July 2, 2019
In the May 2019 issue of the National Association Fire Investigator newsletter, Timothy L. Morse, Ph.D., P.E., CFEI, senior managing engineer at Exponent, Joel E. Sipe, Ph.D., P.E., CFEI, senior managing engineer at Exponent, and Scott A. Wright, Ph.D., P.E., CFEI, principal engineer at Exponent, had published their article, "Rooftop Solar Fire Investigation."
Rooftop solar photovoltaic (PV) installations in the United States have becoming increasingly common in the past two decades. Solar PV systems are made up of individual solar cells, which are silicon based wafers that each produce approximately 0.5 V. Cells are combined into a module, which is typically a polymer laminate backed glass plate with an aluminum frame. Modules are connected in strings (also called panels) using cables.
The widespread installation of solar PV arrays on rooftops has raised concerns over fire hazards. There are currently two fire test standards for roof mounted PV modules: UL 1703 Standard for Flat-Plate Photovoltaic Modules and Panels and FM 4478 Roof Mounted Rigid Photovoltaic Modules. These standards also rely on ASTM E108 Standard Test Methods for Fire Tests of Roof Coverings. In addition to adding fuel load to the roof, in the form of polymeric backing materials and encapsulants, the presence of the modules can result in increased flame spread rates between the modules and the roof. In 2013 the UL 1703 test standard was modified to address these concerns.
In addition to changing the fire dynamics, PV systems have multiple potential failure modes that can present ignition hazards. There have been multiple cases where fire causes have been associated with electrical faults in the wiring of PV arrays. In addition to fire ignition concerns regarding the panels, PV systems include additional electrical components that may be subject to failure, including their inverters. In addition, a PV system represents a different source of electrical energy into the building electrical system (in addition to the normal electrical service). Failures can occur if the PV system is not integrated properly.
PV arrays in a house fire pose a challenge for firefighters as well. A typical firefighting approach is to shut-off electric service to a house to eliminate electrical hazards before making entry. However, the presence of PV system typically means that there is not a single shut-off point, as the household electrical system could remain energized even if the electrical utility service is disconnected. Newer systems have addressed this issue through rapid shutdown functionality that is now required by the National Electrical Code (NEC), but older systems do not have this functionality. PV systems may also incorporate batteries, which is another source of electrical energy. These challenges often cause firefighters to approach a house fire involving PV arrays more defensively, leading to more fire damage but maintaining firefighter safety.
The presence of multiple sources of electrical energy has implications for fire investigation as well. In a typical household configuration, all electrical energy comes from the electrical utility, and if that source of energy is removed, there will be no further electrical activity or electrical faulting/arcing as a fire progresses. However, in a house with a PV system, the routing of the PV conductors, and their status throughout the fire must be considered. Due to electrical faults, neutral and ground wires throughout a house may become unintentionally energized by the PV system.
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