Explosion-proof markings and types

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Overview of Explosion Protection Basics – Classification of Hazardous Areas: Common Data Continuous presence of hazard Intermittent presence of hazard Presence of hazard in the event of an accident Greater than 1,000 hours/year, 10–1,000 hours/year, 0.1–10 hours/year. International Electrotechnical Commission: Zone 0 (gases), Zone 1 (gases), Zone 2 (gases); European electrical standards: Zone Z(10) (dusts), Zone Y(11) (dusts). North America (USA/Canada): Zone 1 (gases and dusts). China: Zone 0 (gases), Zone 1 (gases), Zone 2 (gases), Zone 10 (dusts), Zone 11 (dusts). The classification of hazardous areas represents a measure of the actual risk present in a particular area, which determines the appropriate level of explosion protection required for that area. (In European standards, only the UK (zones Z and Y) and Germany (zones 10 and 11) have regulations regarding the classification of areas with dust hazards; proposals from the International Electrotechnical Commission (IEC) are under consideration.) Temperature group (Group T): This refers to the maximum surface temperature of electrical equipment in relation to the ignition temperature of the gas (assuming an ambient temperature of 40°C). The ignition energy is independent of the ignition temperature; for example, the ignition energy of hydrogen is 20 UJ with an ignition temperature of 560°C. All flammable gases and their temperature groups are listed in Part 1 of standard Bs5345. Temperature groups: IEC79-8 – T 1, T 2, T 3; T 4, T 5, T 6. GB3836.1 – T 1, T 2, T 3, T 4, T 5, T 6. Maximum surface temperature: 450℃, 300℃, 200℃, 135℃, 100℃, 85℃. Explosion-proof marking: In accordance with the requirements of GB 3836 standard, the explosion-proof marking for explosion-proof electrical equipment includes the explosion-proof type + equipment category + (gas group) + temperature group. The explosion-proof types can be classified into flameproof type, increased safety type, intrinsically safe type, positive pressure type, oil-immersed type, sand-filled type, encapsulated type, type n, special type, and dust explosion-proof type, based on the explosion-proof measures adopted. Their identifiers are shown in Table 1. ?Table 1 Basic types of explosion protection. Explosion protection types, explosion protection type markings: Flameproof type – Ex d; Sand-filled type – Ex q; Increased safety type – Ex e; Encased type – Ex m; Positive pressure type – Ex p; n-type – Ex n; Intrinsic safety type – Ex ia, Ex ib; Special type – Ex s; Immersed in oil type – Ex o; Dust explosion protection type – DIP, ADIP B. 2 Equipment categories: Electrical equipment for explosive gas environments is classified as follows: Class I: Electrical equipment for use in coal mines ； ?Class II: Electrical equipment for other explosive gas environments other than coal mines. Class II flameproof “d” and intrinsically safe “i” electrical equipment are further divided into Classes IIA, IIB, and IIC. Electrical equipment for flammable dust environments is divided into: Type A dust-tight equipment ； Type B dust-tight equipment ； ?Type A dust-proof equipment ； Type B dust protection equipment. ?3 The detonation propagation capacity of explosive gas mixtures in a given gas group indicates the level of their explosive hazard; the greater the detonation propagation capacity of an explosive mixture, the higher its danger level. The detonation propagation capability of an explosive mixture can be expressed by the maximum test safety gap. At the same time, the ease with which explosive gases, liquid vapors, and mists can be ignited also indicates the level of their explosion risk, which is expressed as a minimum ignition current ratio. Class II explosion-proof electrical equipment or intrinsically safe electrical equipment is further divided into Classes IIA, IIB, and IIC based on their maximum test safety gap or minimum ignition current ratio suitable for explosive gas mixtures. As shown in Table 2. Table 2 Relationship between the groups of explosive gas mixtures and the maximum test safety gap or minimum ignition current ratio. Gas Group Maximum Test Safety Gap – MESG (mm) Minimum Ignition Current Ratio – MICR IIA MESG ≥ 0.9 MICR > 0.8 IIB 0.9 > MESG > 0.5 0.8 ≥ MICR ≥ 0.45 IIC 0.5 ≥ MESG 0.45 > MICR 4 Temperature Group The ignition temperature of an explosive gas mixture is the upper limit temperature at which it can be ignited. Electrical equipment is classified into groups T1 to T6 based on its maximum surface temperature, such that the maximum surface temperature of electrical equipment in each