Electronic Enclosures: Answers to Common Questions

by Ravi Jain, Bud Industries Inc.

If you want to learn more, please visit our website.

Here are answers to commonly asked questions we hear from people selecting an electronic enclosure.

Which material is best for my application?

Electronic enclosures are made from plastic, die-cast aluminum, sheet metal, and fiberglass. Choosing the right material for your application depends on the environment in which the enclosure will be used. Engineering considerations include impact resistance, cooling and shielding, weight, corrosion resistance, and of course, cost.

Impact Resistance
Impact resistance, or more generally, material strength, is a primary consideration when selecting a material for your enclosure. Think about the end-use location. For example, will the enclosure be located high on a pole or on a machine where workers could bump it with a ladder? Think about the contents. Does it house critical factory controls or an optional tablet PC? What is the weight of the internal components?

• For high impact resistance, steel is a classic material favored in some industrial enclosure applications, such as large cabinets housing electrical components.
• Die-cast aluminum is another tough material. The casting process creates enclosures with thick walls and radiused corners that effectively distribute loads. It is more affordable than steel.
• Fiberglass enclosures gain high tensile strength by strengthening plastic with glass fibers.
• Fiberglass-reinforced polycarbonate plastic includes 10 percent glass fibers. It adds peace of mind in enclosure applications where moderate impact resistance is useful.

Cooling
Heat management is a constant in electronic design. Metal enclosures are effective at absorbing heat and dissipating it to the air outside the enclosure. Choices include steel, die-cast aluminum, sheet metal aluminum, and extruded aluminum.

Shielding
Metal enclosures naturally provide some level of EMI (electromagnetic interference) and RF (radio frequency) shielding. If an enclosure needs total EMI/RP protection, then the gasket needs to include a wire screen that covers the gap between the body and lid of the enclosure. (Bud&#;s IPS Series die-cast aluminum enclosure offers this type of shielded gasket.)

Weight

• Fiberglass is a good choice for large enclosures because it weighs far less than steel yet offers high impact resistance.
• Plastic is the lightest enclosure material, well suited for portable and handheld enclosures.
• Polycarbonate plastic enclosures weigh less than enclosures made of ABS plastic.
• Aluminum weighs less than steel, but is less rigid

Corrosion Resistance

• • Unlike steel, plastic, fiberglass, and aluminum impart natural corrosion resistance to enclosures.
  • Powder coating steel makes Increases its corrosion resistance
  • For extreme corrosion resistance where high strength is needed, stainless steel is the safest choice.

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Cost
In one way or another, your enclosure choice typically involves a cost/benefit tradeoff. Because the cost of an enclosure represents a significant share of a bill of materials, specifiers should select the least costly material that will meet the requirements.

• ABS plastic has the lowest cost of all enclosure materials. Compared to polycarbonate plastic, it is heavier, less impact-resistant, and not UL rated.
• Polycarbonate plastic allows low-cost enclosures, especially when IP and NEMA ratings are required but at a higher than ABS
• Fiberglass enclosures are not inexpensive, but offer superior UV and weather resistance.
• Die cast enclosures are also low cost but share with plastics the very high tooling costs for modification of dimensions.
• There are various sheet metal enclosures which are often higher priced than plastic but do allow for more ready modifications of major dimensions.

Learn More
Selecting the ideal material is critical. This topic is covered in detail in our website article, &#;Choosing the Right Material for your Enclosure.&#; It includes descriptions and selection tips for:

• Steel Enclosures
• Aluminum Sheet Metal Enclosures
• Extruded Aluminum Enclosures
• Die-Cast Aluminum Enclosures
• Polycarbonate Plastic Enclosures
• ABS Plastic Enclosures
• Fiberglass Enclosures
• Fiberglass Reinforced Polycarbonate Enclosures

How do modifications affect UL ratings?

Electronic and electrical applications require openings in the enclosure for switches, indicators, displays, signal lines, and power wires. This creates something of a dilemma. You may source a UL approved enclosure, but cutting an opening in an enclosure will remove its UL rating. This means that you need to recertify your end design. Fortunately, UL is aware of the situation, and it has stated that by using an initially UL rated box with UL components (like switches, displays, and connectors) the UL approval process for your design will go much easier. See tips how to recertify on our enclosures FAQs page.

How to cool a cabinet

In normal operation, the biggest threat to the reliability of electronic equipment is heat, which can shorten the life of electrical components. Servers, computers, network switches, routers, and communications equipment all require a reasonable operating temperature. When these devices are located in a cabinet, various cooling strategies are used.

