What Is Ex t Dust Protection? A Complete Guide to Protection by Enclosure

Ex t, formally known as Protection by Enclosure, is an explosion protection method for electrical equipment used in explosive dust atmospheres.

Unlike gas protection concepts such as Ex d, Ex e, or Ex i, Ex t is specifically designed for environments where combustible dust can form an explosive atmosphere.

The protection principle combines two important requirements: preventing hazardous dust from entering the equipment and controlling the equipment's maximum surface temperature so that it does not become an ignition source.

The method is covered by IEC 60079-31, which specifies requirements for equipment using protection by enclosure and surface-temperature limitation in explosive dust atmospheres. The current IEC publication is IEC 60079-31:2022.

What Does Ex t Mean?

The designation Ex t comes from the IEC 60079 explosion-protection system.

• Ex = explosion protection marking

• t = protection by enclosure for explosive dust atmospheres

The protection concept is based on the physical integrity of the enclosure together with controlled surface temperatures.

IEC 60079-31 specifies the design, construction, and testing requirements for equipment protected by enclosure and surface-temperature limitation for use in explosive dust atmospheres.

How Ex t Protection Works

Ex t protection can be understood through two fundamental functions.

1. Limiting Dust Ingress

The enclosure is designed to restrict the entry of combustible dust into the equipment.

This reduces the possibility that dust will accumulate around internal electrical components that could become ignition sources.

The required ingress protection depends on the applicable equipment design, protection level, dust group, and certification requirements. It should not be assumed that every Ex t product requires the same IP rating.

2. Limiting Surface Temperature

The external and, where applicable, internal temperatures of the equipment must remain within the limits established by the protection requirements.

This is particularly important because combustible dust can form layers on equipment surfaces. A surface that becomes excessively hot can create an ignition hazard even when the dust is not present as a dense airborne cloud.

Therefore, Ex t design considers both dust ingress and thermal behavior.

Why Combustible Dust Is Dangerous

Many industrial materials can form combustible dust atmospheres when sufficiently finely divided.

Examples include:

• Grain dust

• Flour dust

• Sugar dust

• Wood dust

• Coal dust

• Carbon-containing powders

• Some chemical and pharmaceutical powders

• Certain metal dusts

When combustible dust is dispersed in air at a concentration within its explosive range, an ignition source can result in a dust explosion.

The actual ignition characteristics depend on the specific dust. Therefore, equipment selection should be based on the hazardous-area classification and the properties of the actual dust involved rather than assuming that all combustible dust behaves the same way.

Dust Hazardous Area Zones

Hazardous areas containing combustible dust are classified into three zones:

Zone 20

An area where an explosive atmosphere in the form of a cloud of combustible dust in air is present continuously, for long periods, or frequently.

Zone 21

An area where an explosive atmosphere in the form of a cloud of combustible dust in air is likely to occur occasionally in normal operation.

Zone 22

An area where an explosive atmosphere in the form of a cloud of combustible dust in air is not likely to occur in normal operation, but if it does occur, will persist for a short period.

The zone classification describes the likelihood and duration of the explosive dust atmosphere. It does not by itself determine the complete equipment specification.

Dust Groups: IIIA, IIIB, and IIIC

Equipment for explosive dust atmospheres uses the dust-group classification system of IEC 60079.

IIIA – Combustible Flyings

IIIA covers combustible flyings, such as combustible fibers or flyings that can form hazardous accumulations or atmospheres.

IIIB – Non-Conductive Dust

IIIB covers non-conductive combustible dust.

Examples can include many organic or processing dusts, depending on their actual properties.

IIIC – Conductive Dust

IIIC covers conductive dust.

Conductive dust can create additional electrical hazards because it may enter or accumulate on electrical equipment and create conductive paths.

IIIC should not simply be described as "higher risk." It identifies a different dust property that can impose more demanding equipment requirements.

Ex ta, Ex tb, and Ex tc

Ex t equipment is divided into three protection levels:

Protection Level	Group III EPL	Typical Zone Compatibility
Ex ta	Da	Zone 20, Zone 21, Zone 22
Ex tb	Db	Zone 21, Zone 22
Ex tc	Dc	Zone 22

This is a practical compatibility relationship. The complete equipment marking, certificate, installation conditions, and applicable project requirements must still be checked.

IECEx material gives the general dust-zone/EPL relationship as Zone 20 → Da, Zone 21 → Db or Da, and Zone 22 → Dc, Db, or Da.

