IEC 60079-31 Explained: Equipment Dust Ignition Protection by Enclosure "t"

Ex t equipment uses enclosure protection to reduce ignition risks in combustible dust atmospheres.

Combustible dust can create serious fire and explosion hazards when dispersed in air or accumulated around equipment. Electrical equipment installed in these environments must be suitable for the classified area and designed to prevent it from becoming an ignition source.

IEC 60079-31 – Explosive Atmospheres – Equipment Dust Ignition Protection by Enclosure "t" specifies requirements for electrical equipment protected against ignition hazards from combustible dust by means of an enclosure.

The protection concept relies on enclosure construction, protection against dust ingress, and control of equipment surface temperature. Correct installation and maintenance are also important to preserve the intended protection throughout the equipment lifecycle.

1. Where IEC 60079-31 Fits

IEC 60079-31 is an equipment protection standard. It does not classify the hazardous area itself; that assessment is addressed by IEC 60079-10-2.

The relationship between the standards can be summarized as follows:

Dust Hazard Assessment → Area Classification → Equipment Selection → Installation → Inspection and Maintenance

Standard	Main purpose
IEC 60079-10-2	Classification of areas where combustible dust atmospheres may occur.
IEC 60079-31	Equipment dust ignition protection by enclosure "t".
IEC 60079-14	Design, selection, and installation of electrical installations.
IEC 60079-17	Inspection and maintenance of electrical installations.
IEC 60079-19	Repair, overhaul, and reclamation of Ex equipment, where applicable.

2. What Is Ex t Protection?

Ex t is a protection concept for equipment used in combustible dust atmospheres. It uses an enclosure to limit dust ingress and requires compliance with the applicable ignition protection and temperature requirements.

Unlike Ex d, which uses a flameproof enclosure designed to withstand an internal explosion and prevent flame transmission, Ex t relies on the protection characteristics specified for dust ignition protection by enclosure.

Important aspects include:

• Suitable enclosure construction.
• Protection against dust ingress.
• Compliance with applicable surface-temperature limits.
• Suitable cable entries and enclosure interfaces.
• Maintenance of the certified protection characteristics.

3. Why Dust Ingress Matters

Dust entering an enclosure may accumulate near electrical components or other potential ignition sources. Deposits can also interfere with heat dissipation and contribute to overheating.

Depending on the equipment and dust characteristics, dust ingress can:

• Allow combustible material to reach internal components.
• Contribute to internal dust accumulation.
• Affect heat dissipation and temperature conditions.
• Compromise the intended protection concept.

Enclosure integrity must therefore be maintained after installation. Damaged seals, incorrect cable glands, unsuitable replacement parts, or improperly refitted covers can affect the equipment's protection characteristics.

4. Relationship Between Ex t and Dust Zones

IEC 60079-10-2 defines the methodology for classifying combustible dust hazardous areas. The resulting zone is an important input when selecting suitable equipment.

Zone	General description	Typical corresponding dust EPL
Zone 20	Explosive dust atmosphere present continuously, frequently, or for long periods.	Da
Zone 21	Explosive dust atmosphere likely to occur occasionally during normal operation.	Db
Zone 22	Explosive dust atmosphere not likely during normal operation and, if it occurs, present only briefly.	Dc

These are typical zone-to-EPL relationships. Equipment suitability must be confirmed from the complete marking and certificate, including any special conditions of use.

5. Enclosure Protection and IP Rating

Ingress protection is an important consideration for Ex t equipment because the enclosure must limit dust ingress to the level required by its protection concept and certification.

IP ratings are defined under IEC 60529. For dust protection, the first digit indicates protection against solid particles. An IP6X rating indicates a dust-tight enclosure under the specified test conditions, while IP5X indicates dust-protected performance under those conditions.

The required IP rating depends on the applicable edition of IEC 60079-31, the equipment protection level, and the product's certification. Do not assign a universal minimum IP rating solely from the zone designation.

When evaluating equipment, check that the complete enclosure arrangement—including covers, joints, seals, cable entries, and accessories—maintains the required protection when installed according to the manufacturer's instructions.

