Inside a grain silo, the air smells of dampness and warm grain. A layer of dust covers the surfaces, barely a millimeter thick. But once disturbed by a gust of air, conveyor vibrations or the opening of an inspection hatch, that millimeter can turn into a cloud of combustible particles reaching five meters into the air. All it takes is an ignition source: a hot surface, a spark from an electrical circuit or an electrostatic discharge.

An operator panel installed in such a location operates in constant proximity to potentially explosive material. If its surface temperature exceeds the dust’s ignition threshold, if its enclosure allows particles to enter, or if its electrical circuits can generate a spark with sufficient energy, the device itself can become an ignition source.

Selecting an operator panel for a potentially explosive atmosphere starts with understanding ATEX markings. A digit, a letter, a temperature class, an ingress protection rating. Each symbol conveys information about the physics of an explosion, translated into the language of a technical specification.

What Is the ATEX Directive, and Who Does It Apply To?

ATEX is an abbreviation of the French Atmosphères Explosibles, meaning “explosive atmospheres.” The term refers to two European Union legal acts governing safety in areas where such atmospheres may occur.

Directive 2014/34/EU, known as ATEX 114, is aimed at manufacturers, importers and distributors. It specifies the technical requirements for equipment and protective systems intended for use in potentially explosive atmospheres. The manufacturer must ensure that its product bears the CE marking and the Ex symbol and has an EU-type examination certificate issued by an independent notified body. This directive replaced the earlier Directive 94/9/EC.

Directive 1999/92/EC, known as ATEX 137 or ATEX USER, applies to employers and facility owners. It requires them to designate hazardous areas, carry out risk assessments and prepare an Explosion Protection Document. This document must include a list of hazardous areas, their zone classifications and a description of the protective measures in place. Employers are also responsible for employee training and for maintaining equipment in a condition that ensures safety.

The division of responsibilities is clearly defined. The manufacturer designs and certifies the equipment. The employer classifies the zone and selects suitable equipment. Employees follow safety procedures, including prohibitions on open flames and mobile phone use in designated areas, as well as the use of personal protective equipment. The ATEX directives apply throughout the EU and EFTA countries. In Poland, their provisions have been implemented through regulations issued by the relevant ministers.

What Distinguishes Gas Zones 0, 1 and 2?

Zones for gases, vapors and mists are classified according to one main criterion: how frequently and for how long an explosive atmosphere is present in a given location.

• Zone 0This is an area where an explosive atmosphere is present continuously, for long periods or frequently. An indicative threshold is more than 1,000 hours per year. Typical locations include the interiors of tanks, chemical reactors and pipelines carrying flammable substances. Equipment operating in Zone 0 must belong to Category 1G and provide the highest level of protection, designated EPL Ga. EPL stands for Equipment Protection Level. Ga ensures that the equipment remains safe even in the event of two simultaneous, independent faults.
• Zone 1This zone covers areas where an explosive atmosphere may occur occasionally during normal operation. In time-based terms, this corresponds to approximately 10–1,000 hours per year. Typical locations include areas near ventilation openings, filling and emptying points, and leaking valves. The required equipment category is 2G, with EPL Gb, which provides safety in the event of one fault or frequent disturbances.
• Zone 2This category includes areas where an explosive atmosphere is not normally present. If one does occur, it lasts only briefly and results from a fault, such as a ruptured pipeline, damaged seal or unexpected leak. The indicative threshold is less than 10 hours per year. Category 3G equipment with EPL Gc is permitted here. It is designed to ensure that ignition sources are absent during normal, fault-free operation.

The extent and classification of a zone depend on two factors: the nature of the emission source (continuous, periodic or accidental) and the effectiveness of ventilation. Well-designed ventilation can reduce the extent of a hazardous zone and, in extreme cases, reduce it to a negligible size. Detailed rules for zone classification are set out in EN 60079-10-1.

What Are Dust Zones 20, 21 and 22?

Dust zones follow the same basic principle as gas zones. Classification is based on the probability and duration of an explosive atmosphere. In this case, the hazard comes from clouds of combustible dust suspended in the air. However, there is an important additional factor: settled dust. A layer of dust on the panel enclosure, supporting structure or floor is material that can be stirred up into a cloud at any moment.

• Zone 20This is an area where a cloud of combustible dust is present continuously or for long periods, exceeding 1,000 hours per year. Typical locations include the interiors of silos, mills, dust filters and mixers. Required equipment: Category 1D, EPL Da.
• Zone 21This zone covers areas where an explosive dust atmosphere may occur occasionally during normal operation, approximately 10–1,000 hours per year. Examples include areas beneath silos, transfer points and locations near leaking installations. The required category is 2D, EPL Db. The enclosure must provide an IP6X protection rating, meaning it is completely dust-tight. The digit 6 in the IP code indicates that dust cannot enter the enclosure under the specified test conditions.
• Zone 22These are areas where a dust cloud is not normally present and, if one occurs, it lasts only briefly — less than 10 hours per year. Examples include areas around equipment containing dust, spaces beneath conveyor belts and inspection hatches. Category 3D equipment with EPL Dc is permitted. The required ingress protection rating is at least IP5X.

