BY MATTHEW PAISS
The United States is at the threshold of an energy transition. It has been said that the amount of solar energy that hits the earth in one hour could power the entire planet for one year. As a result of the 2007 California Solar Initiative (www.cpuc.ca.gov/puc/energy/solar/), our state is preparing for a massive increase in solar electric installations, which could result in as many as one million solar roof systems in the next decade.
More and more solar electric or photovoltaic (PV) panels are being installed on the roofs of homes, barns, outbuildings, commercial structures, and even on free-standing carports, places where firefighters would most likely cut ventilation openings. How will this affect our operations and safety?
As always, the fire service must stay on top of new technology and adapt to the new developments in building and vehicle materials while maintaining safety and effectiveness. Although we have had little impact on the extensive use of gusset plates and lightweight trusses in construction, we are definitely “at the table” when it comes to PV design and installation guidelines.
Firefighters must understand the particular hazards they face when fighting a fire in a PV-equipped structure. The potential firefighter hazards on such structures are primarily from tripping on conduits and electrical shocks. The basic information provided here will help in identifying when a residential PV system is present, its basic operation, what can hurt us, and what is safe. Much has been written lately on this topic. Some information, unfortunately, has been inaccurate, and I hope to clear up some of the confusion.
PV SYSTEM BASICS
There are two types of solar panels used in residences. Photovoltaic (PV) panels generate electricity (photo 1), and thermal panels, which occupy a much smaller area than PV panels, just heat water and do not generate electricity (photo 2). Thermal panel systems that include a rooftop storage tank can weigh up to 1,500 pounds, depending on the number of panels (photo 3). In most thermal systems, the panels are on the roof, and the storage tanks are at ground level. There are usually between four and six panels in a water-heating system. The only hazard they present is the dead load. Although the water inside these panels may be hot (usually no more than 180ºF), there is no reason to open or cut into these panels. Shutting off the water and power to the structure will eliminate any pressure in these water-heating systems. Pool-heating panels are plastic, have no weight issues, and can be cut through as needed. This article deals only with PV panels, also known as modules in the industry. Although commercial PV systems operate similarly, the sizes, voltages, and currents involved are dramatically different.
(2) Photos by author.
(3) Photo courtesy of Rheem Manufacturing Company.
PV systems have three primary components: modules, inverters, and conduit. Most of the modules installed today are comprised of many silicon cells wired together and enclosed in an aluminum frame with a glass cover. Each module is roughly 30 × 50 inches in area and weighs around 30 pounds. A group of modules mounted on a structure is called an array (photo 4). An array’s weight load on a roof is usually less than five pounds per square foot; the members of a truck company put a far greater concentration of weight on the roof than the PV array. The modules generate electricity from sunlight, have no moving parts, are generally rated at between 125 and 200 watts each, and produce between 24 and 48 volts of direct current (DC) power. When modules are attached to each other in a series, the voltage increases. Most residences will have anywhere from 15 to 40 panels, depending on the home’s electrical needs. These arrays will generate anywhere from 2,000 to 5,000 watts (two to five kilowatts) of power in optimal sunlight conditions, at between 120 and 600 volts DC. The current ranges between five and nine amps. PV systems will cause the electric meter to run backward on a sunny day when there is more power being generated than the house is using. At nighttime, the meter just spins forward again, using up whatever “credits” were gained during the day.
INVERTERS AND DISCONNECTS
Since the modules produce DC power, they are wired to an inverter, which converts the voltage to alternating current (AC) and then feeds the electricity directly back into the main power distribution panel. Photo 5 shows, from left to right, the main electrical meter and circuit breakers, the AC disconnect, the inverter, and the DC disconnect.
(5) Photos courtesy of Independent Energy Systems.
Since the inverter requires AC from the power company to do its job, shutting off the residence’s main circuit breakers also shuts down the inverter. This means that no AC power is being sent into the house. Similarly, if there is a local power outage on a bright sunny day, the system cannot feed power back into the power company grid because the inverter is down, which is always a disappointment to PV system owners.
Switchblade-type disconnects are often mounted on one or both sides of the inverter to shut off the DC power entering the inverter (the DC disconnect) and the AC power leaving it (AC disconnect). These disconnects are primarily used to enable technicians to safely service the inverter.
Caution: The wires from the array to the inverter are live in the daytime hours, even when it’s overcast. The DC disconnect does not shut off the power in the DC conduit coming from the array; it just keeps it from entering the inverter. The DC conduit is still live between the array and the inverter DC disconnect. As you can see in photo 5, this inverter is mounted close to the main power company service disconnect. Inverters may also be installed in other locations such as garages, basements, and outbuildings.
Why isn’t there a rooftop disconnect to kill the DC power in the conduit? Although a reasonable question, there are some very good reasons they are not installed. One is that when we operate a disconnect, we expect it to do just that! On some PV systems, there could be a separate array on another section of the roof or on a barn or garage that could backfeed power through the inverter into the conduit that you thought was deenergized. There are also some issues with the long-term reliability of components. Some have suggested using newer technology for contactor-type disconnects or considering the high-voltage DC components hybrid vehicle designers have developed. Since we will need to rely on this disconnect 20 to 30 years down the road, it is much safer to just assume that the system is energized in the daytime. Additionally, the California State Fire Marshal’s Office specifically recommends against using rooftop disconnects on residential PV systems. Along the same lines, PV industry technology changes much faster than the fire service, so today’s firefighter hazards may not be the same in five years or even by the time you read this!
If a firefighter attempted to disable the array by putting an ax through a module or through the conduit leading from it to inverter, he would likely experience 520 volts in his hands at the end of an array of 12 44-volt DC modules.
