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Purge & Pressurization of Electrical Enclosures

Class, Division, Zone Compliance

General: When a piece of electrical equipment that does not have a suitable Class, Division, or Zone rating is used in a hazardous location, there is often a need for a quick solution. Purge and Pressurization is a common protection technique that allows the use of electrical equipment that is not suitably rated to be used in a hazardous location. However, this technique is not a magic wand allowing anything to be used.

Purge and Pressurization (herein referred as “Pressurization”) works by ensuring the electrical enclosure is free of flammable atmosphere prior to being energized – by purging the enclosure with a clean protective gas. While energized, the enclosure is maintained with the clean protective gas at a positive pressure. This is done to ensure that if a flammable atmosphere exists external to the electrical enclosure, it cannot enter the electrical enclosure.

Limitations: The obvious but often overlooked limitation of this protection technique is that it can only protect equipment internal to the enclosure. Components that are installed through the door or walls of the enclosure may not be properly protected by the Pressurization. This could include pushbuttons, switches, touch screens, or indicators.

A simple pushbutton or rotary switch which is made of a two-part assembly (an electrical contact assembly internal to the enclosure and a mechanical actuator) may be suitable for Pressurization, provided proper purging of the space around the electrical contact assembly can be ensured. However, with other devices, the purging cannot be ensured. This is because with an electrical component installed through the enclosure wall, one side of the component remains exposed to the potentially flammable atmosphere.

Touch screens also present an issue when installed in a Pressurized enclosure. The screen itself has to be considered similar to a make/break component because of its interface with the operator. These devices cannot be protected by Pressurization.

Whenever an electrical component is installed through the wall of an enclosure, it must be Certified for the Hazardous Location rating of the installation because it is impossible to purge all the air surrounding the component.

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