Fire Concerns with Roof-Mounted Solar Panels

Issue 92: Fire Concerns with Roof-Mounted Solar Panels

By Richard J. Davis, P.E., FSFPE

As companies look to reduce their dependence on fossil fuels, many are turning toward rooftop photovoltaic (PV) power systems, or solar panels, as a source of renewable, clean energy. However, this technology comes with specific risks. One of the many dangers to solar panels is how the panel and its mounting system impact the combustibility of the overall roof system. Some solar panels, for example, include a backing of highly combustible plastic.

 

 

In laboratory-based fire tests of roof assemblies,1, 2 the maximum allowable fire spread is between approximately 20 and 40 ft2 (1.9 and 3.7 m2), depending on whether an A, B or C rating is desired. In actual roof fires with roof-mounted solar panels, fire damage has involved areas of between 1,000 and 183,000 ft2 (93 and 17,000 m2). In the most extreme case the fire spread to the inside and destroyed the entire building (see Fig. 1).

 
Fig. 1. PV roof fire at a refrigerated warehouse in NJ in 2013
(photo courtesy of Vince Lattanzio, NBC Philadelphia)

While the results of a lab test and an actual fire are not always identical, such a wide disparity is reason for concern. Lab tests conducted by at the FM Global Research Campus in West Glocester, RI, USA, confirm these concerns. For such testing, an ASTM E108 test apparatus was utilized, placing PV panels over a commonly used, Class A-rated roof assembly (when the roof alone was tested), starting near the flame-exposed end. This roof assembly failed the test (see Fig. 2). While only one failure mode is required by the test standard, in this test all three of the following failure modes occurred: Fire spread laterally to both edges of the sample, material continued to burn after falling to the floor, and fire spread across the 13 ft (4 m) length of the assembly within 90 seconds.

 
Fig. 2. Fire test of rigid PV panels over Class A-rated roof consisting of an
EPDM cover over polyisocyanurate insulation (photo courtesy of FM Global)

Why did this happen? Regardless of the materials used in the construction of a PV panel, its mere presence changes the dynamics of a fire involving a roof assembly. Research tests done at Underwriters Laboratories 3, 4, 5, 6,7,8, 9, 10, 11 demonstrate that even a cement panel simulating the presence of a PV panel will increase fire spread across a common roof assembly.

 

 

There are three key considerations that affect fire spread along a roof where a roof-mounted PV array is installed:
  1. In a typical roof fire, the flame is primarily vertical, or perhaps somewhat slanted due to wind. Once such flames spread under a PV panel, the flame is redirected much closer to the roof surface and nearly parallel to it. This increases the incident heat flux on the roof surface, often above its critical heat flux.
  2. While the exterior fire classification of a roof is an effective way to rate the exterior fire performance of roof assemblies, even a Class A assembly will offer some fuel contribution to a roof PV fire, with most standing seam metal roof systems being the exception.
  3. While the top surface of a rigid PV panel is usually made of tempered glass, the bottom of the panel may contain combustibles (used to protect the PV circuitry) in the form of polyester-based encapsulants and back sheets (see Fig. 3). If this ignites and the heat re-radiates, fire spread is likely to continue back and forth beneath the roof assembly and the PV back sheet.
Fig. 3. The underside of a rigid PV panel
(photo courtesy of FM Global)

 

PV rooftop fires have been caused by electrical arcs that occurs near the combiner box, where numerous wires from PV panels are connected. This is a location where there is considerable voltage, before the current is converted from DC to AC at the inverter, and where the roof assembly could ignite and result in fire spread under the PV panels.

 

 

Fortunately, there have been some improvements made by manufacturers during the past few years with regard to the electrical components that can reduce the potential for ignition. Some PV panels have micro-inverters on each PV panel, which convert voltage from DC to AC. This can be expensive, but it reduces the probability of ignition.

 

Manual firefighting efforts also can be hampered by the electrical risk associated with PV arrays. While minimum 4 ft (1.2 m) wide aisle spaces between panels at a maximum of 150 ft (46 m) apart have been recommended 12, this does not alleviate all the risk. Disconnecting electrical power from the PV array is complicated, and arrays continue to generate electricity, sometimes even at night. The PV array and the roof assembly should be designed so their construction limits potential fire spread and the entire burden for fire protection is not placed on manual firefighting efforts.