corresponding group T1 to T6 shall not exceed the allowable value for that temperature group. The relationship between temperature groups, equipment surface temperature, and the ignition temperature of flammable gases or vapors is shown in Table 3. ?Table 3: Relationship between temperature groups, surface temperature of equipment, and ignition temperature of flammable gases or vapors? Temperature category? Maximum surface temperature of equipment according to IEC/EN/GB 3836? T [°C] Ignition temperature of flammable substances [°C] T1 450 T > 450 T2 300 450 ≥ T > 300 T3 200 300 ≥ T > 200 T4 135 200 ≥ T > 135 T5 100 135 ≥ T > 100 T6 85 100 ≥ T > 85 Examples of explosion-proof markings? To further clarify the notation used for explosion-proof markings, the following examples are given for gas-explosion-proof electrical equipment:? If the electrical equipment is of Type I explosion-proof design, the explosion-proof marking is ExdI. If the electrical equipment is of Type II explosion-proof design, with gas group B and temperature category T3, then the explosion-proof marking is ExdIIBT3. If the electrical equipment is of Category II intrinsically safe type ia, with gas group A and temperature group T5, then the explosion protection mark is: E*aIIA T5. ? For Category I special types: ExsI. For flameproof electrical equipment used in mines to handle flammable gases other than methane – under normal conditions, these are gases belonging to Group II, Class B, and temperature class T3 – the explosion protection mark is: ExdI/IIBT3. ?Furthermore, for the following special cases, the contents of the explosion protection mark can be adjusted appropriately:?(1) If an electrical device adopts more than one composite type, the main explosion protection type should be indicated first, followed by the other explosion protection types. For example, a motor of Category II, Group B, intrinsically safe type with an enhanced safety junction box in Group T4 has an explosion protection mark of: ExdeIIBT4. ?〖JP3〗(2) If electrical equipment is permitted to be used only in an environment with one type of flammable gas or vapor, its marking may be indicated by the chemical formula or name of that gas or vapor; in such cases, it is not necessary to specify the gas group and temperature group. For example, Category II refers to flameproof electrical equipment designed for use in ammonia environments, and its explosion protection mark is: ExdII(NH3) or ExdII (Ammonia). Conversely, by using Table 2, manufacturers can determine the temperature category of the product based on the operating environment of the explosion-proof electrical equipment, and then design the surface temperature or internal temperature of the equipment’s enclosure according to that temperature category. Users of explosion-proof electrical equipment can easily select the temperature category of such products based on the type of explosive gases or vapors that may be present in the area. For example, if isobutane (with an ignition temperature of 460 °C) is present in the environment, explosion-proof electrical products of Group T1 can be selected ； If butane and ether (ignition temperature 160 °C) are present in the environment, explosion-proof electrical products of Group T4 must be selected. ?For dust-explosion-proof electrical equipment: Equipment of type A that can be used in Zone 21 has a maximum surface temperature of ?T??A of 170 ℃; its explosion-proof rating is DIP A21 TA170 ℃ or DIP A21TA, ?T??3 ； ?If it can be used in Zone 21 for Type B equipment, the maximum surface temperature T?B is 200 °C. Its explosion protection rating is: DIP B21 T?B200 °C or DIP B21TB, T?3?6. Requirements for marking: (1) The mark should be placed in a visible location on the main body of the electrical equipment ； ?(2) The markings must remain clear and durable even in the presence of potential chemical corrosion. Markings such as Ex, explosion-proof type, category, and temperature group can be applied in relief or intaglio on a prominent position on the enclosure. The material used for the markings should be resistant to chemical corrosion, such as bronze, brass, or stainless steel. ?7 Examples of commonly used markings for explosion-proof electrical equipment internationally? Table 4? International Electrotechnical Commission (IEC)? Ex d [ia] IIC T5? Where:? Ex – Explosion protection? d – Protection type (flameproof)? [ia] – Equipment with intrinsically safe outputs? II – Equipment category? C – Gas group? T5 – Temperature group〖〗 CENELEC (European Committee for Electrotechnical Standardization)? EEx d [ia] IIC T5? Where:? EEx – European marking? d – Protection type (flameproof)? [ia] – Equipment with intrinsically safe outputs? II – Equipment category? C – Gas group? T5 – Temperature group US (NEC 505) (United States)* ?Class I, Zone 1, A Ex d [ia] IIC T5 ? Where:? Class I – Allowed category? Zone 1 – Location where use is permitted? A – U.S. **standard? Ex – Explosion protection? d – Protection type (flameproof)? [ia] – Intrinsically safe? II – Equipment category? C – Gas group? T5 – Temperature group US (NEC 500) (United States)? Explosion proof with I.S. outputs, Class I, Division 1, Groups A, B, C, D, T5? Where:? Explosion proof with I.S. outputs – Protection type (other than I.S. can be selected)? Class I – Allowed category? Division 1 – Allowed rating (other than Division 2 can be selected)? Groups A, B, C, D – Allowed gas groups? T5 – Temperature group * NEC is the U.S. electrical code; in 1996, Chapter 505 was added to this code to align with the hazard classification methods used in the internationally accepted IEC standard system. NEC 500 is designated as suitable for the North American hazardous location classification system. (Explosion-proof classification of explosion-proof electrical equipment 1. Conditions for an explosive mixture to explode: An explosion occurs when a substance, through physical or chemical changes, suddenly changes from one state to another, releasing enormous amounts of energy in the form of light, heat, or mechanical work.) Here, only the explosion of explosive mixtures is discussed, that is, the explosion of explosive mixtures formed by all flammable gases, vapors, and dusts in combination with air. For such explosions to occur, three conditions must be met simultaneously: first, there must be explosive or flammable materials ； Second, there must be an oxidizing substance, mainly oxygen in the air ； Third, there must also be a source of ignition (such as sparks, arcs, and dangerous temperatures), which provides the energy necessary to ignite the mixture. Only when all three conditions are present simultaneously is an explosion possible; without any one of these conditions, neither combustion nor explosion will occur. Therefore, appropriate measures should be taken to prevent explosions by ensuring that not all three conditions are present at the same time. Since explosive mixtures are commonly found in places related to production, processing, storage, and transportation in industries such as coal, petroleum, chemicals, textiles, and food processing, an explosion in such areas can cause severe damage. Therefore, various explosion-proof technical methods have been adopted to prevent the formation of explosive hazardous environments and their explosions. ?2. Basic explosion protection types?(1) Flameproof type “d”. The flameproof explosion protection type involves enclosing all those components of the equipment that could ignite explosive gas mixtures within a housing. This housing is capable of withstanding internal explosions caused by combustible mixtures that penetrate through any joints or gaps in the housing, without being damaged; at the same time, it prevents ignition of an explosive environment outside resulting from one or more gases or vapors (refer to standard GB 3836?2). ? All components that may generate sparks, arcs, and dangerous temperatures are placed within an explosion-proof enclosure, which separates the interior space of the equipment from the surrounding environment. The flame-proof enclosure contains gaps, and due to the breathing action of electrical equipment and gas penetration, explosive gas mixtures may form inside it. When an explosion occurs, the enclosure is able to withstand the resulting explosive pressure without being damaged. Moreover, the gaps in the enclosure’s structure help to cool the flame, reduce its spread rate, or interrupt the chain reaction that drives its progression, thereby preventing the flame or dangerous flame products from passing through these gaps and igniting the surrounding explosive environment – thus achieving the purpose of flame prevention. ? Flameproof type “d” is divided into Class I, and IIA, IIB, IIC according to the types of explosive gas environments in which it may be used. ?This explosion-proof type of equipment is suitable for Zones 1 and 2. ?