Key Principles and Practices in the Design of Equipment with Flameproof Enclosure ‘d’

The design of equipment with flameproof enclosure ‘d’ according to EN 60079-1 is crucial for ensuring safety in explosive environments. This standard is a fundamental resource for engineers and designers working with devices intended for explosive zones. Understanding and implementing its requirements is key to achieving explosion protection. This article outlines the critical aspects of this standard, offers practical design tips, and highlights best engineering practices in line with the ATEX Directive 2014/34/EU.

What is Flameproof Enclosure ‘d’ in the Context of EN 60079-1?

Flameproof enclosure ‘d’ is a method of protecting electrical equipment used in explosive atmospheres of gases, vapors, or mists. Its primary goal is to enclose all potential ignition sources, such as electrical contacts or heating elements, within a robust casing. This casing must withstand an internal explosion of a flammable mixture that may have entered it, while also preventing the flame from escaping into the surrounding explosive atmosphere.

A key component of this protection method is the use of flameproof joints (flame paths). These are precisely crafted gaps between enclosure parts that, while not gas-tight, are narrow and long enough to cool explosive gases below the ignition temperature of the external explosive atmosphere as they pass through.

The PN-EN 60079-1 standard distinguishes three levels of equipment protection (EPL – Equipment Protection Level) for flameproof enclosures:

• ‘da’ (EPL ‘Ma’ or ‘Ga’): This highest level of protection is intended for devices operating in the highest risk zones (zone 0 for gases or zone 20 for dusts – although ‘d’ primarily pertains to gases, EPL ‘Ma’ refers to Group I devices, such as those used in mines). Requirements for this level are the most stringent, including limited internal volume and increased number of test cycles.
• ‘db’ (EPL ‘Mb’ or ‘Gb’): This is the most commonly encountered protection level for Ex ‘d’ devices, designed for operation in zone 1 (gases) or Group I (mines). Most construction and testing requirements in the standard pertain to this level.
• ‘dc’ (EPL ‘Gc’): The lowest protection level for Ex ‘d’, intended for zone 2 (gases). It mainly concerns devices with disconnectable contacts and has specific, less stringent construction and testing requirements compared to ‘da’ and ‘db’ levels.

It is crucial to remember that PN-EN 60079-1 supplements and modifies the general requirements found in PN-EN IEC 60079-0. This means that when designing Ex ‘d’ devices, one must always apply the requirements of both standards, with PN-EN 60079-1 taking precedence in case of conflicts.

Design of Equipment with Flameproof Enclosure ‘d’ – Construction Requirements

The most critical element of the flameproof enclosure is the flameproof joints. The standard precisely defines their parameters, which depend on the gas group (related to MESG – Maximum Experimental Safe Gap) and the internal volume of the enclosure.

Types and Dimensions of Flameproof Joints

The standard defines different types of joints and provides minimum lengths (L) and maximum gaps (i) in detailed tables (Tables 2, 3, 4, 5).

• Flanged, cylindrical, and cylindrical-flanged joints: These are the most commonly encountered types of joints. Requirements for them vary significantly depending on the gas group (I, IIA, IIB, IIC) and the internal volume of the enclosure. For example, for group IIC (gases with small MESG, such as hydrogen or acetylene), allowable gaps are significantly smaller than for group IIA (e.g., propane).
• Threaded joints: Must meet requirements regarding thread pitch, execution, fitting, and the number of engaged threads. The standard refers to ISO standards for metric threads and ANSI/ASME for NPT threads.
• Tapered, partially cylindrical, serrated, multi-stage joints: The standard specifies requirements for these specific types of joints, often referring to the need for testing to confirm their effectiveness.

Practical tip: When designing joints, always consider manufacturing tolerances. The dimensions provided in the standard are limit values that must be met by the finished product. The standard allows the use of anti-corrosion greases on joint surfaces, but they must meet specific conditions and not negatively affect flameproof properties. Galvanic coatings are permissible, but thicker layers require additional verification.

Openings and Fastening Elements

Openings in joint surfaces (e.g., for screws) must be appropriately distanced from the joint edge to avoid creating an easy path for the flame. Fastening elements (screws, threaded pins) accessible from the outside must be secured against accidental loosening, e.g., by using special closures or natural construction protection. The standard also specifies requirements regarding materials for fastening elements (plastics and light alloys are not allowed) and the minimum thickness of the enclosure wall in the case of non-through holes.

Enclosure Materials

The choice of materials is crucial for the enclosure’s resistance to explosion pressure and its resistance to the effects of the explosive atmosphere and environmental conditions.

• Mechanical strength: Materials must ensure that the enclosure withstands the pressures generated during an internal explosion, confirmed by testing.
• Material restrictions: The standard imposes specific restrictions for certain materials in specific atmospheres:
  • Zinc or zinc alloys with high zinc content are not used due to degradation risk.
  • In atmospheres containing acetylene, copper and copper alloys with more than 60% copper content used outside the enclosure must be coated with a protective layer (e.g., tin, nickel) to prevent the formation of hazardous acetylides.
• Non-metallic enclosures: If the enclosure or its parts are made of non-metallic materials (e.g., glass-reinforced plastics – GRP, which are a popular choice in practice), they must meet additional requirements regarding, among others, resistance to tracking currents, insulation distances, and undergo specific type tests, including flame erosion testing.

