard. In fact, you could argue that opening a rooftop dc disconnect provides firefighters with a false sense of security that could increase the likelihood of an electrical shock. Since turning off a rooftop dc disconnect in a PV system does not de-energize the conductors on either side of the switch, NEC 690.17 requires the following warning label on dc disconnects:
WARNING: ELECTRIC SHOCK HAZARD.
DO NOT TOUCH TERMINALS.
TERMINALS ON BOTH THE LINE AND LOAD SIDES
MAY BE ENERGIZED IN THE OPEN POSITION.
Many PV systems are still dangerous even after you open a rooftop disconnect because there are typically power sources on both sides of the switch. On the one hand, the PV power source energizes the conductors on the PV array side of the switch during daylight hours, typically at voltages greater than 300 Vdc and up to 1,000 Vdc in nonresidential applications. On the other hand, the PV output side of the switch is often connected to the dc input bus of an inverter, which is another potential source of power. This side of the switch remains energized as long as PV output circuits are connected in parallel at a subarray or inverter-input combiner. For example, in a scenario where 15 dc disconnects are installed on a commercial rooftop—one at the location of each source-circuit combiner box—if an emergency response team leaves only one of these 15 in the on position, that could leave all array wiring fully energized and potentially lethal. Even where there are no parallel-connected PV power circuits, large dc input capacitors at the inverter may keep this side of the switch energized long after responders open the rooftop disconnect.
Though the fire service stakeholders who developed the CAL FIRE Guideline discussed rooftop dc disconnects at great length, representatives from the solar industry clarified that these would have little impact on improving safety for first responders. As a result, the published Guideline does not include requirements related to rooftop dc disconnects. However, the rationale for this intentional omission was not published in the Guideline, which has led some jurisdictional authorities and fire departments to adopt additional local requirements for rooftop dc disconnects.