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Keeping solar connections safe

Keeping solar connections safe

The enclosures that house photovoltaic connection circuitry should be designed to last at least 30 years. A few pointers help in specifying them.

Examine most solar array installations and you will generally find what are called solar combiner or re-combiner boxes located between the arrays and the inverter. A combiner box is basically an electrical distribution box containing dc circuit breakers. The combiner gets its name from its role combining the multiple dc inputs coming from the solar panel terminations and converting these into one dc output. The output of the combiner box goes to the inverter. Recombiners, also referred to as array combiners or subcombiners, are used in large PV arrays to combine the outputs of several combiners before reaching a central inverter.

Enclosures are used at every stage of the PV collection process, including the combiners, re-combiners, at points of transition from one wire size to another, all the way to the inverter. In general, enclosures for the solar industry typically carry a UL type 4 or 4X rating. The NEMA rating gives information on the approved mounting locations and orientations. For example, NEMA 4 and 4X ratings permit installation of enclosures outdoors in any orientation, from vertical to horizontal. NEMA 3 and 3R rated products can go outdoors as well, but in not as many mounting orientations. It is also possible to find solar combiner enclosures certified by ETL to UL 1741, a safety standard for distributed generation. ETL is a certifying body for solar applications.

When evaluating electrical enclosures, designers must contemplate the environmental conditions, the amount of solar load on the enclosure itself, and the total cost of implementation. For example, plastic degrades when exposed to sunlight and gets brittle. So outdoor PV enclosures generally must be metal or fiberglass. As you might suppose, solar power wiring components usually reside outdoors, so the electrical enclosures and equipment within see harsh environments. These conditions include UV radiation and temperature extremes, wind with blowing rain, snow, dust and dirt and salt spray in coastal regions.

Ultraviolet light (UV) energy may degrade enclosure appearance and breakdown the physical substrate of some non-metallic materials. Sodium chloride (salt) in the air may cause extensive corrosion and component failure in painted enclosures composed of mild steel. It can also degrade the appearance of stainless steel enclosures over time.

Designers should also note that field modifications of an enclosure can compromise its NEMA rating. So it’s best to use listed commercial products to handle any conductors that enter or exit the enclosure. For example, commercial cord grip bushings can strain-relieve PV source circuit conductors as they enter the combiner enclosure, while still maintaining the enclosure’s NEMA rating.

Metallic vs. non-metallic material

Painted mild steel may be acceptable for most uses indoors, but for outdoor applications, materials such as stainless steel and non-metallic enclosures provide superior protection against corrosion. Painted mild steel will deliver an adequate protection for solar applications in general indoor and outdoor conditions where potential corrosion is not a concern. In wet or coastal environments with salt in the air, mild steel begins to corrode.

Alternatively, stainless steel enclosures are robust and rigid in design and perform well against a wide assortment of chemicals and corrosive agents, such as sodium chloride. Stainless steel maintains a good appearance, is acceptable for use in direct sunlight and in applications with temperature extremes. Though stainless steel costs more initially, it costs less to maintain over the life of the installation.

Non-Metallic Enclosures

Enclosures molded from non-metallic materials, such as fiberglass, polyester or polycarbonate, are widely used in modern solar applications. They often serve where their light weight is important. These materials resist corrosion and have UV stabilizers and flame retardency additives in their formulations that let them perform well outside in sunlight. Each material offers varying amounts of impact resistance and flexibility that let the enclosure withstand impacts without denting and operate in a wide range of temperatures.
Fiberglass enclosures are molded from a thermoset polyester resin with embedded glass fibers for strength and rigidity. This corrosion-resistant material provides superior chemical resistance, withstands a wide range of temperatures, and performs well outdoors. Fiberglass enclosures are typically formed in one of two ways: compression molding or a spray-up process.

In compression molding, a material known as sheet molding compound (SMC) -- basically a pigmented polyester resin with impregnated glass-fibers -- is formed into a sheet of material. UV inhibitors and aluminum trihydrate can also be added to boost UV resistance and flame retardancy to varying degrees. The final formulated material is then laid into a precision-designed mold that, under heat and pressure, forms an enclosure. The glass strands provide a combination of exceptional strength with flexibility.

Although they are durable and strong, fiberglass enclosures are susceptible to a phenomenon called fiberbloom. This happens over a period of years when fiberglass is continually exposed to UV light. The UV rays erode the protective resin gel covering the glass fibers on the outside of the enclosure, eventually exposing the glass fibers which exhibit a snowflake like appearance (fiberbloom).

Fiberbloomed enclosures are still completely functional but don’t look attractive. They also are more difficult to clean and fiberbloomed material may irritate skin so it’s best to handle these enclosures with gloves. Additionally, fiberglass, like painted steel and other materials, will eventually discolor from UV exposure.

Polyester, polycarbonate or hybrid polycarbonate/polyester blends provide new alternatives to fiberglass and stainless steel for corrosion resistance. These materials offer a wide range of performance and may sometimes prove to be more economical. Thermoplastic enclosures are molded by injecting the thermoplastic material into a mold. The finished product is an attractive, uniform enclosure made with a material that exhibits high impact resistance and is an electrical insulator. It also resists chemicals, moisture, and a wide range of corrosive agents and atmospheres.

Polyester and polycarbonate materials rank high in impact resistance. For this reason, they can withstand rough handling and hard impacts without cracking or breaking. Fiberglass, on the other hand, gets its rigidity and strength from the glass, which can be broken by hard impacts. Because polyester and polycarbonate do not include glass, they can be easily modified with common hand tools providing clean cuts with minimal tool wear and essentially no irritating dust.

Non-metallic enclosures weigh less than mild steel and stainless steel versions. If a polyester object weighs one pound, a fiberglass version of the same size will weigh 1.5 lb. The object would weigh 2 lb if made of aluminum, 6.5 lb if fabricated from steel. Of course, enclosures filled with equipment are heavier. Weight is important particularly for enclosures that mount on a wall or pole. With this in mind, designers should always factor in both the weight of the electrical components and the expected mounting configuration.

Managing solar load

Electrical enclosures sitting outside absorb solar energy. The enclosure color can significantly affect the amount of solar energy the enclosure absorbs. The accompanying chart illustrates temperature rise based on enclosure color when exposed to the daytime sun.

Sometimes lighter colors aren’t an option for enclosures because of aesthetics. A solar shield can reduce solar loading in such cases. Solar shielding basically shades the enclosure. It usually takes the form of an added surface that provides roughly one-inch of airspace between itself and the enclosure. This airspace is generally ventilated to let heat generated internally conduct out.

Testing has demonstrated that shielding all sides of an enclosure this way can halve solar loading. However, solar shielding is typically only practical on enclosures made of mild or stainless steel where standoffs can be added to the outside for mounting the shields.

The addition of insulation can also dramatically reduce the solar load. But insulation also keeps heat generated inside the enclosure from escaping. Insulation is a good option when used in conjunction with such active cooling systems as air-conditioners, heat exchangers, or ventilation fans.

Resources

Pentair Technical Products, http://www.pentairtechnicalproducts.com/markets_energy.html

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