by Tinu Oza, Product Line Manager, L-com
NEMA enclosures are necessary wherever equipment needs to be protected from the environment or from tampering. These enclosures are commonly used for installation of many industrial/commercial outdoor and indoor electronic equipment. From circuit breakers to wireless access points, these enclosures allow for basic, standardized protection of any and all internal systems. This article discusses the various NEMA ratings and IP ratings, as well as the numerous considerations involved in the selection and assembly of a NEMA enclosure for installation.
NEMA in a Nutshell
The National Electrical Manufacturers Association, or NEMA, defines the North American standards for electrical enclosures. The NEMA-rated enclosure comes in a number of gradings to best protect the internal circuitry from its respective environment. The variety of environments and applications (e.g., outdoor, industrial, military) each has with it its own specific considerations. Table 1 lists some NEMA ratings as well as their corresponding uses and levels of protection.
NEMA vs IEC IP Standards
The Ingress Protection (IP) standards are defined in the IEC 60509 specification and describe the levels of protection for a device with respect to dirt and water ingress (See Table 2). These ratings differ slightly from NEMA ratings as the NEMA ratings were primarily intended for indoor and outdoor electrical enclosures placed in a static location. On the other hand, IP-ratings are often used for mobile devices such as handsets, tablets, and mobile radios. Also, NEMA ratings often include additional layers of protection from either corrosion, fungus, vapors, or vermin to provide protection from highly specific non-corrosive/corrosive chemical agents as well as protection against explosive atmospheres caused by specific flammable chemicals/particulates.
Table 2 shows a cross-reference chart that provides an equivalent IP rating for a NEMA rating. Note that is the only “direction” in which it should be used as the IP rating only specifies dust and liquid protection but NEMA much more so.
Table 3 lists the IP rating for both dust and liquids. For example, an IP67 rating provides protection for all dust ingress and protection against liquid immersion up to 1 meter.
Protection Against Hazardous Conditions
NEMA ratings that extend to hazardous conditions are primarily geared towards protection against explosive environments and their various flammable agents (Table 4). Classes 7, 8, 9, and 10 are primarily responsible for such hazardous locations with the appropriate respective class and group. It should be noted that explosion-proof enclosures are not necessarily watertight and dust-tight, but are specifically designed to rapidly quench flames that develop by an internal ignition and prevent any spread external to the enclosure.
Selecting a NEMA Enclosure
The choice of enclosure relies heavily on the environment and application. For instance, outdoor access points can be housed in NEMA 3R enclosures and be operational with minimal maintenance (annually), however locations that experience driving rain on occasion (San Francisco, New Orleans, etc.) would do better with a NEMA 4 or NEMA 4X enclosure. Enclosures in hydraulic or pneumatic control installations would require a level of corrosion resistance as well as resistance to splashes/sprays from potentially corrosive chemicals such as hydraulic fluid or coolant. In these cases, NEMA 12, 12K, or 13 rated enclosures would suffice to protect electronic circuitry.
Ensuring Weathertight Seals
Often, the weakest links in the proverbial chain could be seen as the points connecting the internal circuitry to the external environment. This includes the antenna and cabling such as coax, Ethernet, or power cables. These points are potential angles where dust or water can enter the enclosure. As such, they should be properly accounted for. Conduits to and from the enclosure are best placed at the bottom to mitigate any liquids entering the enclosure. External mounting points and watertight grommets that slide over the cables eliminate the need for drilling the enclosure and using silicone caulking, or some other type of sealant, on the interior and exterior of the hole in order to mount the device.
Sizing an Enclosure
When installing electronic equipment inside a NEMA enclosure, there are a series of potential sizing issues that can crop up without the appropriate planning. Understanding the dimensions of any and all equipment within the available enclosure space as well as the space that the connectors take up is critical to the installation, operation, and subsequent maintenance of the electronics. While dimensions can be readily ascertained from a data sheet, oftentimes the space that mating/un-mating the connector heads take up can be overlooked. The considerations vary per connector head; where serial ports and Ethernet jackets require removal via push-pull methods, coaxial interfaces are often threaded and therefore require torsion for mating/un-mating. Moreover, coaxial cables cannot be bent beyond a certain bend radius without causing signal degradation or a failure. All of these issues have their respective space constraints. Allowing for at least an inch of clearance around the perimeter of connectorized equipment is a simple rule of thumb that could potentially sidestep serious issues down the road.
