Materials matter for enclosure design
More than 100,000 new materials have been developed in just the past 50 years. As a result, material selection for electrical components has become a very complex task for most electrical control designers and panel builders. Designers should become involved with material selection because, many times, the material-of-choice will not be specified by the end user. By gaining an understanding of materials used for electrical enclosures, the designer/specifier can choose the material that will best meet product performance goals. However, it is always best for designers to work in collaboration with materials engineers during this evaluation stage to avoid unnecessarily expensive structures and avoid failure.
Step 1: Review the materials most often used for electrical components.
The material used to protect electrical components can be metallic or nonmetallic, but it must serve its function of protection for the life of the installation. Durability and longevity are key. Three typical types of materials are available: metal, plastic, and composites.
Common metal material choices include carbon steel, stainless steel, and aluminum. Carbon steel, the most prominent choice based on its low initial cost, is typically galvanized or painted to prolong the service life. Premium metals such as stainless steel and aluminum are used where long life, corrosion resistance, and weatherability are critical (e.g., in protecting controls for junction boxes for utility power PV installations).
Thermoplastics such as polycarbonate, polyester, and polyvinyl chloride offer a degree of corrosion protection beyond painted carbon steel. Thermoplastics, though, are more susceptible to ultraviolet (UV) and weathering degradation over time. Certain stabilizers can be added, but ultimately the nature of the thermoplastics will yield to extended weathering.
UV has been a concern among nonmetallic manufacturers for many years. The rate at which the UV degradation occurs will vary depending on the heat, humidity, and latitude where the product is installed. However, additives and unique formulations have been introduced to help reduce failure and have eliminated most of the concerns from end users.
Thermoset materials—polyester resins combined with glass, for example—create a composite fiberglass-reinforced polyester (FRP) that is exceptionally durable and weather resistant. Like thermoplastics, FRP provides a greater degree of corrosion than painted carbon steel and can be a direct replacement in many stainless steel applications, yet it will perform better than metal and plastics in extremely harsh environments.
Step 2: Compare thermal, electrical, and environmental factors to help narrow the choices.
Regarding thermal conductivity, nonmetallic materials are often selected for enclosures because they are insulated and do not dissipate heat generated in the cabinet. Nonmetallics also do not conduct heat and therefore work as insulators from high ambient temperatures. Metal enclosures are faced with a nearly opposite consideration, but both choices require some forethought in the final installation regarding heat dissipation.
Electromagnetic interference (EMI), caused by energy emanating from high-voltage equipment, and radio frequency interference (RFI), caused by radio waves, are two methods that impair a system’s ability to properly function electronically. Thermoplastic and thermoset composite materials are ideal choices for harsh or aesthetically pleasing environments, but because they are nonmetallic they provide no EMI/RFI shielding. Metallic materials, on the other hand, are inherently effective in their ability to shield internal components from stray electromagnetic waves or radio frequencies. However, composite and thermoplastic materials can be modified to provide a degree of protection against this interference by coating the inside with a highly conductive nickel or copper coating or by imparting the conductivity directly into the composite material itself using carbon fibers or other highly conductive metallic flakes.
The foremost motivating characteristic influencing selection of the material choice is environmental. The consideration envelops temperature, chemical, moisture, and concern for the physical world of the permanent installation. Whether the environment is hostile or passive, an attempt is made to match capabilities of the material and the anticipated ambient environment. An over-specified material will work effectively in a natural environment, but there are severe repercussions for using an under-specified enclosure in a hostile environment, thus making environment the overriding consideration.
– Roger Schroder is director of SpecTech Enclosures and Materials; Edited by Mark T. Hoske, content manager, CFE Media, Control Engineering, Consulting-Specifying Engineer, and Plant Engineering.
SpecTech helps specifiers learn more about materials to properly specify products most suited for highly defined applications, and earn Professional Development Hour credits. A two-day instructional event, held in Belding, Mich., is accredited for 16 PDHs in qualifying states for engineers who need to earn credits to maintain professional licenses.