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

Combustible dust can create explosion hazards that are just as severe as those associated with flammable gases and vapors. Industries such as grain handling, food processing, pharmaceutical manufacturing, and woodworking frequently operate in environments where combustible dust clouds and dust layers are present.

To safely operate electrical equipment in these environments, equipment must be designed to prevent dust ingress and avoid becoming an ignition source.

For this reason, the explosion protection industry relies on IEC 60079-31 – Explosive Atmospheres – Equipment Dust Ignition Protection by Enclosure "t".

This standard specifies the requirements for electrical equipment protected by enclosures that prevent the ingress of combustible dust and limit surface temperatures to avoid dust ignition.

Equipment certified according to IEC 60079-31 is commonly identified by the marking Ex t and is widely used in dust hazardous areas classified as Zone 20, Zone 21, and Zone 22.

This article explains what IEC 60079-31 is, why it is important, and the major concepts that engineers should understand when selecting and applying Ex t equipment.

Quick Answer

What Is IEC 60079-31?

IEC 60079-31 is the international standard that governs electrical equipment protected against combustible dust ignition by enclosure.

The standard specifies requirements for:

• Protection against dust ingress.
• Surface temperature limitations.
• Equipment construction.
• Testing requirements.
• Equipment marking.
• Equipment documentation.
• Verification of enclosure integrity.

The objective of the standard is to ensure that equipment installed in combustible dust atmospheres cannot become an ignition source under normal operating conditions.

What Is Ex t Protection?

Ex t is an explosion protection concept that prevents combustible dust from entering electrical equipment and ensures that the external surface temperature of the equipment remains below the ignition temperature of the surrounding dust.

Unlike Ex d or Ex e protection, Ex t protection does not rely on containing explosions or preventing sparks from occurring internally.

Instead, Ex t protection primarily relies on:

• Suitable enclosure construction.
• Ingress protection against dust.
• Controlled surface temperatures.
• Proper installation practices.

Because combustible dust can accumulate on equipment surfaces, temperature control is one of the most important aspects of Ex t protection.

Why Is IEC 60079-31 Important?

Combustible dust can ignite in two different ways:

• Ignition of suspended dust clouds.
• Ignition of accumulated dust layers.

A dust explosion can occur when combustible dust becomes suspended in air and encounters an ignition source.

Similarly, thick dust layers can accumulate on equipment surfaces, causing heat retention and potentially leading to smoldering ignition.

IEC 60079-31 establishes requirements intended to prevent both of these scenarios.

Who Uses IEC 60079-31?

The standard is widely used by industries that process, transport, or store combustible dust materials.

Typical users include:

• Electrical engineers.
• Instrumentation engineers.
• Hazardous area specialists.
• EPC contractors.
• Equipment manufacturers.
• Inspection organizations.
• Facility operators.
• Regulatory authorities.

The standard is particularly important in industries where combustible dust hazards may not be immediately obvious.

Where Is Ex t Equipment Used?

Equipment certified according to IEC 60079-31 is commonly used in facilities handling combustible dust, including:

• Grain terminals.
• Flour mills.
• Sugar processing plants.
• Wood processing facilities.
• Food manufacturing plants.
• Coal handling facilities.
• Pharmaceutical manufacturing facilities.
• Chemical powder processing plants.

In many of these facilities, combustible dust hazards exist continuously and therefore require appropriately certified equipment.

Relationship Between IEC 60079-31 and Zone Classification

IEC 60079-31 works together with IEC 60079-10-2, which defines combustible dust hazardous area classification.

Zone	Description
Zone 20	Combustible dust cloud is present continuously or for long periods.
Zone 21	Combustible dust cloud is likely to occur occasionally during normal operation.
Zone 22	Combustible dust cloud is not likely to occur during normal operation and, if it occurs, exists only briefly.

Equipment selected for these zones must provide an appropriate Equipment Protection Level (EPL) and comply with the requirements of IEC 60079-31.

Scope of IEC 60079-31

IEC 60079-31 specifies the requirements for electrical equipment protected against ignition hazards arising from combustible dust atmospheres.

The standard applies to equipment that relies on enclosure protection to prevent combustible dust from entering the equipment and to ensure that surface temperatures remain below ignition limits.

The requirements cover:

• Equipment construction.
• Enclosure integrity.
• Ingress protection requirements.
• Temperature limitations.
• Type testing.
• Equipment marking.
• Documentation requirements.
• Installation considerations.

The standard applies to electrical equipment intended for use in Zone 20, Zone 21, and Zone 22 hazardous areas.

Major Requirements of IEC 60079-31

Although IEC 60079-31 contains numerous technical requirements, the major topics generally include:

• Protection by enclosure.
• Ingress protection requirements.
• Surface temperature limitations.
• Dust layer considerations.
• Equipment Protection Levels (EPL).
• Testing requirements.
• Equipment marking requirements.

These requirements are intended to ensure that equipment cannot ignite combustible dust atmospheres during normal operation.

Protection by Enclosure

The fundamental principle of Ex t protection is the use of an enclosure that prevents combustible dust from entering the equipment and coming into contact with potential ignition sources.