Testing – Verification of Compliance with the Standard

Compliance of the device with EN 60079-1 requirements is confirmed through detailed type tests and product tests.

Type Tests

Type tests (conducted on prototypes or representative samples) include:

• Determination of explosion pressure (reference pressure): A test involving initiating an explosion of a gas mixture inside the enclosure and measuring the maximum pressure. The procedure varies depending on the gas group and enclosure volume. The standard also considers specific conditions, such as the presence of internal elements (e.g., rotating machines), which may affect pressure increase.
• Overpressure test: Verification of the mechanical strength of the enclosure to pressure (static or dynamic), which is higher than the reference pressure. The enclosure must not undergo permanent deformations or damage compromising flameproof integrity.
• Internal ignition non-propagation test: A key test confirming the effectiveness of flameproof joints. It involves initiating an explosion inside the enclosure, which is placed in a test chamber with the same explosive mixture. External ignition is not allowed. The standard specifies test conditions (mixture type, number of cycles, gap modifications) depending on the gas group and joint type.

Product Tests

Product tests (conducted on each unit or production batch) aim to detect potential manufacturing defects that could lower the protection level. This is most often a static overpressure test, although the standard allows alternative methods, especially for welded structures.

Specific Components and Applications

The PN-EN 60079-1 standard also contains detailed requirements for specific components and types of devices:

• Input devices (cable glands, bushings, blanking elements): Must be constructed and tested to maintain the flameproof integrity of the enclosure. The standard provides detailed requirements for their construction, seals (e.g., cable glands with sealing rings or filling compound), and testing. They can be certified as Ex devices or components, facilitating their use in various enclosures.
• Rotating electrical machines (motors): Requirements include the construction of bearings and shafts passing through the enclosure and special pressure testing procedures (at rest and in motion). If powered by an inverter, additional requirements for bearings (shaft currents) and temperature class determination must also be considered.
• Batteries: The use of cells and batteries in flameproof enclosures is allowed only if specific requirements regarding cell type (must not emit gases under normal operating conditions) and appropriate protective measures (e.g., against short-circuiting, reverse charging) are met.
• Switchgear (Group I): For devices used in mines (Group I) with flameproof enclosures and remotely controlled switching elements, the standard introduces requirements for disconnection means and mechanical locks for doors and covers, which must ensure power disconnection before opening the enclosure.

Best Design Practices in Engineering Practice

Designing in accordance with EN 60079-1 is not only about meeting formal requirements but also about applying an engineering approach focused on safety.

• Comprehensive Risk Analysis: Although the standard focuses on the ‘d’ type of protection, always start with a full explosion risk analysis for the specific application and hazard zone. This allows for the proper selection of gas group and protection level (EPL).
• Design Documentation: Maintaining detailed and precise documentation, including manufacturing drawings with key flameproof joint dimensions, material specifications, manufacturing and assembly technologies, is essential for the ATEX certification process and subsequent traceability. The standard clearly indicates what information should be included in the technical documentation.
• Collaboration with a Notified Body: Early contact and consultations with an accredited notified body (e.g., for category 1 and 2 devices for gases) can significantly streamline the certification process and help resolve interpretative uncertainties of the standard.
• Production Quality Management: The precision of flameproof joint execution is critical. Implementing appropriate quality control procedures in the production process (e.g., joint dimension control, material verification) is essential to ensure the repeatability and safety of each product. The standard refers to product testing and batch testing as a way to verify production quality.
• Usage and Maintenance Instructions: According to the requirements of PN-EN IEC 60079-0, the device must be accompanied by detailed instructions for installation, use, and maintenance, including information about flameproof joints (e.g., the impossibility of their repair, if specified). In the case of specific operating conditions (e.g., minimum distance from obstacles for flanged joints), they must be clearly indicated in the instructions and labeling.
• Consistency with Other Standards: When designing Ex ‘d’ devices, remember to maintain consistency with other safety standards, such as PN-EN ISO 13849-1 regarding the functional safety of control systems.

Designing equipment with flameproof enclosure ‘d’ is an advanced engineering challenge that requires a deep understanding and rigorous application of the EN 60079-1 standard in conjunction with EN IEC 60079-0. It is crucial to precisely execute flameproof joints, select appropriate materials, and successfully pass type and product tests. By applying best design practices, maintaining thorough documentation, and collaborating with experienced specialists and certification bodies, it is possible to effectively bring safe devices intended for explosive atmospheres to market, while also meeting the requirements of the ATEX Directive 2014/34/EU.

FAQ: Design of Equipment with Flameproof Enclosure ‘d’