Which Enclosure When and Where?

Feb. 12, 2009
What Are the Guidelines for Choosing Between Explosion-Proof and Purged Enclosures?

We're a panel builder that's branching out to work with machine builders whose machines often are installed in explosive dust and other hazardous areas. We need to design the electrical and control panels accordingly. Are there formulae or rules-of-thumb that we can use to calculate when it makes sense to use a purged enclosure instead of an explosion-proof enclosure? The machine builder's customers often have both options in the same facility, so it's not just a case of their clear preference for one over the other.

—from January ’09 Control Design


Classic Gas
This is the classic argument of purge vs. explosion proof. Some customers might have a preference, but there are clear advantages and disadvantages for either protection method. Over the years, explosion-proof enclosures have been the tried and true method to install general-purpose equipment in a hazardous location. This is a low-maintenance solution that does not in itself contain any electronics or moving parts and is suitable for installation of most equipment. But explosion-proof enclosures can become impractical for large-enclosure requirements. The weight of these enclosures can become cumbersome with size, not to mention very pricey. Also, since the design of these enclosures consists of a number of bolts and screws that secure the enclosure cover, it can be an effort just to get inside for maintenance purposes. I think the biggest drawback is that while they do meet haz-loc requirements by containing an explosion to the interior of the enclosure, they do nothing to protect the equipment operating inside. So should an ignition occur, it is highly probable that the equipment inside the enclosure will need to be fixed or replaced, and there really isn’t any indication that this type of event occurred.

When considering the use of a purge and pressurization system, of course the No. 1 requirement is the availability of a compressed air source of clean instrument air. Though most purge systems are available with air filters, it is highly recommended that the air source be free of contaminates such as dirt or water condensation. An inert gas such as nitrogen can also be used, but this might not be a readily available resource for your customer. Some purge systems contain electronics which might require maintenance over time, and there is also some planning that needs to be done to ensure that the proper purge system is chosen based on enclosure size and environmental conditions. That being said, I think the benefits outweigh the disadvantages. For one, purge systems are the only technology that meets the demand for standard enclosures to be mounted inside hazardous locations. So your enclosure design does not have to change if haz-loc installation becomes a requirement. Sometimes it is even an afterthought. A purge system is generally a simple add-on just by adding a few hole penetrations to the enclosure for the air inlet and purge attachments. Other benefits are that you typically can have an alarm indication upon purge failure, the purge system can provide additional cooling for your equipment and, since the intention of a purge system is to prevent an explosion from occurring, you have less worry of your equipment being damaged under a fault.

In the end, both methods have their appropriate place. Explosion-proof is a better route if the enclosure requirements are less than 1 ft3. Any larger than that leaves you working with a cumbersome and expensive solution, where purge systems become the better fit.

Brian Alvarado, product manager, visualization & system solutions

Continental Differences
“Explosion-proof enclosure” is a common term that has many meanings. These enclosures are meant to be used in hazardous areas, where flammable substances, such as gas, vapor or dust, are present and, when mixed with air, can explode if they come in contact with an ignition source. An explosion-proof rating is given only to a single piece of equipment for a specific class, division and group. Equipment installation is the responsibility of the end user and the National Electrical Code defines requirements of an installation.

When specifying panels to be used in a hazardous area, it is critical to understand which standard applies to your application. Two of the most prominent standards in the world are UL (North America) and CENELEC (European Union). In the U.S., if the product is certified through testing by UL, Factory Mutual or CSA, then the explosion-proof requirements for NEC are satisfied. In 2003, Europe (CENELEC) adopted the ATEX directive which defines the technical and legal requirements for products intended for use in potentially explosive atmospheres. Once the correct standard is identified, it is possible to review the standard and determine which type of panel is required. In many cases, if the selection of a proper panel becomes difficult, most manufacturers can help guide you.

The explosion-proof-enclosure method of protection is based on the explosion containment concept. In this type of enclosure, the energy source is permitted to come in contact with the dangerous air and flammable substance mixture. In effect, the explosion is allowed to take place, but it must remain confined in the enclosure. These enclosures are built to resist the excess pressure created by an internal explosion, impeding the propagation to the surrounding atmosphere.

This protection method is one of the most widely used and is suitable for electrical apparatuses located in hazardous locations as well as in applications where high levels of power are required, such as motors and transformers, i.e., parts that may generate a spark. However, high maintenance and calibration costs may make the use of this method less cost-effective.

Purging, also known as pressurization, is a protection method based on a segregation concept. This method does not allow the hazardous air and substance mixture to enter the enclosure containing the ignition source, i.e., electrical parts.

In the U.S., there are limitations for the use of purged systems. These limitations are based on the volume of the enclosure, group of the substance and the application class (Class I or II). These systems include a purging/pressurization device to monitor flow and pressure and an enclosure with a NEMA 4, 4X or 12 rating. This technique is based on filling the enclosure with clean air or an inert gas, with or without continuous flow, at sufficient pressure to prevent the entrance of combustible substance. In Europe, the enclosures would be designed to an ATEX standard, and the term “pressurization” refers to a purging technique for Zones 1 and 2.

Ultimately, the choice comes down to standard conformity, adherence to company policies and total cost of ownership. A clear understanding of the environment, relative standards and product options will help you make the right choice.

Bernardino Martínez, project manager, engineered solutions business, control products & solutions
Rockwell Automation

Pay Your Way Through Customs
As a manufacturer of custom equipment, I understand your frustration with major enclosure manufacturer's services. As someone who has visited several of the major manufacturer's facilities and collaborated with many of their associates over a 20-year period, please know these companies routinely pass off or toss out the vast majority of custom-enclosure inquiries they receive because they fully realize they are not in the best position to satisfy these applications.

Alternatives can be obtained by searching the term "custom metal enclosures" in your favorite search engine. Be careful to avoid the term "custom enclosure," which will mostly resolve to car and home stereo system speaker boxes. Using the first term, you will find a half-dozen or so well-qualified companies scattered around the country, each with a slightly different methodology for manufacturing an enclosure to your exact specifications. However, please understand that while you may gain some additional satisfaction with service and delivery from custom enclosure manufacturers, prices will be much higher that standard enclosures due to the limited if not one-off volume you are requesting. The fast delivery and low pricing for standard enclosures is attained by mass-production techniques that require no engineering or programming and little if no human interface during production, once the design is finalized.

In contrast, a custom enclosure often requires an intense level of effort from beginning to end, depending on the degree of modification you require for each new unit you have produced and how many if any similarities between each variation of the enclosure are incorporated from project to project. A real key to your future satisfaction will be harmonization and compartmentalization of your designs, regardless of who you go with. Think in terms of building blocks or modules that can be reused from application to application to eliminate as much engineering and design effort as possible. Then use standard enclosures wherever possible and stack on the custom bits and pieces only where needed. The end result is minimized engineering and production costs, which you can then use to maintain competitive pricing and sustain a reasonable degree of aesthetic value.

Mike Baucom, executive director
Bebco Industries

May’s Problem
We upgraded to high-efficiency single-speed AC motors on our metal-cutting machines about five years ago. We are wondering if now is a good time to revisit the motor market and seek improved solutions. What significant improvements, if any, have occurred in the past five years with respect to single-speed AC motors in the 1-25 hp range?

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