Dusts are divided into three subgroups according to particle characteristics and electrical conductivity:

• IIIA covers combustible flyings, such as cotton and rayon fibers.
• IIIB covers non-conductive dusts, such as sugar, flour and wood flour.
• IIIC covers conductive dusts, such as powdered aluminum, iron and graphite. Subgroup IIIC presents the most demanding conditions because conductive dust entering an enclosure can cause a short circuit and ignition.

The temperature classes T1–T6 used for gas zones are not applied to dust zones. Instead, the manufacturer specifies the maximum surface temperature of the equipment directly, for example T80°C or T135°C.

Two safety margins apply when selecting equipment. The panel’s surface temperature must not exceed two-thirds of the minimum ignition temperature of the dust cloud. It must also be at least 75°C below the ignition temperature of a dust layer up to 5 mm thick. If a layer thicker than 5 mm may accumulate on the panel enclosure, the safety margin must be increased further in accordance with EN 60079-14.

How Does the Zone Affect the Choice of Keypad or Operator Panel?

Each zone imposes a specific combination of requirements that an operator panel must meet. In industry, an operator panel is commonly referred to as an HMI (Human-Machine Interface): a display with a touchscreen or keypad through which an operator controls a machine or industrial process. Reading the marking on the nameplate involves checking several parameters at once.

• Equipment category and zone hierarchyCategories 1G and 1D indicate equipment intended for Zones 0 and 20. Categories 2G and 2D are intended for Zones 1 and 21. Categories 3G and 3D permit operation in Zones 2 and 22. The letter G indicates suitability for gas atmospheres, while D indicates suitability for dust atmospheres.The hierarchy principle applies: equipment from a higher category may be used in a lower-risk zone. For example, a Category 1G panel may be installed in Zone 2. The reverse is not permitted. Installing a Category 3G panel in Zone 1 violates the requirements and creates a safety hazard.
• Explosion subgroupFor gases, the subgroups are designated IIA, IIB and IIC. IIA covers gases that are comparatively difficult to ignite, such as propane, acetone and benzene. IIB covers substances presenting an intermediate level of ignition hazard, such as ethylene. IIC is the most demanding group and includes hydrogen, acetylene and carbon disulfide.Equipment marked IIC covers the requirements of groups IIA and IIB as well. The reverse does not apply: a panel marked IIA is not suitable for a zone where hydrogen is present.The same hierarchy applies to dust subgroups IIIA, IIIB and IIIC. Equipment marked IIIC can be used with all three dust subgroups. A panel marked IIIA cannot be used where conductive dusts are present.
• Temperature classIn gas zones, equipment is classified from T1 (maximum surface temperature of 450°C) to T6 (maximum of 85°C). The gas’s autoignition temperature must be higher than the panel’s maximum surface temperature. T6 is the most restrictive class and also the most widely applicable because it meets the temperature requirements of all the other classes. In dust zones, as explained above, the manufacturer specifies the maximum surface temperature directly.
• Equipment Protection Level (EPL)EPL connects the ATEX framework with international IEC standards, which is important when exporting equipment outside Europe. Ga and Da represent the highest levels of protection and are intended for Zones 0 and 20. Gb and Db correspond to Zones 1 and 21. Gc and Dc are sufficient for Zones 2 and 22.
• The letters “X” and “U” in a certificateThe letter “X” at the end of an ATEX certificate number indicates that additional conditions for safe use apply. These may include a prohibition on cleaning with a dry cloth due to the risk of electrostatic discharge, the requirement to use specific cable glands or a minimum clearance around the enclosure. The details are provided in the type-examination certificate and must be reviewed before installation.The letter “U” indicates that the item is an Ex component rather than a complete, stand-alone product. A membrane keypad marked “U” must be installed in a certified enclosure so that the complete assembly can meet ATEX requirements and bear the CE marking. The component itself is not approved for independent use in a potentially explosive atmosphere.

