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The greater utilization of photovoltaic systems across the U.S. has made traditional firefighter tactics for suppression, ventilation and overhaul more complicated. This change leaves firefighters vulnerable to potentially unrecognized exposures, particularly due to risk of electrocution.

Although the electrical and fire hazards associated with electrical generation and distribution systems are well known, PV systems present unique safety considerations.

Under a U.S. Department of Homeland Security Assistance to Firefighters grant, recent research focused on the fire service concerns of interacting with PV systems during suppression, ventilation and overhaul operations.

The objective of this work was to document firefighter vulnerability to electrical and casualty hazards when personnel are mitigating a fire involving PV modules and support systems. It involved a technical panel of fire service, PV and research experts who were selected based on their previous experience with research studies, fire service practices, PV system technology, professional affiliations and dissemination to the fire service.

Design of the experiments was based on the PV system’s exposure to a fire and the resulting hazards for firefighters. The research included experiments to develop empirical data for understanding the magnitude of these hazards and unsafe conditions, including the following:

• Assessment of PV power using a variety of light sources;
• Shock hazard due to severing of conductors and assessment of potential shock hazard from damaged PV modules and systems;
• Shock hazard due to the presence of water and PV power during suppression activities; and
• Shock hazard due to direct contact with energized components during firefighting operations, emergency disconnect and disruption techniques.

In order to aid firefighters’ understanding of the significance of the results of this project, hazards were quantified into four levels using milliAmpere (mA), the base metric unit for electric current. Safe levels are defined as 0-2 mA, “perception” levels are defined as 2.1-40 mA, “lock on” levels are defined as 40.1-240 mA, and electrocution can result at levels greater than 240 mA.

 

Assessment with light sources

Sunlight is the main source of illumination for a PV system, but the ability of other sources of light to energize a PV system has been unknown. Therefore, experiments were conducted evaluating three types of lighting:

• Artificial light from fire trucks that used scene lighting during a nighttime fire event;
• Light from an exposure fire; and
• Light from a low ambient source, such as a full moon.

If a fire is burning on or near a roof that has PV modules installed, one concern is that light from the fire could produce enough illumination to cause the PV array to produce a hazardous level of electricity. For this experiment, two of the framed modules (230 W, nominal 37 V open circuit, 7.5 A short circuit) were mounted side-by-side on a mobile cart that could be moved various distances in a dark room, with overhead lights only left on for egress.

It is often assumed that once the sun sets, the PV system is no longer generating electricity, and, thus, no danger of an electrical hazard is present. In order to validate this assumption, an experiment was conducted from noon to noon during a full moon phase.

The results of the experiment indicate that when illuminated by artificial light sources, such as fire department light trucks or an exposure fire, PV systems are capable of producing electrical power sufficient to cause a lock-on hazard.

 

Severing of conductors

During firefighting operations where PV systems are involved, a firefighter may be subjected to an electrical shock hazard due to the cutting of live electrical PV conductors or raceways. Several experiments were conducted to demonstrate the potential electrical hazards from the severing of conductors in PV systems using cable cutters, an axe, rotary and chainsaws.

A wire connector was attached to accessible metal hardware on each device to represent a firefighter coming in contact with the metallic portions of the tool. In addition, PV modules representing metal rack-mounted, building-integrated and laminate-on-metal-roof technologies were investigated

The results of the experiments indicate that a firefighter may be subjected to an electrical shock hazard due to damaged PV system components, as live electrical parts may become exposed. Some of this damage may occur during the fire or overhaul operations.

Additionally, damage to wiring and modules from tools may result in both electrical and fire hazards. The hazard may occur at the point of damage or at other locations depending on the electrical path. Metal roofs, in particular, present unique challenges in that the surface is conductive, unlike other types, such as shingle, ballasted or single-ply roofs. Care must always be exercised during ventilation and overhaul.

 

Suppression techniques

Safe firefighting activities normally require that the building’s electrical power be disconnected before water is applied to a building fire during suppression activities. The following experiments were conducted to investigate potential shock hazard from damaged energized equipment and direct impact from hose streams.

Although some guidelines already exist for safe approach distances for firefighters with hose streams near live electrical equipment, guidelines based on voltage and current levels expected in PV installations had not previously been explored.

