Zone 0 vs Zone 1 vs Zone 2: What's the Difference?

Zone classifications indicate the likelihood and duration of the presence of an explosive gas atmosphere, helping engineers select suitable equipment for hazardous areas.

Understanding the differences between Zone 0, Zone 1, and Zone 2 is fundamental to hazardous area engineering.

These classifications are used for areas where flammable gases, vapors, or mists may form an explosive atmosphere. The zone describes how frequently and for how long an explosive atmosphere is expected to be present.

The classification then becomes an important input for equipment selection, installation, inspection, and maintenance.

In this guide, we explain what Zone 0, Zone 1, and Zone 2 mean, how they differ, and how the zone classification affects the selection of explosion-protected equipment.

What Are Zone 0, Zone 1, and Zone 2?

Under the IEC 60079 system, areas where an explosive gas atmosphere may occur are classified into three zones:

• Zone 0 – explosive gas atmosphere present continuously, frequently, or for long periods.

• Zone 1 – explosive gas atmosphere likely to occur during normal operation.

• Zone 2 – explosive gas atmosphere not likely to occur during normal operation and, if it does occur, exists only for a short period.

The difference between the zones is therefore primarily about the frequency and duration of the explosive atmosphere.

The zone number should not be interpreted as a complete plant risk rating. Area classification describes the presence of an explosive atmosphere and does not by itself quantify the consequences of an ignition.

Zone 0 Explained

Zone 0 is an area in which an explosive gas atmosphere is present continuously, for long periods, or frequently.

Typical locations can include internal spaces of process equipment where a flammable atmosphere is expected to be present for extended periods.

Examples may include:

• Inside certain process vessels

• Inside storage tanks

• Inside certain separators

• Other internal process spaces containing a persistent flammable atmosphere

Because the explosive atmosphere is expected to be present for a significant amount of time, equipment used in Zone 0 requires an appropriate level of protection and certification for that application.

Zone 1 Explained

Zone 1 is an area in which an explosive gas atmosphere is likely to occur during normal operation.

This does not mean that the atmosphere is continuously present. Instead, the process conditions are such that an explosive atmosphere can reasonably be expected during normal operation.

Typical examples may include areas around certain:

• Pump and compressor seals

• Sampling points

• Vent or breathing points

• Process equipment connections

• Other identified sources of release

Equipment installed in Zone 1 must have a protection level and protection concept appropriate for the classified area and the specific hazardous substance.

Zone 2 Explained

Zone 2 is an area in which an explosive gas atmosphere is not likely to occur during normal operation and, if it does occur, will exist only for a short period.

Zone 2 can occur around release sources where a flammable gas or vapor release is considered unlikely during normal operation but may occur under abnormal conditions.

Examples may include:

• Areas surrounding certain Zone 1 locations

• Well-ventilated outdoor process areas

• Areas where releases are unlikely during normal operation

Zone 2 is still a hazardous area. The lower frequency of explosive atmospheres does not mean that ordinary equipment can automatically be installed without considering the applicable requirements.

Zone 0 vs Zone 1 vs Zone 2

Is Zone 0 More Dangerous Than Zone 1 or Zone 2?

Zone 0 represents the greatest frequency or duration of an explosive gas atmosphere among the three zone classifications.

However, it is better not to describe Zone 0, Zone 1, and Zone 2 simply as "high risk," "medium risk," and "low risk."

Area classification primarily describes the likelihood and duration of an explosive atmosphere. It does not provide a complete assessment of the consequences of an explosion, toxicity, process hazards, or other risks at the facility.

IEC 60079-10-1 specifically states that the classification methodology does not generally take the consequences of ignition into account. :chatgpt-content-reference{index="6"}

How Is the Zone Determined?

The zone is not selected simply by looking at the type of equipment installed.

Hazardous area classification considers the conditions that can produce and disperse an explosive atmosphere.

Source of Release

A source of release is a location from which a flammable gas, vapor, or mist can be released.

Examples include:

• Flanges and connections

• Valves

• Pump and compressor seals

• Drains

• Vents

• Sampling points

Frequency and Duration of Release

The expected frequency and duration of a release are key factors in determining the resulting zone.

A release that is continuous or occurs for long periods can result in a different classification from a release that is only expected under abnormal conditions.

Ventilation

Ventilation affects how quickly a released flammable substance is dispersed.

Natural or artificial ventilation can therefore influence the extent of the classified area and must be considered as part of the classification assessment.

Process Conditions

Pressure, temperature, release rate, substance properties, equipment configuration, and operating conditions can all influence the classification.

IEC 60079-10-1 is specifically concerned with classification of areas where flammable gas or vapor hazards may arise and provides a basis for equipment selection and installation. :chatgpt-content-reference{index="7"}

How Does Zone Classification Affect Equipment Selection?

The zone is one of the first parameters checked when selecting Ex equipment, but it is not the only parameter.

A suitable equipment selection should consider the complete hazardous-area specification.

1. Zone

Determine whether the equipment will be installed in Zone 0, Zone 1, or Zone 2.

2. Equipment Protection Level

The required EPL must be appropriate for the classified area and the applicable protection concept.