(2) Increased safety type “e”: The increased safety explosion-proof type is an explosion-proof design that involves taking additional measures for electrical equipment that does not generate arcs or sparks under normal operating conditions, in order to enhance its safety and prevent the occurrence of dangerous temperatures, arcs, and sparks in its internal and external components. It does not include equipment that generates sparks or arcs under normal operating conditions (see GB 3836?3 standard). In electrical equipment that does not generate sparks, arcs, or dangerous temperatures under normal operation, measures are taken to reduce or control the operating temperature, ensure the reliability of electrical connections, enhance insulation performance, and improve the protection level of the enclosure. These actions aim to reduce the likelihood of contamination caused by dirt and the entry of moisture, thereby minimizing the risk of ignition failures and enhancing the safety and reliability of the equipment under normal operating conditions as well as in the face of specified faults (such as motor rotor jamming). 〖JP〗? This type of equipment is primarily used in Zone 2 hazardous areas; some variants can be used in Zone 1, such as increased-safety low-voltage asynchronous motors equipped with appropriate protective devices, junction boxes, etc. ?(3) Intrinsically safe type “i”: The intrinsically safe explosion-proof design means that all circuits within the equipment are such that any electric sparks or thermal effects generated under standard conditions (including normal operation and specified fault conditions) cannot ignite the specified explosive gas environment. 〖HTH〗Electrical equipment of “iɑ” category 〖HT〗are intrinsically safe electrical devices that cannot cause ignition under normal operating conditions, as well as under conditions of the presence of a single fault or any combination of faults ； 〖HTH〗Electrical equipment of “ib” category 〖HT〗are intrinsically safe electrical devices that cannot cause ignition under normal operation or when a fault condition occurs (refer to GB 38 36?4 standard). Intrinsic safety is achieved by restricting the energy in the circuit; through proper control of circuit parameters, the potential spark energy is reduced to below the level necessary to ignite the specified gas mixture, and the surface temperature of wires and components is kept below the ignition temperature of that gas mixture. ?This explosion-proof type can only be used in low-voltage equipment; such equipment is suitable for Zones 0, 1, and 2 (Ex ia) or Zones 1 and 2 (Ex ib). ?(4) An explosion-proof type of positive-pressure “p” electrical equipment. It is an electrical device that ensures safety by maintaining the pressure of the protective gas inside its enclosure at a level higher than the pressure of the explosive environment surrounding it (see GB 3836?5 standard). Different methods can be used for the protection type of positive pressure equipment. One method is to maintain a static positive pressure within the system, while another is to keep a continuous flow of air or inert gas to prevent flammable mixtures from entering the enclosure. Both methods require flushing the enclosure with a protective gas before starting the equipment, in order to remove any flammable gases that may have entered the enclosure due to the non-positive pressure conditions inside the equipment, thereby preventing the formation of flammable mixtures within the enclosure. The key to these methods is monitoring the system and performing regular ventilation to ensure its reliability. Devices of this type can be used in Zone 1 or Zone 2 hazardous areas depending on the protection method. ?(5) Oil-immersed “o” oil-immersed explosion-proof type involves submerging the entire device or its components in oil (a protective fluid), thereby preventing explosive gases above the oil surface or outside the enclosure from causing an explosion (refer to GB 3836?6 standard). This is an old explosion-proof technical method mainly used for switching equipment. The resulting arc and sparks are submerged in oil. ?This type of equipment is suitable for hazardous areas in Zone 1 or Zone 2. ?(6) The sand-filled \"q\" type, a dust-proof design, is a type of electrical equipment protection mechanism in which sand particles or other powder materials with specified properties are filled inside the enclosure; this prevents arcs or high temperatures generated within the enclosure from igniting the explosive gas environment around it under specified operating conditions (see GB 3836?7 standard). This explosion-proof design fixes the conductive components that could ignite an explosive gas environment and completely buries them in sand-filled material, thereby preventing the spread of sparks, arcs, and dangerous temperatures, and stopping them from igniting the external explosive gas environment. It is usually used for components in Ex“e” or Ex“n” devices and for heavy-duty traction battery packs. ?This type of equipment is suitable for hazardous areas in Zone 1 or Zone 2. ?