The enclosure must:

• Prevent excessive dust ingress.
• Maintain its integrity during service.
• Withstand environmental conditions.
• Protect against mechanical damage.
• Maintain the required degree of protection.

Unlike gas protection concepts such as Ex d, Ex t protection primarily focuses on preventing dust ingress and controlling equipment temperatures.

Ingress Protection Requirements

Ingress protection (IP) is one of the most important requirements of IEC 60079-31.

The required IP rating depends on the intended Equipment Protection Level (EPL) and zone classification.

EPL	Typical Zone	Minimum IP Rating
Da	Zone 20	IP6X
Db	Zone 21	IP6X
Dc	Zone 22	IP5X or IP6X depending on the application

Maintaining the specified ingress protection rating is essential because dust entering the enclosure may accumulate and become an ignition source.

Surface Temperature Limitations

Combustible dust can ignite when exposed to elevated temperatures.

For this reason, IEC 60079-31 requires equipment surface temperatures to remain below:

• The ignition temperature of a dust cloud.
• The ignition temperature of a dust layer.

The maximum permitted surface temperature of equipment is normally specified as part of the equipment marking.

Examples include:

• T85°C
• T100°C
• T120°C
• T135°C
• T200°C

Selecting equipment with an inappropriate temperature rating may create a serious ignition hazard.

Dust Layers Versus Dust Clouds

One of the unique aspects of combustible dust protection is that equipment must be evaluated against both dust clouds and dust layers.

Dust layers can:

• Act as thermal insulation.
• Increase equipment surface temperatures.
• Smolder and create ignition sources.
• Become dispersed and form explosive dust clouds.

The thickness of dust layers may therefore significantly influence equipment selection and operating limitations.

Equipment Protection Levels (EPL)

IEC 60079-31 uses Equipment Protection Levels to indicate the degree of protection provided by equipment.

EPL	Zone	Protection Level
Da	Zone 20	Very high protection
Db	Zone 21	High protection
Dc	Zone 22	Enhanced protection

The EPL forms an important part of the equipment marking and assists engineers in selecting equipment suitable for the intended hazardous area.

Type Testing Requirements

Equipment certified according to IEC 60079-31 must undergo testing to verify its suitability for use in combustible dust atmospheres.

Testing may include:

• Ingress protection testing.
• Temperature testing.
• Thermal endurance testing.
• Mechanical strength testing.
• Impact testing.
• Resistance to environmental conditions.

These tests help demonstrate that the equipment can safely operate under the intended service conditions.

Equipment Marking Requirements

Equipment certified according to IEC 60079-31 includes specific markings that indicate its suitability for combustible dust atmospheres.

A typical marking may appear as:

Ex tb IIIC T85°C Db

This marking indicates:

• Ex – Explosion protected equipment.
• tb – Protection by enclosure for dust.
• IIIC – Conductive dust group.
• T85°C – Maximum surface temperature.
• Db – Equipment Protection Level.

Understanding these markings is essential for selecting equipment suitable for combustible dust hazardous areas.

Advantages of IEC 60079-31

IEC 60079-31 provides a standardized method for protecting electrical equipment used in combustible dust atmospheres and significantly reduces the risk of dust ignition.

Without a dedicated standard for dust protection, equipment manufacturers and end users would have no consistent methodology for evaluating enclosure performance and temperature limitations.

Key advantages of IEC 60079-31 include:

• Provides internationally recognized requirements for Ex t equipment.
• Protects against dust ingress and dust accumulation.
• Addresses both dust clouds and dust layers.
• Supports safe equipment selection for Zone 20, Zone 21, and Zone 22.
• Provides clear marking requirements.
• Supports global certification schemes such as IECEx and ATEX.
• Improves reliability in dusty industrial environments.
• Reduces the likelihood of combustible dust explosions.

For many industries, IEC 60079-31 forms an essential part of their combustible dust risk management strategy.

Limitations of IEC 60079-31

Although IEC 60079-31 provides comprehensive requirements for dust protection, several limitations should be considered.

• Protection depends heavily on maintaining enclosure integrity.
• Damaged gaskets can compromise protection.
• Improper installation may reduce the effective IP rating.
• Dust accumulation may require additional maintenance.
• Equipment surface temperatures may change under abnormal conditions.
• Regular inspection is necessary to maintain protection.

For these reasons, Ex t equipment should always be installed and maintained in accordance with the manufacturer's instructions and applicable standards.

The Importance of Surface Temperature

One of the most critical aspects of combustible dust protection is controlling equipment surface temperature.

Combustible dust can ignite even when no electrical sparks are present.

Potential ignition mechanisms include:

• Hot external surfaces.
• Smoldering dust layers.
• Internal overheating.
• Mechanical friction.
• Abnormal operating conditions.

Because dust layers can act as thermal insulation, equipment operating temperatures may become significantly higher than expected.

For this reason, IEC 60079-31 places significant emphasis on temperature testing and temperature marking requirements.

The Importance of Dust Layers

A common misconception is that only suspended dust clouds create explosion hazards.

