Enclosures help keep solar-energy projects safe

The National Fire Protection Association (NFPA) consistently ranks electrical fires as among the top five most-frequent types of fires in commercial buildings. Electrical fires also tend to produce a higher percentage of property damage than other sources of combustion.

Unfortunately, solar panels can become an ignition source for building fires. Arc-faults pose the greatest fire risk to photovoltaic (PV) installations. The need to detect and extinguish arc-faults in PV systems has given birth to a new protection device called a DC PV Arc-Fault Circuit Interrupter (DC PV AFCI). Designed to detect the signature of an arc-fault in the power line it monitors, the DC PV AFCI kills the current flowing through the line upon detecting an arc-fault condition.

The 2011 U.S. NEC contains a new provision, Article 690.11, that specifies the need for arc-fault protection in rooftop PV systems that use direct current circuits at voltages of 80 V or higher. Unfortunately, no DC PV AFCI currently exists that can perform this task. While AFCIs exist for ac circuits, the characteristics of dc power changes the game. For example, it is much harder to extinguish an arc in a dc circuit because dc has no zero-crossing voltage like that found in ac circuits. In response, solar-industry vendors are already developing technologies that not only meet this requirement but surpass it to provide additional protection against PV-system fires.

If a fire should start, controlling its spread becomes paramount in limiting the damage it causes. Obviously, the selection of materials used in the construction of the PV system and, of course, its enclosure, plays a critical role in controlling a fire’s spread. Fire testing programs carried out by Underwriters Laboratories (UL), the NFPA, and other facilities play a key role in understanding how a material reacts when it burns.

Plastic materials undergo two flammability tests to measure their burning qualities. The first test, outlined in UL 94, determines a material’s tendency either to extinguish or spread the flame once ignited. While UL 94 sets standards for the U. S., it complies with international standards IEC 60707, 60695-11-10, and 60695-11-20 along with ISO 9772 and 9773.

The second test program measures the ignition resistance of the plastic to electrical-ignition sources. Material resistance to ignition and surface tracking is described in UL 746A, which also complies to the test procedures described in IEC 60112, 60695, and 60950.

UL flame ratings group materials into categories based on their flammability. UL 94 looks at how the materials burn in both vertical and horizontal positions.

For the horizontal test a specimen supported in a horizontal position is tilted 45°. The specimen contains two marks, one at 1 in. from the burn edge and another at 4 in. A flame is applied to the end of the specimen for 30 sec or until the flame reaches the 1-in. mark. If the specimen continues to burn after the removal of the flame, the time needed to burn between the 1 and 4-in. marks is recorded. If the specimen stops burning before the flame spreads to the 4-in. mark, the time of combustion and damaged length between the two marks is recorded. Three specimens are tested for each thickness of the material.