Solar photovoltaic systems in the USA: allay safety fears
In the line of fire
9 April 2015
The use of photovoltaic (PV) systems is growing dramatically in the USA, at a rate of 30% annually. Located on and near commercial properties and residential homes, they convert sunlight into usable power and are seen as a sustainable, clean technology and supported by various levels of government
Use and types of PV modules have evolved over the years and those in use include legacy PV module manufacturing (solar cells connected together and packaged tightly behind a glass sheet), and also hybrid thin silicon PV films adhered to curtain walls. Installation involves several components – isolation switches, inverters, modules, battery banks and mounting racks, which are eventually tied to the power grid via a utility interactive connection (see figure 1). PV modules can be found:
- mounted on rooftops;
- integrated into building components (roof shingles, glass panes);
- rack mounted;
- integrated to discrete energy dependent products (lighting, signs, fans);
- as PV farms.
Figure 1: PC components and product safety standards
The increased manufacture and use of PV systems are being spurred by governmental incentives at local or national level and green conservation requirements (LEED, Green Codes (International Green Construction Code). But although attractive from an environmental standpoint, their use is not without safety and fire concerns, particularly with mountings and connections. The safety issues have become major discussion points for technical committees involved with the development of product standards and installation codes.
Concerns have seen changes to the global independent safety science company Underwriters Laboratories (UL)
1703 standard for safety of flat-plate photovoltaic modules and panels to address new materials being introduced into PV panels, ground construction requirements and an assembly level fire test. In addition, UL 1699B, outline of investigation for outlet branch circuit arc-fault circuit-interrupters was developed to cover the proper evaluation. The National Electrical Code and International Building Code also include revisions to address issues such as proper fire rating of roofing, grounding connections, and use of isolation circuit devices.
UL has conducted multiple research projects over the past 7 years related to PV system safety concerns. The issues addressed were:
- effect of rack-mounted PV arrays on the fire rating of roofs
- mitigation of electrical hazards encountered by the fire service.
UL’s research on fire propagation of roof-mounted PV arrays was carried out in co-operation with US PV associations, manufacturers and other industry stakeholders. The multiple phases focused on the fire rating of generic roof assemblies and the impact of PV module installations variables. These covered separation height, roof slope and inclination to the roof. Details included:
- investigation of rack-mounted PV modules on roof decks to determine the effect of PV modules mounted at angles (positive and negative) to steep and low sloped roofs; the impact of PV modules mounted at zero clearance to the roof surface and with the ignition source directed toward the surface of the roof or the PV surface; and the heat exposure to a roof surface from Class A, B, C brands and common materials such as leaf debris and excelsior (wood wool)
- the investigation of mitigation measures such as flashing at the leading edge of the roof with control of separation between the roof and flashing; and use of non-combustible back sheet
- evaluation of a revised burning-brand performance criteria of UL 1703 for steep sloped mounting systems
- evaluation of a steep slope PV/roof assembly and low slope PV/roof assembly performance criteria of UL 1703 spread-of-flame test requirements.
The intent was to provide quantitative data to support future standard and code revision proposals. Results may also be used to provide the solar industry with a set of installation practices that meet fire performance requirements.
UL received a grant through the US Department of Homeland Security to examine potential PV system fire safety considerations and provide data to help the fire service understand the inherent risks. Greater PV use has complicated traditional tactics for suppression, ventilation, and overhaul, leaving firefighters vulnerable to potentially unrecognised exposure. The data will enable the fire service to develop safety solutions and response tactics for a fire involving a PV modules system. Testing explored multiple issues including:
- electrical resistance of firefighter personal protection equipment (PPE)
- potential circuit paths through hose streams
- power generation during low light conditions
- potential electrical safety hazards because of damaged modules and wiring
- techniques to depower PV modules
- fire experiments.
Two series of large-scale tests were carried out. The first involved a functioning PV array consisting of 26 framed modules rated 230W each, for a total rated power of 5,980W, mounted above a Class A roof supported by wood trusses. This array was used as a test fixture for the non-fire experiments.
The second series comprised fire experiments to represent the ignition of debris accumulation on a roof. Three PV technologies were subjected to fire conditions: a rack-mounted metal-framed glass on polymer module; building-integrated PV shingles; and a flexible laminate attached to a standing metal seam roof.
Electrical leakage current produced during simulated fire suppression activities in the tests was measured. This served to establish a benchmark to develop correlation data between PV system voltage and known leakage current thresholds for human exposure to electric shock (see figure 2).
Figure 2: Current thresholds for human exposure
Data analysis also indicated that portions of arrays and individual modules remained partially to fully functional even with significant damage. For example, of 20 rack-mounted modules, 5 were destroyed, 3 remained partially functioning, and 12 fully operational. In experiments with the 68 shingle modules, 35 were destroyed and the remainder were fully operational. Testing of membrane modules attached to standing metal seams demonstrated that out of 21 modules, 7 were destroyed and the remaining deemed fully operational. This information has direct implications for firefighters during overhaul operations.
High-level findings of the research projects included:
- The electric shock hazard because of water application is dependent on voltage, water conductivity, distance, and spray pattern. A slight adjustment from a solid stream toward a fog pattern (such as a 10° cone angle) reduced measured current below perception level. A distance of 6m was determined to reduce potential shock hazard from a 1,000V DC source to a level below 2mA, which is considered safe. It was noted that pooled water or foam may become energised because of damage in the PV system.
- When illuminated by artificial light sources, such as fire department light trucks or an exposure fire, PV systems can produce electrical power sufficient to cause a lock-on hazard.
- The gloves and boots worn by firefighters afford limited protection against electrical shock provided the insulating surface is intact and dry. They should not be considered equivalent to certified electrical PPE.
- Tarps offer varying degrees of effectiveness to interrupt the generation of power from a PV array, independent of cost. Heavy, densely woven fabric and dark plastic films can reduce the power to near zero. As a general guide, if light can be seen through a tarp, it should not be used. Caution should be exercised during the deployment of tarps on damaged equipment, because a wet tarp may become energised and conduct hazardous current if it contacts live equipment. Also, typical firefighting foam should not be relied on to block light.
Alfredo Ramirez is a Codes and Advisory Services Manager at Underwriters Laboratories