Important: An IP rating by itself does not establish that equipment is certified for Ex t applications.

6. Surface Temperature and Dust Layers

Maximum surface temperature is a critical selection parameter for equipment installed in combustible dust atmospheres. The permitted temperature must be suitable for the relevant dust and the applicable certification requirements.

Unlike gas applications, dust assessments must pay particular attention to deposited dust layers. Accumulated dust can act as thermal insulation, reducing heat dissipation and increasing the temperature of equipment surfaces.

The temperature assessment may need to consider:

• Ignition characteristics of a suspended dust cloud.
• Ignition characteristics of a dust layer.
• Dust layer thickness and accumulation conditions.
• Equipment loading and operating temperature.
• Ambient conditions and certification restrictions.

Surface-temperature markings may use values such as T85°C, T100°C, T120°C, or T135°C. These are examples, not a list of universally acceptable temperatures. The appropriate limit must be established from the actual dust hazard assessment and equipment documentation.

7. Dust Groups and Equipment Protection Levels

Dust equipment markings may identify a dust group and an Equipment Protection Level (EPL). These describe different aspects of equipment suitability.

The dust groups are generally distinguished as follows:

• IIIA: Combustible flyings.
• IIIB: Non-conductive dust.
• IIIC: Conductive dust.

For dust atmospheres, the relevant EPL designations are Da, Db, and Dc. These correspond to different protection levels and are considered alongside the area classification.

Selection must account for the actual dust group, zone, EPL, temperature requirements, and complete equipment marking. A product should not be accepted based on only one of these parameters.

Related reading: Dust Groups IIIA vs IIIB vs IIIC Explained and EPL Da vs Db vs Dc Explained.

8. How to Read an Ex t Marking

Consider this illustrative marking:

Ex tb IIIC T85°C Db

Marking element	Meaning
Ex	Equipment intended for use in explosive atmospheres.
tb	Dust ignition protection by enclosure, protection level "b".
IIIC	Conductive dust group.
T85°C	Maximum surface temperature indicated by the marking.
Db	Equipment Protection Level for dust applications.

This is an illustrative example, not a claim that any particular product carries this marking. Actual equipment markings may include additional information, such as gas protection, ambient temperature limits, certificate references, and special conditions of use.

9. Testing and Certification

Equipment evaluated against IEC 60079-31 is subject to the applicable conformity assessment and testing requirements. Depending on the equipment and applicable standard edition, assessments may address enclosure ingress protection, temperature, mechanical strength, and other relevant characteristics.

When reviewing a product, check the actual certificate and supporting documentation rather than assuming every product undergoes an identical set of tests.

Certification evidence should be assessed alongside the equipment marking, manufacturer's instructions, and project requirements. An IP rating by itself does not establish that a product is certified for Ex t applications.

10. Practical Ex t Equipment Selection Checklist

For engineering and procurement teams, the equipment review should connect the product's certification and marking to the actual hazardous-area requirements.

Check	What to verify
Hazardous area	Applicable dust zone from the approved hazardous-area classification.
EPL	Da, Db, or Dc as required by the application.
Dust group	IIIA, IIIB, or IIIC suitability.
Temperature	Maximum surface temperature compatible with the dust hazard.
Enclosure protection	Required IP rating and maintenance of enclosure integrity.
Cable entries	Suitable cable glands, entry arrangements, and installation instructions.
Environment	Ambient temperature, corrosion, impact, and other service conditions.
Certification	Applicable certificate, complete marking, and any special conditions of use.

This checklist is useful during RFQ review and technical bid evaluation. If a vendor proposes an alternative product, confirm that the alternative meets every applicable requirement rather than comparing IP ratings alone.

11. Cable Glands and Enclosure Interfaces

Cable entries form part of the enclosure protection system. The selected gland and installation arrangement must be suitable for the equipment, cable, environmental conditions, and required protection characteristics.