(7) “n” type explosion-proof electrical equipment: This type of electrical equipment does not ignite the explosive gas environment around it during normal operation, and it is also unlikely to develop faults that could lead to ignition (see GB 3836?8 standard). Normal operation of an “n”-type electrical device means that the device meets the design specifications both electrically and mechanically, and is used within the limits specified by the manufacturer, so that no sparks, arcs, or dangerous temperatures can occur. ?This type of electrical equipment is only suitable for Zone 2 hazardous areas. ?(8) Encased “m” type – The encased explosion-proof design involves sealing electrical components that could generate sparks, arcs, or dangerous temperatures capable of causing an explosion in explosive mixtures, within an encapsulating material (compound), thereby preventing them from igniting the surrounding explosive mixtures (see GB 3836?9 standard). By using potting measures, short circuits in electrical components can be prevented, and electrical insulation can be secured. This eliminates sources of ignition such as sparks, arcs, and dangerous temperatures in the circuit, prevents the intrusion of explosive mixtures, and controls surface temperatures under normal and fault conditions. ?These devices are suitable for hazardous areas in Zones 1 and 2. ?(9) Hermetically sealed “h”: This type of explosion-proof equipment features a hermetically sealed enclosure. That is, explosive gas mixtures in the environment must not enter the interior of the equipment enclosure. Hermetically sealed enclosures are sealed using melting, extrusion, or bonding methods; such enclosures are usually non-removable in order to ensure permanent hermeticity (see GB 3836?11 standard). ?This explosion protection measure falls under the category of \"n\" type explosion protection measures; GB 3836?11 has been replaced by GB 3836?8—2003. ?(10) For special types of explosion-proof electrical equipment, “s” refers to **explosion-proof types that are not covered by the standards; for such types, the competent authorities may temporarily issue provisional regulations, and electrical equipment that has been tested and certified by designated explosion-proof testing institutions as having explosion-proof properties can be used. ?These devices are developed based on actual usage and can be applied in corresponding hazardous areas. ?(11) Dust explosion-proof electrical equipment for electrical devices used in environments with flammable dusts operates by limiting the maximum surface temperature of the enclosure and by using \"dust-tight\" or \"dust-proof\" enclosures to prevent dust from entering, thereby avoiding the ignition of flammable dusts (see standard GB 12476?1). Such devices house the live components within a casing with certain protective capabilities, thereby preventing dust from entering and isolating the ignition source from the dust to avoid explosions. Equipment is classified as Type A or Type B based on the difference in its dust-proof casing design. Devices are classified into grades 20, 21, and 22 based on the dust protection level of their enclosures; for example, DIP A20, DIP A21, DIP B20, and DIP B21. Devices of this type are suitable for dust-hazardous areas of Zones 20, 21, or 22 depending on their classification. ?In practical use, it is easy to observe that many explosion-proof electrical products employ multiple explosion-proof protection methods within a single product. For example, lighting fixtures may employ increased safety protection (enclosures and terminal boxes), explosion-proof protection (switches), and encapsulated protection (ballasts). This allows manufacturers to adopt the most suitable composite explosion protection method. It is important to note that the order in which the explosion-proof methods listed on the product’s nameplate usually indicates the product’s design; for example, if a product is labeled as Ex d e, it is likely to be of the flameproof type with additional safety components. If another product is labeled as Ex ed, it is highly likely that it does not have an explosion-proof enclosure (such as stainless steel or reinforced polyester glass), but rather contains an explosion-proof switch or components installed therein. Both products may be suitable for Zone 1, but they use different explosion-proof protection measures to achieve the same goal. Users can choose products of the appropriate explosion-proof type that offer the best balance in terms of cost, performance, and safety, based on their actual needs and the information they have.