Internal components are typically mounted to a heat sink, a metal structure that conducts heat away from the component and releases the heat via radiation and convection. Basically, it transfers the heat to the air. The strategy for cooling a cabinet is to remove the hot air and replace it with cool air. Cabinets are well ventilated, allowing for natural airflow. When natural airflow is insufficient, designers add blowers and fans to flow air through the cabinet.

While fans are effective and low in both initial and operational cost, fans alone cannot remove enough heat in some critical applications. Cabinets with air conditioning and liquid cooling systems provide cooling in these applications, although at high cost.

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Engineers specifying a cabinet rack must understand where to locate cooling features inside the cabinet. The considerations include:

• Chimney effect
• Fans versus blowers
• Backpressure
• Ventilation
• Dust
• Locations in the cabinet
• Air conditioning
• Airflow calculations

All of these considerations are discussed in our white paper, The Basics of Enclosure Cooling.

How to prevent condensation in an enclosure?

Condensation occurs when humidity in the air collects on a cool surface, such as water drops collecting on the outside of a cold glass of lemonade on a hot day. While condensation is not a problem for lemonade, it poses a serious threat to electronic components. Moisture can cause corrosion leading to premature failure. It can even create damaging electrical shorts.

Condensation is a concern in humid locations that are subject to swings in temperature. It is not typical in indoor locations, and it is not even common to all outdoor locations. Nevertheless, a designer may not know all the locations settings which an end product will be used, and so schemes to prevent condensation should be employed.

Different strategies are used to mitigate condensation. Each has advantages and disadvantages.

Heating the cabinet.
Sometimes designers add heating elements to the bottom of enclosures (large cabinets, not small box enclosures). The heating elements assure the interior components never get cold enough for condensation to form (above the dew point). Unfortunately, heat can shorten the life of electrical components and increase nuisance tripping of overcurrent protection devices. In most electronics applications, designers try to remove heat.

Air conditioning the cabinet.
Air condition both cools the air removes moisture from cabinets. It prevents condensation and maximizes component life. On the other hand, it adds a great deal of expense and complexity, which is not practical in most applications. Both air conditioning and heating systems require thermostat controls, use a lot of energy, and will eventually require maintenance.

Sealing the enclosure.
A tight seal, available in enclosures rated IP65/NEMA 4 and higher, will prevent moisture from entering the enclosure. This is the usual strategy for box enclosures and small cabinets. The downside is that warm air cannot escape the enclosure. Also, atmospheric pressure changes caused by changing weather can create a pressure difference large enough to cause gaskets to fail over time.

Using IP rated vents.
To allow air pressure inside an enclosure to equalize with the outside air pressure, a designer may install a vent. The vent also allows heat to escape from the enclosure. Recently, IP rated vents have become available. These use special breathable fabric that permits airflow but blocks moisture that could otherwise cause condensation.

Using a vortex cooler.
If compressed air service is available on-site, then designers can employ a technique to use that air for cooling. The vortex cooler spins the air into a vortex. The vortex separates the cool air, which can be directed through the cabinet. These systems are loud and consume a lot of air. Just because compressed air is already there, cooling isn&#;t free, as the compressor costs money to run. One advantage of vortex cooling is that the positive air pressure helps keep out dust.

What do enclosure flammability ratings mean?

Any plastic has its origin in petrochemicals and is ultimately flammable. Fortunately, flame retardant additives make plastic resistant to fire, a feature that is important in many enclosure applications, especially in manufacturing and hazardous locations.

The UL 94 standard defines the various levels of flame resistance. Flammability ratings let the specifier know how the material in a plastic enclosure will behave if exposed to fire. Will it protect against fire, or will it contribute to it? The ratings are how you know.

The main two things measured by the standard are how fire spreads on the wall of the enclosure and if drips of melting plastic are in flame. Although the standard is for enclosures specifically, the tests are done on a sample of the material.

Our blog post, &#;Flammability 101 for Plastic Electronic Enclosures,&#; describes each UL enclosure flammability rating and associated test:

• HB
• V-2
• V-1
• V-0
• 5VB
• 5VA

The tests are performed on horizontal samples or vertical samples. The tests specify burning time before self-extinguishing. Read our blog post for how to specify an enclosure flammability rating.

Now that we have answered your common questions about enclosures, find the enclosures that you need by searching by size, type, and other characteristics.

If you want to learn more, please visit our website custom sheet metal enclosures.