The 2012 International Building Code, while still not yet widely adopted in the United States, requires that rooftop rack-mounted PV systems have the same fire classification as required of the roof assembly—Class A, B, or C depending on the building type and location. Class A roofing materials or assemblies have a greater ability to resist fire spreading and to resist burning embers. While residential roofs have not generally been required to be Class A, some areas with high fire hazards are moving in that direction for new construction or significant reroofing projects. For example, Colorado Springs, Colorado, and many cities in California have local agency ordinance requirements for residential Class A roofs. Some roofing materials are considered Class A, like slate, clay, concrete roof tiles, and steel, although they must be installed correctly (for example, eliminating gaps between the roof covering and decking where birds could build nests) to earn this rating. Class A fiberglass-reinforced asphalt composition shingles are also available, while other types of asphalt or wood shingles will typically have a lower Class B or C rating.
Most glass-front, plastic-backed modules have a Class C fire rating, but modules with glass on both sides may meet higher Class A rating requirements. Trina’s TSM PDG5 and Silicon Energy’s Cascade Series are both fire Class A-certified, glass-front and glass-backed modules.
Besides their improved fire resistance rating, some glass-on-glass modules have the benefit of allowing dappled light to pass through for structurally integrated arrays like patio or walkway covers. Looking up at the back of a module that is letting light into an atrium and seeing the PV cells instead of an opaque plastic backsheet is considered by many people an aesthetic improvement and architecturally interesting. Another advantage of glass-on-glass modules is superior protection for the back of the module as compared to plastic, and enhanced resistance to sheer stresses.
Frameless versions of glass-on-glass modules are available (including many thin-film modules) that have no metal frames to ground, so the labor and material costs of grounding module frames to racking are eliminated. (Note that the rails still have to be grounded.)
The benefits of AC module and microinverter systems include simplified array design with no string sizing necessary and MPPT for every module, rather than a whole array or string. Unlike with a string inverter, one module or inverter failure does not affect the whole system. In addition, module-level data monitoring capabilities allow easy troubleshooting of an underperforming array or module, and arrays are more easily scalable, as modules can be added without dealing with the constraints of series strings.
Both system types are extremely safe to install and operate, compared to systems with string inverters, because DC voltages are kept to a one-module maximum; all equipment connectors are touch-safe; and DC voltages will generally stay below the 50 V limit associated with shock hazards. Shutting off the main service AC disconnect or PV system disconnect also immediately deenergizes all the PV system conductors except module leads, as the inverters shut off immediately without the presence of grid voltage. One of the benefits of AC modules includes even-quicker installation, since the inverter is preinstalled—there is no DC field wiring, and no DC arc-fault protection necessary. (Metal module frames and any other metal equipment like junction boxes or racks still need equipment grounding.)