Protection by Enclosure - Ex t

Ex t EN 60079-31 Explosive atmospheres. Equipment dust ignition protection by enclosure "t"
Type of protection for explosive dust atmospheres where electrical equipment is provided with an enclosure providing dust ingress protection and a means to limit surface temperatures

Ex & Triangle En 60079-13, introduced in 2009 is a specific standard for dust using 'Protection by Enclosure' i.e. keep the dust out.
Based on the principle, for Dust, that a good ingress protection rating by definition should be sufficient to prevent an explosive dust mix forming within the enclosure. Therefore the primary concern is the temperature rise of that enclosure1.

Split into 3 protection levels Ex ta, Ex tb and Ex tc for Categories 1D, 2D and 3D respectively. Ex ta is the only2 Category 1 protection method other than Ex ia.

If the primary protection method is to keep the dust out then generally it does not matter what goes into the enclosure3 as long as the heat dissipation is considered. Any items going through the enclosure walls i.e. coming in contact with the dust must be suitably certified.

Although Ex tc certification with undefined components inside the enclosure is being offered by a small number of manufacturers Ex tb with undefined contents is rare for standard style enclosures (i.e. Ex e style). The only offering for Ex tb enclosures allowing un-certified internal components seems to be flameproof style enclosures.


Notes


  1. Ex ta has additional requirements for arcing and sparking parts and not only must the marked maximum surface temperature must be measured on the outside of the enclosure but also internally 

  2. Ex da for Category 1G does exist but is a special case for gas detectors only 

  3. There may be limitations for some specific components e.g. batteries. 

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