 There are several design choices that can limit fire spread if ignition occurs:

  1. Use a complete system (PV panels, securement, and roof assembly) that has been tested to simulate actual field conditions. FM Approval is available, 13 which includes testing for fire exposure as well as wind and hail.
  2. If the existing roof has aged, it is recommended that a new roof be installed before installing a PV system. Choose roof assemblies that limit potential fuel contribution in the event of an exterior fire. Appropriate options include metal roof systems, as well as noncombustible materials (such as gypsum cover boards, mineral wool or expanded glass roof insulation) installed directly below single-ply or multi-ply roof covers. In some cases, coatings may need to be applied to the top of the roof cover.
  3. Where existing roofs will remain, investigate the need for a coating to be applied to the top of the roof cover that will improve performance with regard to exterior fire exposure. Construction materials that melt at low softening temperatures and can flow when burning (such as expanded or extruded polystyrene insulation or multi-ply roof covers) may require protection such as a gypsum cover board installed over the insulation or a coating over the roof cover.
  4. To prevent an exterior fire from entering the building, protect building expansion joints by securing mineral wool or other fire-resistant compressible insulation between wood nailers, covered by steel flashing.
  5. Evaluate the potential for fuel contribution from the underside of the PV panel. The underside of the panel may have a glass backing, aluminum or fluoro-polymer-based back-sheet as an alternative to a polyester-based back-sheet.

Most importantly, it is best to use a PV panel that has passed a fire test with the proposed roof assembly.

 
For additional information, see FM Global Property Loss Prevention Data Sheet 1-15, Roof Mounted Solar Photovoltaic Panels.

 

Richard Davis is with FM Global

  1. ASTM E108, Standard Test Methods for Fire Tests of Roof Coverings, ASTM International, West Conshohocken, PA, 2011.
  2. UL 790, Standard for Standard Test Methods for Fire Tests of Roof Coverings, Underwriters' Laboratories, Northbrook, IL, 2004.
  3. Backstrom, B. & Tabaddor, M.  "Effect of Rack Mounted Photovoltaic Modules on the Fire Classification Rating of Roofing Assemblies," Underwriters' Laboratories, Northbrook, IL, 2010.
  4. Backstrom, B. & Tabaddor, M.  "Effect of Rack Mounted Photovoltaic Modules on the Flammability of Roofing Assemblies – Demonstration of Mitigation Concepts," Underwriters' Laboratories, Northbrook, IL, 2010.
  5. Backstrom, B. & Sloan, D.  "Effect of Rack Mounted Photovoltaic Modules on the Fire Classification Rating of Roofing Assemblies - Phase 2," Underwriters' Laboratories, Northbrook, IL, 2012.
  6. Backstrom, B. & Sloan, D.  "Characterization of Photovoltaic Materials – Critical Flux for Ignition/Propagation - Phase 3," Underwriters' Laboratories, Northbrook, IL, 2012.
  7. Backstrom, B. & Sloan, D.  "Report of Experiments of Minimum Gap and Flashing for Rack Mounted Photovoltaic Modules - Phase 4," Underwriters' Laboratories, Northbrook, IL, 2012.
  8. Backstrom, B. & Sloan, D.  "Considerations of Module Position on Roof Deck During Spread of Flame Tests - Phase 5," Underwriters' Laboratories, Northbrook, IL, 2012.
  9. Backstrom, B. & Sloan, D.  "Validation of 42” PV Module Setback on Low Slope Roof Experiments - Project 7," Underwriters' Laboratories, Northbrook, IL, 2012.
  10. Backstrom, B. " Validation of Roof Configuration 2 Experiments - Project 9," Underwriters' Laboratories, Northbrook, IL, 2012.
  11. Backstrom, B. & Fischer, C." Report on Spread of Flame and Burning Brand Performance of Generic Installations," Underwriters' Laboratories, Northbrook, IL, 2012.
  12. "Solar Photovoltaic System," Los Angeles City Fire Department Requirement No. 96,Los Angeles, CA, February, 2009.
  13. Class Number 4478, " Approval Standard for Rigid Photovoltaic Modules," FM Approvals, Norwood, MA, 2012.

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