Once enclosure requirements are determined, it is important to select enclosures that are certified to validate performance to specific standards.
In North America, the National Electrical Manufacturers Association (NEMA), Underwriters Laboratory (UL) and Canadian Standards Association (CSA) issue type ratings for enclosures based on similar performance criteria. However, the way these organizations certify performance varies greatly.
NEMA doesn’t require enclosures to be tested by qualified evaluators, nor does it send site inspectors to ensure that manufacturers adhere to prescribed manufacturing methods and material specifications, as UL and CSA do. Instead, NEMA allows manufacturers to self-certify that their products meet NEMA compliance.
In Europe, the International Electrotechnical Commission (IEC) issues type ratings for enclosures, and all devices with electrical/electronic components receive an ingress protection (IP) rating. It is possible to cross reference IP and North American standards for dust and water entry, however NEMA, UL and CSA rating types add requirements for corrosion, ventilation, drainage openings, mounting methods, gasket types and more.
Furthermore, using an IP-rated enclosure in a North American application is not allowed. An enclosure with a NEMA, UL or CSA rating is required.
Enclosure Integrity
After the appropriate enclosure type rating has been selected, it is critical to maintain the enclosure’s integrity by installing only those devices—such as gaskets or accessories mounted through openings in the enclosure wall—that are UL listed or recognized, or CSA certified as having the same environmental rating.
For example, using an unrecognized gasket could cause UL or CSA to ignore it for rating purposes. Similarly, UL does not consider silicone or other sealants as listed or recognized components. Other common problems include using glue or tape to connect conductive parts, rather than an agency approved mechanical fastening method.
Once the requirements for an enclosure are determined, how do buyers know they are getting what they want? Manufacturers verify compliance to core enclosure standards through performance and reliability testing.
For example, one manufacturer uses a test procedure matrix for all new products during the design phase and provides verification to NEMA, UL and CSA. When customers purchase an enclosure, the UL or CSA mark is their verification of testing to meet type standards.
For metallic enclosures, key tests include metal material thickness tests, type rating, gasket tests, nonmetallic material tests, grounding and bonding tests and “above and beyond” customer needs tests. IP tests also are conducted for all products.
Composite material testing covers nonmetallic enclosures including those made of fiberglass, polycarbonate and polyvinyl chloride (PVC). The material must be agency recognized, which includes an ultraviolet (UV) resistance test and strength testing performed by the agency. Thickness dependent testing includes an impact test, a flame test and conduit tests.
Most enclosure manufacturers have in-house programs that perform a variety of tests. Environmental testing includes cold composite impact and ice build-up tests. A heat and humidity chamber creates an accelerated aging environment, and is used for heat and humidity aging and 30-day gasket swell tests.
Electrical system limits are tested using variable frequency and variable voltage. Additional testing and certification above and beyond basic enclosure standards is conducted for all components not covered by other testing. This includes cycle testing of all moving parts, static load testing and what is called “user abusive” testing. Solar testing to measure heat gain based on color, finish and material as well as seismic, thermal load, electromagnetic compatibility (EMC), paint qualification and vandal resistance testing also is performed.
Understanding the differences between rating types is the first step to selecting the proper enclosure for an application. Testing helps ensure consistent and reliable enclosure performance.
Robert Lau, [email protected], is technical services manager for product development at Hoffman.