Review the following:

• Correct cable gland type and size.
• Applicable certification and suitability for the dust application.
• Compatibility with the enclosure entry and cable construction.
• Correct installation and sealing arrangement.
• Maintenance of the required ingress protection.

The enclosure, gland, seals, and installation method should be evaluated together. A suitable enclosure can lose its intended protection if its cable entries are incorrectly selected or installed.

12. Installation, Inspection, and Maintenance

Ex t protection must remain effective after installation. The condition of the enclosure and its interfaces should be checked in accordance with the applicable inspection requirements, manufacturer's instructions, and site maintenance procedures.

Relevant inspection points may include:

• Enclosure covers, joints, and fasteners.
• Gaskets and sealing components.
• Cable glands and entry interfaces.
• Corrosion or mechanical damage.
• Equipment markings and identification.
• Dust accumulation that may affect thermal conditions.

Repairs or replacement of seals and other components must follow the applicable manufacturer's instructions and certification requirements. Do not assume that an improvised modification will preserve the certified protection characteristics.

13. Common Ex t Application Mistakes

• Selecting equipment based on IP rating alone.
• Ignoring the required dust group or EPL.
• Choosing an unsuitable maximum surface temperature.
• Overlooking dust accumulation around equipment.
• Using incompatible or incorrectly installed cable glands.
• Damaging or incorrectly refitting enclosure seals.
• Ignoring certificate conditions and ambient limits.
• Failing to review enclosure condition during maintenance.

These mistakes can compromise equipment suitability even when the original product was correctly certified. The intended protection must be preserved throughout installation and service.

14. IEC 60079-31 vs IEC 60079-10-2

These standards are related but perform different functions.

IEC 60079-10-2	IEC 60079-31
Classifies areas where combustible dust atmospheres may occur.	Addresses equipment dust ignition protection by enclosure "t".
Determines Zone 20, Zone 21, or Zone 22 based on the hazard assessment.	Defines applicable equipment protection requirements.
Provides an input to equipment selection.	Provides requirements for the equipment selected for the application.

The area classification should be established first. Equipment selection then needs to demonstrate suitability for that area and the applicable dust, temperature, environmental, and installation requirements.

Frequently Asked Questions

What is IEC 60079-31?

It specifies requirements for electrical equipment using enclosure "t" protection against ignition hazards in combustible dust atmospheres.

What does Ex t mean?

Ex t is dust ignition protection by enclosure. The equipment must meet the applicable enclosure, temperature, marking, and certification requirements.

Can Ex t equipment be used in Zone 20, Zone 21, or Zone 22?

Suitability depends on the equipment's certified protection level, marking, and conditions of use. Check the certificate and applicable project requirements rather than relying on the Ex t designation alone.

Does IP66 mean equipment is suitable for Ex t applications?

No. IP66 describes ingress protection under specified test conditions. It does not, by itself, establish Ex certification or suitability for a particular dust zone.

Why are dust layers important?

Dust deposits can act as thermal insulation and increase surface temperature. They may also become airborne and contribute to a combustible dust cloud.

Conclusion

IEC 60079-31 addresses equipment protection against ignition hazards in combustible dust atmospheres through enclosure "t" protection.

For engineering and procurement teams, the assessment should cover the complete equipment marking, EPL, dust group, maximum surface temperature, enclosure protection, cable entries, certification, and service conditions. An IP rating alone is not sufficient evidence of Ex t suitability.

Correct installation and maintenance are equally important. The enclosure, seals, and cable entries must remain in suitable condition so that the equipment's intended protection characteristics are preserved throughout its service life.

References

• IEC 60079-31 – Explosive Atmospheres – Equipment Dust Ignition Protection by Enclosure "t".
• IEC 60079-10-2 – Explosive Atmospheres – Classification of Areas – Combustible Dust Atmospheres.
• IEC 60079-14 – Explosive Atmospheres – Electrical Installations Design, Selection and Installation.
• IEC 60079-17 – Explosive Atmospheres – Electrical Installations Inspection and Maintenance.
• IECEx System – International Certification System for Equipment Used in Explosive Atmospheres.