Q&A on explosion-proof knowledge

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1. How does our country classify explosive hazard areas? 　　Answer: Our country uses a method equivalent to that of IEC for classifying explosive hazardous areas. **Standard GB 50058-92 specifies that explosive gas hazardous areas are classified into three levels – Zone 0, Zone 1, and Zone 2 – based on their degree of hazard, while explosive dust hazardous areas are divided into two levels: Zone 0 and Zone 11. See Table 4-1 for details. 　　Table 4-1 Classification of Hazardous Areas in China Explosive Substances Area Classification Definition of Areas Gases Zone 0: An environment where explosive gas mixtures are present continuously or for an extended period. Zone 1: An environment where explosive gas mixtures may occur during normal operation. Zone 2: An environment where explosive gas mixtures cannot occur during normal operation, or if they do appear, it is only for a short time. Dusts Zone 10: An environment where explosive dusts are present continuously or for an extended period. Zone 11: An environment where accumulated dust may occasionally be disturbed, resulting in the formation of explosive dust mixtures. 2. How are explosive hazardous areas classified internationally? 　　Answer: Internationally, the major industrial countries generally have two different views regarding the classification of explosive-hazardous areas. 　　A system represented by the IEC (International Electrotechnical Commission), which includes countries such as Germany, the United Kingdom, Italy, Japan, and Australia; it classifies gases into Zones 0, 1, and 2, and dusts into Zones 10 and 11. Its definition is essentially the same as that of IEC (see China’s definitions for various regions; China adopts IEC standards on an equivalent basis). 　　Another classification is used in North America, such as in the United States and Canada; it is represented by the definitions in NEC (the National Electrical Code of the United States). Gases are classified into Zone 1 and Zone 2 (with no Zone 0), and dusts are also classified into Zone 1 and Zone 2. 　　The correspondence between the two is roughly as follows:
Gases: IEC Zone 0, Zone 1 – NEC Zone 1
IEC Zone 2 – NEC Zone 2
Dusts: IEC Zone 10 – NEC Zone 1
IEC Zone 11 – NEC Zone 2
The English term for “Zone” in IEC is “Zone” ； 　　The English term for NEC’s “District” is Division. 3. How many types of explosion-proof structural designs are there for explosion-proof electrical equipment in our country? List its name and logo. 　　Answer: According to **standard GB 3836—83**, there are 8 types of explosion-proof structural designs for explosion-proof electrical equipment in China, which are listed as follows. 　　Structure type Code Structure type code 　　Flame-proof type d Oil-filled type o 　　Increased safety type e Sand-filled type q 　　Inherently safe type i Spark-free type n 　　Positive pressure type p Special type s 4. What is a flame-proof instrument? What are its features? 　　Answer: Explosion isolation, also known as pressure-resistant explosion protection, involves enclosing the instrument components that could ignite explosive substances within a specially strong enclosure capable of withstanding the explosive pressure from any internal explosive mixtures, and preventing the spread of such explosions to areas outside the enclosure. In other words, an explosion may occur inside the housing of an explosion-proof instrument, but it will not spread outside the housing. Therefore, strict explosion-proof requirements apply to the joints between various components of such instruments – such as the number of thread turns on the instrument cover, the precision of the threads, the zero point setting, the gaps between the range adjustment screws and the instrument housing, as well as the gaps between the sensing components and the conversion components of the transmitter, and the wire connections. 　　Apart from being relatively bulky, flameproof instruments are otherwise quite simple and do not require any associated equipment such as safety barriers. However, before opening the panel cover, the power must be turned off first; otherwise, in the event of sparks being generated, they would be exposed to the atmosphere, posing a danger. 5. What are intrinsically safe instruments? What are its features? 　　Answer: Inherently safe instruments are also known as safety spark instruments. Its feature is that, whether under normal conditions or in fault conditions, the sparks generated by the circuits and systems, as well as the temperatures reached, will not ignite explosive mixtures. Its explosion protection is primarily achieved through the following measures: ① The instrument circuit is constructed using new types of integrated circuit components, allowing it to operate at lower operating voltages and lower operating currents ； 　　②Use safety barriers to separate the circuits in hazardous areas from those in non-hazardous areas, thereby limiting the energy that can be transmitted from non-hazardous areas to hazardous areas ； 　　③The connection wires of the instruments should not exhibit excessive distributed inductance and distributed capacitance, in order to reduce the energy stored in the circuit. 　　