A restricted-breathing enclosure is one that is designed to limit the exchange of air between the inside of the enclosure and the outside environment. It is almost gas-tight, but not quite.

The reason this works is because restricted-breathing enclosures are intended only for Zone 2 areas where gas can only be present rarely, and for brief periods. During a gas exposure event, the enclosure is designed to restrict the exchange of atmosphere inside and outside the enclosure. If done properly, the amount of flammable gas that leaks into the enclosure is small, and dilited by the interior air to the point where it cannot result in an explosive atmosphere. Once the external gas has dissipated, any flammable gas trapped inside the 'nR' enclosure eventually leaks out and dissipates.

Restricted-breathing enclosures have some advantages in that they can protect equipment that cannot qualify as non-sparking. Thus, restricted-breathing enclosures can protect motors, relays, lamps and other higher-power, normally-arcing equipment that is unsuited to other types of protection.

Restricted-breathing is largely a mechanical design technique, rather than an electrical one, making it attractive to electrical designers (and unattractive to mechanical designers). It can also appear intuitively easier to understand, and it is possible to purchase commercial off-the-shelf restricted-breathing enclosures for a ready-made solution.

These enclosures are not perfect and have many limitations. They must undergo a battery of durability tests, and normally requires a test port to facilitate this. There are also restrictions on the kinds of equipment that may be installed, meaning that off-the-shelf enclosures do not suit every situation.

Restricted-breathing was originally lumped into IEC 60079-15 alongside types "nL", "nC" and "nA", but none of these techniques were especially related to each other. Accordingly, there has been discussion about moving various type "n" techniques (including, presumably type "nR") to a more appropriate standard. This has already been done with types "nL" (in 2006) and "nA" (in 2018), so it is entirely possible type "nR" will undergo a similar transformation in the future. Exactly which standard is "better", however, is obviously up for debate, since no other standard relies primarily upon gas-tight sealing as a protective method.

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Go to Understanding IEC 60079.