The inverters are designed with very good ground fault interruption (GFI). If there is any ground fault between the wires and any section of the metal conduit, the modules themselves, or their mounting racks, the inverter trips and opens the circuit. This is also the case when the inverter is shut down. When there is an open circuit, the panels will still put out full voltage (in the daylight), but there is no current flowing unless they find a ground, like a firefighter and his ax.
Labels on the main service panel will indicate the presence of a PV system when we can’t find an inverter (photos 6, 7). The labeling may be on the outside or inside of the main panel. What you are looking for is the dedicated breaker for the inverter. It may be labeled “Solar Disconnect” or some variation. This breaker may be in a subpanel inside the structure as well, but there will always be a label on the main electrical panel stating the presence of a second generating source on-site. There may also be labeling along the conduit run (photo 8). The labels may be the only identifiers you will ever see, as the array may not be visible and the inverter may be in the fire. This makes securing the electrical utilities that much more important. Look for labels!
(8) Photos by author.
On the West Coast, vertical ventilation is one of the truck company’s primary tasks after search. Understandably, this is not true in all areas of the country. The array’s location and size may affect performing a coordinated vertical ventilation operation with the crews on the hoseline inside. Removing modules is unsafe and delays ventilation. Additionally, although the modules can bear the weight of a person, they can be extremely slick if wet, and standing or stepping on them is not recommended. There is no reason for a firefighter to stand on the array.
Regarding ventilation, you must decide the correct site to cut based on fire and building conditions. Once you locate it, though, ensure that you cut only the roofing material. Since the conduit from the array to the inverters may be energized, you want to stay clear of it. Look out for an exterior conduit that runs along the roof; it may be painted the same color as the roofing and be hard to see. It is a serious and definite tripping hazard.
If the conduit run penetrates the attic space, it may or may not be attached to the bottom of the rafters. So it is important that you be skilled in rolling the rafters and keeping your cuts shallow. Don’t cut all the way through, or you may cut the conduit (photos 9-11).
(9) Photo courtesy of Independent Energy Systems.
(11) Photos by Craig Allyn Rose.
If it is a daytime fire and ventilation is not needed or is complete and salvage operations are underway, you can put a tarp over the array. Covering the entire array with a totally opaque material, such as heavy dark canvas tarps or black plastic, will prevent the array from generating electricity (photo 12). Note that blue plastic tarps do not work. The covering material must cover the entire array, including subarrays mounted on other roof pitches or outbuildings. However, even if enough opaque covering materials are available, wind and structural conditions may eliminate this option. But it’s just another tool for you to consider.
(12) Photo by author.
If the conduit is intact, it is still grounded from the array to the inverter, so any wires that may be shorted to it from high heat melting the insulation off the wires inside are not energizing it. It is safe to touch. This is one of the benefits of using a metallic conduit; it acts as a separate ground wire, directing any current to ground. But if there has been any roof collapse, assume that the conduit is no longer grounded; exercise caution, and avoid contact with it.
If the array cannot be tarped, the crews inside must be careful when pulling ceiling, since they may contact the conduit with their hooks. On the West Coast, there is no reason to see metal conduits in an attic space on a residential structure, since Romex is all that is required to be used for typical residential wiring. If I see metal conduit in a residential attic, that’s a red flag that a PV system may be present. Keep in mind that in commercial structures, metallic conduit may be used everywhere.
If your department carries noncontact voltage detectors, remember that they detect only the presence of AC voltage, not DC voltage. There is no noncontact detector on the market for detecting DC voltage.
Clearly, in daytime fires where the attic has been exposed to severe heat or fire and DC conduit runs through it, take care to avoid contact with the conduit.
Finally, remember that in a nighttime fire in which the attic space has been exposed to severe heat damage, the conduit and wires inside may have become compromised. It is possible that when the sunlight contacts the array the next day, it could result in some arcing. It’s a good idea to recheck a structure in the morning for arcing or rekindling from arcing until a qualified solar contractor can respond to disconnect the array connections. If you need to turn breakers back on (if the incident was minor), go ahead. Remember to close any disconnects that were opened. The system should start up normally without any additional steps. Bear in mind that you should do this only if you are sure that the PV system was not damaged or the cause of the incident.
Remember the following six main safety points when dealing with fires in PV-equipped structures:
- Daytime = Danger; Nighttime = No Hazard
- Inform the incident commander that a PV system is present.
- Securing the main electrical panel does not shut down the PV panels; in the daytime, electricity continues to flow from the panels to the inverter.
- At night, apparatus-mounted scene lighting does not produce enough light to generate a dangerous amount of electricity from the arrays.
- Cover all PV panels with 100-percent opaque material to block sunlight and stop the generation of electricity.
- Stay away from the panels and conduit. Don’t break, remove, or walk on the PV panels.
Determining the presence of a PV system is the key to understanding these systems and not getting hurt. There is a much greater risk to firefighters from the line voltages that come into every structure than from PV systems. In fact, on a recent fire as I was on the roof, I noticed that the utility service drop was easily within contact of our firefighters. The wires’ insulation was missing in some spots. Since this was before the utility service drop entered the meter, the current would not be protected by circuit breakers but by the limits of the wire, which could be as much as 1,000 amps!
As I speak with firefighters around the country about solar PV systems, eventually the conversation turns personal and some ask, “How can I install this in my home or firehouse?” Clearly, there is great interest in reducing utility bills, our dependence on oil, and greenhouse gases. These systems are here to stay, and the next one could be on your firehouse!
MATTHEW PAISS is a 13-year veteran with the San Jose (CA) Fire Department and a fire engineer with Truck Co. 13. Before entering the fire service, he was a service engineer in the semiconductor industry. Paiss teaches firefighter classes in photovoltaic (PV) safety and has degrees in fire science and solar technology. He is a member of the California State Fire Marshals PV Task Force.