Thus, experiments were conducted using different nozzles, water pressure, conductivity, voltages and distances. The electric shock hazard due to application of water is dependent on voltage, water conductivity, distance and spray pattern.

The research found that slight adjustments in water stream during firefighting and distance impacted the risk of shock. In addition, pooled water or foam may become energized due to damage in the PV system.

During structural firefighting operations, electrical enclosures may be directly struck - intentionally or unintentionally - by a hose stream. To evaluate the potential shock hazard, electrical outdoor enclosures were subjected to hose stream tests. Outdoor weather exposure-rated electrical enclosures are not resistant to water penetration by fire hose streams. A typical enclosure will collect water and present an electrical hazard.

 

Effect of direct contact

During a fire event, a PV array - including modules, components and their associated wiring - may be subjected to thermal and mechanical stresses that can result in damaged energized devices and wiring. Direct contact with these exposed energized PV system components could lead to a firefighter’s exposure to an electrical shock hazard.

In order to address this concern, two sets of experiments were conducted on functioning PV arrays. The first set represented a room of fire within a building that transitioned to a structure fire beneath the array.

The second set of experiments represented two fire conditions: a room of fire venting out an open window and a debris fire under an array. The room utilized wood pallets as a fuel source, while the debris fire used pine needle straw.

Two additional experiments were conducted using the same fire test fixture and metal-frame modules to explore a confined fire directed from inside the bunker to the roof through a window and a fire originating on the roof from material and debris located under the modules.

These additional experiments were designed to terminate the fire before the roof collapsed, thus providing a means to investigate tactical challenges for the fire service in performing overhaul operations with a partially damaged but potentially electrically hazardous roof array.

Results from the experiments indicate that severely damaged PV arrays are capable of producing hazardous conditions ranging from perception to electrocution. Damage to the array may result in the creation of new and unexpected circuit paths. These paths may include both array components (such as module frames, mounting racks and conduits) and building components (such as metal roofs, flashings and gutters).

Responding personnel must stay away from the roofline in the event that the modules or sections of the array slide off the roof. Fires under an array - but above the roof - may breach roofing materials and decking, allowing fire to propagate into the attic space.

Hence, caution must be exercised during all operations, both interior and exterior. One should contact a local professional PV installation company to mitigate potential hazards.

 

Emergency disconnect

Lastly, a main test array consisted of 26 framed PV modules serving as a test bed with a 5,980 W total rated power. With all 26 framed modules wired in series, the maximum open-circuit voltage was 964 V and, at maximum power, the voltage would be 792 V. The roof structure also included an open testbed area where additional modules and/or different PV technology devices were mounted and wired to the main array for test purposes.

During firefighting operations, the best a firefighter can often do to de-energize parts of a PV system is to open all PV disconnecting means, realizing that the conductors and components between the PV modules and the disconnect will remain energized. Currently, the only effective way to de-power is to block illumination of the PV module to provide a safe work environment.

However, turning off an array is not as simple as opening a disconnect switch. Depending on the individual system, there may be multiple circuits wired together to a common point, such as a combiner box. All circuits supplying power to this point must be interrupted to partially de-energize the system. As long as the array is illuminated, parts of the system will remain energized.

Tarps or foam may be used to cover the modules in the array to block light. Tarps offer varying degrees of effectiveness to interrupt the generation of power from a PV array. The research did find that heavy, densely woven fabric and dark plastic films reduce the power from PV systems to near zero.

However, caution should be exercised during the deployment of tarps on damaged equipment, as a wet tarp may become energized and conduct hazardous current if it comes into contact with live equipment. The use of tarps that completely block light presented the best option. The research showed that firefighting foam, in contrast, should not be relied upon to block light.

In summary, the results of these experiments are intended to provide a technical basis for the fire service and PV installation industries so that they can better examine their equipment, tactics, standard operating procedures and training content.

The two industries have made progress in working together to begin addressing firefighter safety issues, and this research provides further intelligence and findings for consideration as these discussions and partnerships continue. R

 

Robert Backstrom is a research engineer within Underwriters Laboratories’ corporate research department. He can be contacted at (847) 664-2250 or robert.g.backstrom@ul.com.