Which Enclosure When and Where?

Are There Formulas or Rules-of-Thumb That Must Be Followed When Installing Machines in Explosive Dust and Other Hazardous Areas?

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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 formulas 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

Answers

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 consists of a number of bolts and screws that secure the enclosure cover, it can be an effort just to open it 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, the No. 1 requirement is of course 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 also can be used, but this might not be a readily available resource for your customer. Some purge systems contain electronics that might require maintenance over time, and there is also some planning required 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 on 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 places. 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,
Pepperl+Fuchs

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 that 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 can generate a spark. However, high maintenance and calibration costs might make the use of this method less cost-effective.

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