In reality, dust layers can be equally dangerous because they may:

• Retain heat.
• Create smoldering ignition sources.
• Increase equipment surface temperatures.
• Become dispersed and form explosive dust clouds.
• Contribute to secondary explosions.

The thickness of accumulated dust can therefore have a direct impact on equipment selection and safe operating temperatures.

Maintenance Considerations for Ex t Equipment

Because Ex t protection relies heavily on enclosure integrity, proper maintenance is essential throughout the equipment lifecycle.

Typical inspection activities include:

• Checking enclosure condition.
• Inspecting gaskets and seals.
• Verifying cable gland integrity.
• Removing excessive dust accumulation.
• Checking equipment markings.
• Inspecting for corrosion and mechanical damage.

Failure to maintain enclosure integrity may significantly reduce the effectiveness of Ex t protection.

Common Mistakes When Using Ex t Equipment

Experience throughout the industry has shown that several installation and maintenance errors occur repeatedly.

Examples include:

• Using equipment with insufficient IP protection.
• Ignoring dust layer accumulation.
• Installing equipment with damaged gaskets.
• Using incorrect cable glands.
• Opening equipment without properly restoring seals.
• Selecting equipment with an unsuitable temperature rating.
• Ignoring housekeeping requirements.
• Installing equipment in a zone for which it is not certified.

Many of these deficiencies can compromise dust protection and create serious ignition hazards.

Relationship Between IEC 60079-31 and Other Standards

IEC 60079-31 forms part of the broader combustible dust protection framework and should be used together with several other IEC 60079 standards.

Standard	Purpose
IEC 60079-10-2	Classification of combustible dust hazardous areas.
IEC 60079-31	Equipment protection by enclosure for dust atmospheres.
IEC 60079-14	Selection and installation of Ex equipment.
IEC 60079-17	Inspection and maintenance of Ex installations.
IEC 60079-19	Repair and overhaul of Ex equipment.

Together, these standards provide a complete framework for managing combustible dust hazards throughout the lifecycle of industrial facilities.

Why Engineers Should Understand IEC 60079-31

Combustible dust hazards are often underestimated because they exist in industries that are not traditionally associated with explosive atmospheres.

A basic understanding of IEC 60079-31 helps engineers:

• Select suitable Ex t equipment.
• Interpret equipment markings.
• Understand temperature limitations.
• Recognize the importance of dust layers.
• Review installation requirements.
• Improve maintenance programs.
• Reduce the likelihood of dust ignition incidents.

Because combustible dust explosions can be catastrophic and are often preventable, IEC 60079-31 remains one of the most important standards for engineers working in industries where combustible dust is present.

Frequently Asked Questions (FAQ)

What is IEC 60079-31?

IEC 60079-31 is the international standard that specifies the requirements for electrical equipment protected against combustible dust ignition by enclosure. The standard establishes requirements for enclosure protection, temperature limitations, testing, and equipment marking.

What does Ex t mean?

Ex t is an explosion protection concept that prevents combustible dust from entering electrical equipment and ensures that equipment surface temperatures remain below the ignition temperature of the surrounding dust atmosphere.

Can Ex t equipment be used in Zone 20?

Yes. Ex t equipment can be used in Zone 20 provided it has the appropriate Equipment Protection Level, typically EPL Da, and complies with the requirements of IEC 60079-31.

Why is the IP rating important for Ex t equipment?

The IP rating indicates the ability of the enclosure to prevent dust ingress. If dust enters the enclosure, it may accumulate and create ignition hazards or impair equipment performance.

Why are dust layers important?

Dust layers can act as thermal insulation, causing equipment temperatures to rise. They can also become dispersed and form explosive dust clouds that may result in secondary explosions.

What does Ex tb IIIC T85°C Db mean?

The marking indicates equipment protected by enclosure for dust atmospheres, suitable for conductive dust (IIIC), with a maximum surface temperature of 85°C and an Equipment Protection Level of Db for use in Zone 21 applications.

Conclusion

IEC 60079-31 is one of the most important standards for protecting electrical equipment used in combustible dust atmospheres. The standard establishes requirements for enclosure integrity, dust ingress protection, surface temperature limitations, and equipment marking.

Unlike many gas protection concepts, Ex t protection relies heavily on preventing dust ingress and controlling surface temperatures to avoid ignition of dust clouds and dust layers.

A proper understanding of IEC 60079-31 helps engineers select suitable equipment, maintain enclosure integrity, and recognize the hazards associated with combustible dust accumulations.

Because combustible dust explosions can occur in a wide variety of industries and often have devastating consequences, IEC 60079-31 remains an essential standard for engineers, equipment manufacturers, and facility operators responsible for managing combustible dust hazards.

Technical Review

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 Erection.
• IEC 60079-17 – Explosive Atmospheres – Electrical Installations Inspection and Maintenance.
• IECEx System – International Certification System for Equipment Used in Explosive Atmospheres.
• International Electrotechnical Commission (IEC).
• EN IEC 60079-31 – Equipment Dust Ignition Protection by Enclosure "t".