EX EXPLAINED #6: EX T – PROTECTION BY ENCLOSURE FOR COMBUSTIBLE DUST
You'll see it marked Ex ta, Ex tb or Ex tc, most often on motors, luminaires and enclosures used in food processing, milling, woodworking, powder handling and similar industries.
The Principle: Keep Dust Out, Keep Surfaces Cool
Ex t works on two fronts at once: preventing dust getting into the enclosure, and limiting how hot the enclosure's surfaces get.
Ingress protection. Dust is kept out by the enclosure's IP rating. Zone 20 and Zone 21 equipment (Ex ta and Ex tb) require IP6X, fully dust-tight. Zone 22 equipment (Ex tc) requires at least IP5X, dust-protected, allowing a very limited amount of ingress that isn't sufficient to interfere with safe operation (but must be IP6X if for use in conductive dust environments).
Surface temperature. Unlike gas, where ignition risk is judged mainly against a cloud of gas around the equipment, dust presents two separate ignition risks that both have to be assessed: a dust cloud, and dust that has settled into a layer on top of the equipment. A settled layer acts as an insulator, trapping heat, so the permitted surface temperature is set with this in mind and is derated further as the assumed layer gets thicker.
The Levels: ta, tb and tc
Ex t protection is divided into three levels, aligned with the dust zones.
Ex ta provides EPL Da and is suitable for Zone 20, areas where a combustible dust cloud is present continuously or for long periods, such as inside a vessel or hopper.
Ex tb provides EPL Db and is suitable for Zone 21, areas where a dust cloud is likely to occur occasionally during normal operation.
Ex tc provides EPL Dc and is suitable for Zone 22, areas where a dust cloud is not likely, or if it does occur, only briefly.
Remember, EPL Da equipment can be used in a Zone 20, 21 or 22; EPL Db can be used in a Zone 21 or 22; EPL Dc can only be used in a Zone 22.
Why This Matters for Rotating Machinery
A motor operating in a dusty environment faces a particular risk: heat generated internally, or from bearing friction, has to be managed so that no external surface exceeds its permitted temperature once a dust layer is taken into account. Cooling fins and fan covers are common places for dust to settle and build up, and a motor that's accumulated a thick coating of dust can run hotter at the surface than its rating assumes, even if the motor itself is operating normally.
Why This Matters for Repair and Maintenance
Seals and gaskets are the main protection. Because Ex t depends on the enclosure's IP rating, any seal, gasket or cable entry disturbed during repair must be correctly reinstated. A degraded seal that would be a minor issue on standard equipment can compromise an Ex t rating entirely.
Keep surfaces clean and accessible for inspection. Dust accumulation is an operational and maintenance issue as much as a design one. Regular cleaning schedules, appropriate to the site's dust hazard, help ensure the equipment continues to operate within its certified temperature limits. Good housekeeping also helps to reduce Zone extent and may sometimes remove a Zone entirely.
Cooling paths must be maintained. If a repair affects fan covers, cooling fins or ventilation paths, the equipment's ability to control surface temperature may be affected, not just its IP rating.
Repairs follow BS EN IEC 60079-19. As with the other concepts in this series, repair and overhaul of Ex t equipment must be carried out by competent personnel, referencing the equipment's original certification to confirm that dimensions, seals and thermal performance remain within specification.
Documentation and traceability apply as they do throughout this series. Repairs must be recorded, and the user informed in writing if the equipment can no longer be confirmed as meeting its original certification.
The Bottom Line
Where flameproof, increased safety and intrinsic safety were nominally built around gas and vapour hazards (Ex i is permitted for use in potentially flammable dust atmospheres based on its very low energy and thus ability to create surfaces hot enough to cause smoulder or ignition), Ex t addresses a different problem: keeping dust out, and keeping surfaces cool enough that neither a dust cloud nor a settled layer can ignite. For rotating machinery, that makes routine cleaning and seal condition just as much a part of maintaining certification as anything done on the workshop bench.
What's Next?
That completes our tour of the core Ex protection concepts. In the next post, we'll bring the series together and look at what all of this means in practice when it comes to repair, responsibility and choosing a competent repairer.