Outside of this, there is also the consideration of enclosure hardware that can limit the effective space for electronic equipment within the enclosure. For instance, some iterations of enclosures have internal flanges that limit the height of the equipment within the enclosure, causing it to be unable to be closed securely. While the addition of components such as a mounting plate, DIN rails and heating/cooling fans allow for simpler mounting and improved system reliability, they still come with their own sizing considerations as these components take up space.
Enclosures are typically composed of either metallic or non-metallic material. Metallic enclosures typically feature galvanized steel or stainless steel and are fabricated in a hot- or cold-rolled steel process. The seams are continuously welded to protect against dust, dirt, water, and non-corrosive agents. Non-metallic enclosures include ABS (plastic), Fiberglass Reinforced Polyester (FRP), and Polycarbonate. Gasketing materials can vary between rubbers such as neoprene, urethane, and silicone. In many cases, it is important to ensure that all these materials can withstand corrosive conditions while ensuring a dirt-tight and watertight seal. Even with a strong seal, condensation can occur within an enclosure in humid environments. Typically, plastic enclosures are less prone to condensation than metallic enclosures. Moreover, the use of an internal cooling fan or heating system (or both) can function as humidity control within the chamber. Metallic enclosures have far more mechanical strength as well as a much higher inherent immunity against electromagnetic interference (EMI). However, metallic enclosures can be more susceptible to corrosion and humidity.
External mounting systems are typically either on a flat surface/wall, or on a pole. For a wall, an integral mounting flange or wall mount brackets can be used to mount to any flat surface with proper screws into secure studs. Pole mounting systems can also be compatible with these wall mounting systems. Internal mounting systems include either a metallic or non-metallic mounting plate to ensure electronic components can be securely affixed to the enclosure. Additional aluminum mounting brackets with pre-drilled and cut holes can also be utilized to add more specialized components to the enclosure. Figure 1 shows a sample NEMA enclosure for a wireless access point (AP) —in this system, a mounting plate and a mounting bracket are utilized where the power supply for an access point attached to a mounting bracket.
In outdoor applications, indirect lightning strikes—which often have energy within the DC to 1 MHz range—have the ability to travel along coaxial interconnects and power lines, ultimately leading to power surges and failures of the sensitive circuitry installed within the NEMA enclosure. Electrical control boxes in outdoor environments require a level of surge protection; this could be electrical junction boxes in telephone company applications or PoE wireless access points. Surge protection of the supply line is therefore necessary for the powered devices within an enclosure. For coaxial interfaces, DC block/high pass filter or a gas tube replacement surge protector with bulkhead coaxial interconnect can protect the coaxial cable runs and the equipment they are connected to. Ethernet-based surge protection is also necessary for access points or PoE-based equipment. Transient voltage protection for powered devices can be accomplished using high-speed semiconductors that clamp any voltages, a grounding lug for superior grounding, and shielded RJ45 connector heads with metallic housing for additional EMI protection (Figure 2).
NEMA enclosures can also include DIN rails—a standard mounting rail for equipment such as circuit breakers and various industrial control equipment including industrial Ethernet switches, industrial serial converters, and power supplies. Aluminum mounting rails allow for the installation of the specialized DIN rails and subsequent DIN modules securely. Rails can also be adjusted to hold various systems for optimal component and interconnect placement within the enclosure. Figure 3 shows a composite of various NEMA enclosure interiors. The DIN rail mount is shown on the right side and is called out. The DIN rail mount allows you to install a DIN rail into the enclosure, as well as additional options to best service the equipment inside the enclosure.
Convective cooling of internal components can either be accomplished via a passive vented model or selecting a model with a cooling fan or heater. (See Figure 3). An internal thermostat control senses the temperature within the enclosure and turns on/off the fans or heaters based upon preset temperature limits. These systems are particularly ideal in outdoor or indoor industrial environments that experience severe temperature fluctuations or humid environments where the internal buildup of condensation can damage equipment.
Choosing the Right NEMA Enclosure and Subsystems
Selecting the right NEMA enclosure comes with a number of sizing, material, and applicational considerations. Mounting equipment can be simplified through the use of internal and external mounting systems such as a mounting plate, DIN Mmces as well as pole mounting systems. Allowing for enough space to perform scheduled maintenance and access internal ports is paramount, while selecting the proper enclosure per the application is critical to its operational lifetime. Ultimately, a small amount of preplanning can go a long way in reducing maintenance, repair, or replacement costs down the line.