Understanding the Equipment Protection Level

The EPL communicates the level of protection provided by the equipment.

• Da → very high protection level for explosive dust atmospheres
• Db → high protection level
• Dc → enhanced protection level

For equipment selection, the required EPL should be derived from the hazardous-area classification and applicable installation requirements.

Do not select equipment simply because its marking contains the letter "t." The complete marking must be checked.

Ingress Protection and Ex t Equipment

Ingress protection is a critical part of Ex t equipment because the enclosure must restrict the entry of combustible dust.

Depending on the protection level and dust group, applicable Ex t equipment may use dust-protection levels such as IP5X or IP6X. The exact requirement is determined by the applicable standard and certification.

Therefore, a specification should not simply state "IP66" and assume that the equipment is Ex t certified. IP rating and Ex t certification are related but are not the same thing.

The Ex certificate must confirm that the enclosure and its ingress-protection characteristics satisfy the applicable dust-protection requirements.

Dust Layers and Surface Temperature

One of the important differences between gas and dust hazards is the significance of accumulated dust layers.

Dust can settle on an equipment surface and form an insulating layer. This can reduce heat dissipation and cause the surface temperature to rise.

For this reason, Ex t equipment must be evaluated for its temperature characteristics under the applicable conditions.

The maximum surface temperature for dust equipment is commonly expressed directly in degrees Celsius, for example:

T85°C

This should not be confused with gas temperature classes such as T4 or T6.

Typical Ex t Equipment

Ex t protection can be used for a wide range of electrical equipment designed for combustible dust environments.

Examples include:

• Lighting equipment

• Junction boxes

• Control stations

• Motors

• Sensors

• Instrument enclosures

• Electrical panels

• Connection equipment

Typical industries include grain handling, flour processing, sugar production, cement processing, mining, woodworking, and other processes where combustible dust can occur.

Understanding an Ex t Marking

A typical marking may appear as:

Ex tb IIIC T85°C Db

Marking	Meaning
Ex	Explosion protection marking
tb	Protection by enclosure, level "b"
IIIC	Conductive dust group
T85°C	Maximum surface temperature
Db	Equipment Protection Level

The complete marking should be reviewed together with the certificate, ambient-temperature range, installation conditions, and manufacturer's instructions.

Unlike gas equipment markings, dust equipment commonly uses a direct maximum surface-temperature value such as T85°C rather than the T1–T6 gas temperature-class system.

Ex t Cable Entries and Glands

Cable entries are an important part of maintaining the dust-protection integrity of the equipment.

The cable gland, stopping plug, or other entry device must be suitable for the certified enclosure and the applicable installation arrangement.

During specification and installation, verify:

• Ex certification of the entry device where required

• Compatibility with the equipment enclosure

• Cable type and diameter

• Dust ingress requirements

• Ambient temperature

• Installation instructions

IEC 60079-31:2022 includes specific requirements for entry devices associated with type of protection "t," including cable transit devices.

Ex t Maintenance and Inspection

The effectiveness of Ex t protection depends heavily on maintaining the enclosure and its certified installation condition.

Inspection should consider:

• Damage to the enclosure

• Condition of covers and joints

• Dust accumulation

• Condition of cable glands and stopping plugs

• Damaged or missing seals where applicable

• Evidence of unauthorized modifications

• Corrosion that could affect enclosure integrity

• Equipment operating temperature

The objective is not simply to keep the equipment visually clean. The inspection must maintain the conditions on which the certified dust protection depends.

Common Ex t Specification Mistakes

Confusing Ex t With Gas Protection

Ex t is specifically intended for explosive dust atmospheres. It does not replace gas protection concepts such as Ex d, Ex e, or Ex i.

Assuming Any IP66 Enclosure Is Ex t

An IP rating alone does not establish explosion protection certification. Ex t certification and the applicable dust-protection requirements must also be verified.

Using the Gas T-Class for Dust

Dust equipment commonly uses a maximum surface-temperature value such as T85°C. This should not be interpreted using the gas T1–T6 classification.

Ignoring Dust Group

IIIA, IIIB, and IIIC identify different dust characteristics. The equipment marking must be suitable for the actual dust hazard.

Selecting the Wrong EPL

The required EPL must correspond to the hazardous-area classification. For example, Zone 21 generally requires equipment with Db or Da, while Zone 22 can use Dc, Db, or Da.

Ignoring Dust Accumulation

Dust layers can affect heat dissipation and surface temperature. Equipment selection and maintenance therefore need to consider the actual dust accumulation conditions.