For gas atmospheres, the principal EPL designations are:

• Ga

• Gb

• Gc

The relationship between zone and EPL is an important part of equipment selection, but the exact applicability depends on the equipment and protection concept.

3. Protection Concept

The equipment must use a protection concept appropriate for the application.

Examples include:

• Ex d – Flameproof

• Ex e – Increased Safety

• Ex i – Intrinsic Safety

• Ex p – Pressurization

There is no universal rule such as "Zone 1 always requires Ex d." Several protection concepts may be suitable depending on the equipment design, EPL, certification, and application.

4. Gas Group

The hazardous substance must be considered when determining the applicable gas group.

For example, equipment certified for a particular gas group may have different applicability from equipment certified for a more demanding group.

See our guide: Gas Groups Explained: IIA vs IIB vs IIC.

5. Temperature Class

The maximum surface temperature of equipment must be compatible with the ignition characteristics of the hazardous substance.

Common gas temperature classes include T1 through T6.

See: Temperature Classes Explained.

6. Certification and Marking

The complete Ex marking and certificate must be checked to confirm that the equipment is certified for the intended application.

Do not select equipment based only on a product description such as "explosion proof" or "hazardous area approved."

Zone Does Not Equal Equipment Category

A common mistake is treating the zone and equipment category as if they are the same thing.

They are related, but they are not identical classifications.

For example, under the ATEX system, equipment categories are used to express the level of protection provided by equipment, while Zone 0, Zone 1, and Zone 2 describe the classification of the area.

The relationship must therefore be evaluated rather than assuming that:

• Zone 0 = Category 1

• Zone 1 = Category 2

• Zone 2 = Category 3

Those simplified equations can be misleading because equipment suitability depends on the complete product marking and applicable requirements.

Common Mistakes About Zone Classification

• Calling Zone 2 a safe or non-hazardous area.

• Treating Zone 0, Zone 1, and Zone 2 as simple overall risk ratings.

• Assuming the zone alone determines the equipment protection concept.

• Assuming Zone 1 always requires Ex d equipment.

• Ignoring gas group requirements.

• Ignoring temperature class requirements.

• Confusing zone classification with ATEX equipment category.

• Using equipment without checking its complete certification and Ex marking.

• Ignoring ventilation when determining the extent of a hazardous area.

• Confusing gas zones with dust zones such as Zone 20, Zone 21, and Zone 22.

Frequently Asked Questions

Which zone has the highest frequency of explosive atmosphere?

Zone 0. It describes an area where an explosive gas atmosphere is present continuously, for long periods, or frequently.

Is Zone 2 a safe area?

No. Zone 2 is still a classified hazardous area. The explosive atmosphere is not likely during normal operation and, if it occurs, is expected to exist only for a short period.

Can Zone 2 become Zone 1?

Yes. Changes in process conditions, release sources, ventilation, equipment arrangement, or operating conditions can change the area classification. The classification should be reviewed when relevant conditions change.

What equipment can be used in Zone 0?

Equipment must have a protection level and certification suitable for Zone 0 and the specific application. Ex ia intrinsic safety is one example used for suitable instrumentation, but the applicable protection method depends on the equipment and certification.

Can Zone 1 equipment be installed in Zone 0?

Not automatically. Equipment must be certified and have the required protection level for the intended zone. Equipment suitable for Zone 1 cannot simply be assumed to be suitable for Zone 0.

Does Zone 1 always require Ex d?

No. Ex d is one possible protection concept. Other protection methods may also be suitable depending on the equipment, EPL, certification, gas group, temperature requirements, and application.

What is the difference between Zone 0 and Zone 1?

Zone 0 is where an explosive gas atmosphere is present continuously, for long periods, or frequently. Zone 1 is where an explosive gas atmosphere is likely to occur during normal operation.

What is the difference between Zone 1 and Zone 2?

Zone 1 is where an explosive gas atmosphere is likely during normal operation. Zone 2 is where it is not likely during normal operation and, if it occurs, exists only for a short period.

Who determines the hazardous area classification?

Hazardous area classification should be performed by competent personnel using site-specific process information, release-source data, ventilation conditions, substance properties, and the applicable classification standard.

Zone 0 vs Zone 1 vs Zone 2 Summary

Conclusion

Zone 0, Zone 1, and Zone 2 are classifications used to describe the likelihood and duration of an explosive gas atmosphere.

Zone 0 represents continuous, frequent, or long-duration presence. Zone 1 represents an explosive atmosphere that is likely during normal operation. Zone 2 represents an atmosphere that is not likely during normal operation and, if it occurs, exists only for a short period.

The zone is an important starting point for equipment selection, but it is not sufficient by itself. The complete selection must also consider EPL, protection concept, gas group, temperature class, certification, and installation requirements.

Understanding this distinction helps engineers avoid one of the most common mistakes in hazardous-area engineering: selecting equipment based only on the zone number.

References

• IEC 60079-10-1 – Explosive Atmospheres – Classification of Areas – Explosive Gas Atmospheres

• IEC 60079-14:2024 – Explosive Atmospheres – Electrical Installation Design, Selection and Installation of Equipment, Including Initial Inspection

• IECEx System Documentation