Protection Methods

HMI panel manufacturers use various protection techniques identified by Ex symbols:

• Ex i (intrinsic safety) limits the electrical energy in circuits to a level at which any potential spark is too weak to ignite the explosive atmosphere. This method is common in membrane keypads and sensors. It allows lightweight, ergonomic interfaces to be designed without heavy enclosures.
• Ex d (flameproof enclosure) means that the enclosure can withstand an internal explosion and prevent the flame from propagating to the outside. This approach is heavy and expensive and is used where panel components may produce sparks during normal operation.
• Ex p (pressurization) involves filling the inside of a control cabinet with clean air or protective gas at a pressure higher than the surrounding atmosphere. This prevents the explosive atmosphere from entering. It can be an economical solution for large installations containing multiple HMI panels.
• Ex t (protection by enclosure) is used in dust zones. It relies on a sufficiently high IP rating to prevent dust from entering the enclosure.
• Ex m (encapsulation) means that components capable of causing ignition, such as display modules and coils, are embedded in an insulating compound that permanently separates them from the surrounding atmosphere.

Modern panels often combine several protection methods, for example a flameproof enclosure for the main components together with intrinsically safe circuits for an external keypad.

Beyond certification parameters, an operator panel for an Ex zone must also meet practical usability requirements. The touchscreen should respond when operated with work gloves, which is why resistive technology is often used: it responds to pressure rather than the electrical properties of a finger. The display must remain readable at wide viewing angles and in strong light. Ex zones can also be located outdoors, for example on offshore drilling platforms, where intense sunlight and brine impose additional material requirements.

What Are the Most Common Mistakes When Selecting Equipment for an Ex Zone?

Errors in selecting panels for ATEX zones have one thing in common: they result from reading a marking too superficially or overlooking a single parameter.

The first, and potentially most costly, mistake is installing an IIA panel in a zone where hydrogen is present, which requires IIC equipment. The explosion subgroup determines which substances the equipment can safely operate around. An IIA panel is not designed for gases with the highest ignition sensitivity. The certificate does not permit such use, and the consequences of exceeding its limits can be immediate.

The second mistake concerns the temperature class. If the gas in the zone ignites at 100°C, a T4 panel (maximum surface temperature of 135°C) is too hot. A T6 class (maximum 85°C) is needed. On paper, the difference may appear small, but it determines whether the equipment’s surface can become an ignition source.

The third mistake is confusing an Ex component with a complete device. A keypad marked “U” must be installed in a certified enclosure. If installed independently without appropriate integration, it does not constitute a compliant ATEX product, either formally or in practical terms.

Another serious error is failing to match the IP rating to a dust zone. Zone 21 requires complete dust-tightness, IP6X. Installing a panel with IP5X protection in this zone means that fine dust particles may enter the enclosure. With conductive dusts (subgroup IIIC), this can cause a short circuit.

Ignoring temperature safety margins for settled dust can also be dangerous. If a dust layer thicker than 5 mm accumulates on the panel enclosure, the standard safety margins are insufficient. EN 60079-14 requires these margins to be increased, but installers often overlook this because dust accumulation is rarely the first thing they check.

A less frequently discussed mistake is selecting equipment with a higher protection category without analyzing the cost implications. A Category 1G panel installed in Zone 2 is safe and compliant. It is also several times more expensive than a Category 3G panel, which would be sufficient for that zone. Excessive protection does not violate the regulations, but it increases costs without a proportionate improvement in safety.

How Does QWERTY Approach Equipment Selection?

At QWERTY, we design membrane and foil keypads for use across ATEX zones, from Zone 0 to Zone 22. We start with the materials because they must be selected to avoid sparking, prevent the accumulation of electrostatic charge and ensure that surface temperatures do not reach levels capable of igniting the surrounding atmosphere. The entire design depends on these decisions.

The seals in our keypads are designed to meet the requirements of both types of hazardous zones. In dust zones, they protect the interior against particle ingress; in gas zones, they prevent flammable substances from entering. Ex-marked keypads are designed to meet these requirements from the outset, making it easier for the customer to obtain ATEX certification for the complete end product.

We manufacture Ex components marked with the letter “U”. The keypad is intended to be integrated into the customer’s certified enclosure. We do not certify the finished product. We supply a component that enables the certification process. This distinction matters because responsibility for integrating the components into a complete device rests with the final manufacturer.

Backlighting, glass fronts and optical bonding (a process in which layers are bonded to eliminate the air gap between the glass and the display, improving readability and mechanical resistance) can all be implemented within the constraints of Ex safety requirements. Each element is assessed for its impact on explosion-protection parameters before entering production.

Our solutions operate in refineries, chemical plants, mines and offshore drilling platforms. We design for conditions in which an operator panel must withstand salt, moisture, extreme temperatures and vibration while maintaining display readability and precise operation with work gloves.

In the grain silo described at the beginning, the layer of dust is still just one millimeter thick. A single ignition source can still turn it into a five-meter cloud of combustible particles. Between “safe” and “too late” stand several symbols on the operator panel’s nameplate: equipment category, explosion subgroup, temperature class, IP rating and protection method. Understanding these markings is the foundation of a decision on which the safety of the installation — and the people working there — depends.