Using Damaged Cable Glands or Seals

Damaged entry devices, seals, covers, or enclosure components can compromise the dust-ingress protection on which the Ex t protection depends.

How to Select Ex t Equipment

A practical selection workflow is:

1. Determine the dust hazardous area. Identify Zone 20, Zone 21, or Zone 22.

2. Identify the dust. Determine whether the applicable group is IIIA, IIIB, or IIIC.

3. Determine the required EPL. Establish whether Da, Db, or Dc is required.

4. Check the Ex t level. Verify whether Ex ta, Ex tb, or Ex tc provides the required protection.

5. Check maximum surface temperature. Verify the certified temperature against the ignition characteristics of the actual dust and applicable installation requirements.

6. Check ambient temperature. Confirm that the equipment certification covers the actual environment.

7. Check the certificate. Verify the exact equipment model and certification scope.

8. Check entry devices. Verify cable glands, stopping plugs, cable transits, and other accessories.

9. Review special conditions. Check any conditions of use stated in the certificate.

10. Follow installation and maintenance requirements. Maintain the certified enclosure and dust-protection characteristics throughout service.

Ex t vs Gas Protection Concepts

Protection Concept	Primary Hazard	Basic Principle
Ex d	Gas/vapor	Contain internal explosion and prevent flame transmission
Ex e	Gas/vapor	Increased safety construction
Ex i	Gas/vapor	Limit electrical and thermal energy
Ex t	Combustible dust	Enclosure protection and surface-temperature limitation

This comparison is simplified. Some equipment can use more than one protection concept, and the complete certified marking must always be evaluated.

Frequently Asked Questions

What is Ex t protection?

Ex t is protection by enclosure for explosive dust atmospheres. It combines enclosure protection against dust ingress with surface-temperature limitations.

What standard covers Ex t?

IEC 60079-31 covers equipment dust ignition protection by enclosure "t." The current published edition is IEC 60079-31:2022.

Can Ex t be used for gas hazardous areas?

Ex t is specifically a dust protection method. If an installation can contain both combustible dust and explosive gas atmospheres, the additional gas hazard must also be addressed and the equipment must satisfy the applicable protection requirements.

What is the difference between Ex ta, Ex tb, and Ex tc?

They are different levels of protection within the Ex t concept. For Group III applications, Ex ta corresponds to Da, Ex tb to Db, and Ex tc to Dc.

Can Ex tb be used in Zone 20?

Ex tb is associated with EPL Db, whereas Zone 20 generally requires EPL Da. Therefore, Ex tb alone does not provide the EPL normally required for Zone 20.

Can Ex tc be used in Zone 21?

Ex tc is associated with EPL Dc and therefore does not provide the Db or Da level normally required for Zone 21.

What does IIIC mean?

IIIC identifies the conductive-dust group. It should not be interpreted simply as a general "risk level." The dust group is one of the parameters used to determine equipment suitability.

Does Ex t require IP6X?

Not universally. The applicable dust ingress protection depends on the Ex t protection level, dust group, equipment design, and certification. Some Ex t applications use IP5X while others require IP6X.

What does Ex tb IIIC T85°C Db mean?

It identifies protection by enclosure at level tb, suitability for dust group IIIC, a maximum surface temperature of 85°C, and EPL Db.

Is T85°C the same as T6?

No. T6 is a gas temperature class. T85°C is a direct maximum surface-temperature marking used for dust equipment.

Conclusion

Ex t Protection by Enclosure is designed specifically for electrical equipment operating in explosive dust atmospheres.

Its protection principle combines dust-ingress control with surface-temperature limitation. The enclosure helps prevent combustible dust from reaching internal ignition sources, while temperature limits reduce the possibility of ignition from hot surfaces.

The principal Ex t levels are:

• Ex ta → EPL Da
• Ex tb → EPL Db
• Ex tc → EPL Dc

When selecting Ex t equipment, engineers should evaluate the complete application rather than looking only for an IP rating or the words "dust proof."

The important parameters include dust zone, EPL, dust group, maximum surface temperature, enclosure/IP characteristics, certificate scope, cable entries, ambient conditions, and installation requirements.

References

• IEC 60079-0 – Explosive Atmospheres – Part 0: Equipment – General Requirements
• IEC 60079-31 – Explosive Atmospheres – Part 31: Equipment Dust Ignition Protection by Enclosure "t"
• IECEx System – Certified Equipment Scheme