The explosion-proof performance of these safety instruments is not achieved through external measures such as ventilation, inflation, oil filling, or flame isolation; rather, it is accomplished by the circuit itself, making them intrinsically safe. It can be used in all hazardous locations and with all explosive gas and vapor mixtures, and can be repaired and adjusted while powered on. However, it cannot be used alone; it must be combined with intrinsically safe associated devices (safety barriers) and external wiring to form an intrinsically safe circuit in order to achieve its explosion-proof function. 6. Intrinsically safe instruments come in ia and ib types; please explain the difference between them. 　　Answer: Class IA – It will not ignite explosive gas mixtures under normal operating conditions, nor in the presence of one or two faults in the circuit. In type IA circuits, the operating current is limited to below 100mA. 　　IB rating – Under normal operating conditions, as well as in the presence of a fault in the circuit, it prevents the ignition of explosive gas mixtures. In the IB circuit, the operating current is limited to below 150mA. 　　Type IA instruments are suitable for Zones 0 and 1, while type IB instruments are suitable only for Zone 1. In other words, from the perspective of intrinsic safety, type ib instruments are suitable for use in coal mines, while type ia instruments are suitable for use in factories. 7. What is a positive pressure (p-type) instrument? 　　Answer: It is a type of instrument device that fills the instrument enclosure with clean air or an inert gas under positive pressure, or continuously supplies clean air or non-flammable gases, thereby keeping the pressure of the protective gas inside the enclosure above that of the surrounding hazardous environment. This prevents external explosive gas mixtures from entering the enclosure and isolates the potential hazards associated with the electrical components. 8. What is an enhanced safety type (e-type) instrument? 　　Answer: Instrumentation devices that do not generate sparks or dangerous temperatures capable of igniting explosive mixtures under normal operating conditions, and that incorporate structural measures such as sealing to enhance their safety and prevent ignition under normal and specified overload conditions. 9. What is a special-type (S-type) instrument? 　　Answer: It is a special form other than d, e, i, p, and n, or a combination of these forms; explosion-proof instruments that use this structural format are referred to as special-type instruments. 10. Into how many main categories are explosion-proof electrical equipment divided? 　　Answer: It is divided into two main categories: Category I: Electrical equipment for use in coal mines ； Category II: Electrical equipment for use in factories. 11. How many grades are there for Class II explosion-proof electrical equipment? What is the symbol? 　　Answer: According to **standard GB 3836–83**, Class II explosion-proof electrical equipment is divided into three grades, denoted as A, B, and C respectively. The grading criteria are shown in Table 8-2. 　　Table 8-2 Classification Criteria for Class II Explosion-Proof Electrical Equipment Level MESG/mm MICR II A 1.14＞MESG≥0.9 1.0＞MICR＞0.8 II B 0.9＞MESG＞0.5 0.8≥MICR≥0.45 II C 0.5≥MESG 0.45＞MICR Note: MESG—maximum test safety gap for flammable gas mixtures, in mm. 　　MICR—is the ratio of the minimum ignition current of a flammable gas mixture to the minimum ignition current of methane. 　　II A, II B, and II C are also the deflagration ratings for flammable gas mixtures. 12. Explain the following term: Maximum Test Safety Gap MESG ； 　　Minimum Ignition Current MIC ； 　　Minimum Ignition Current is referred to as MICR. 　　Answer: Maximum Test Safety Gap (MESG) – refers to, under specified test conditions, the maximum gap between the joints of a housing filled with a mixture of the gas under test and air at a certain concentration; even after ignition, this gap prevents the explosive gas mixture within the housing from igniting. 　　Minimum Ignition Current (MIC) – the minimum current required to ignite a flammable gas mixture, when 3000 ignition tests are conducted using a spark with 24V DC and a 95mH inductance on a specified testing apparatus. A 5% decrease in this current prevents ignition. 　　Minimum Ignition Current Ratio (MICR) – the ratio of the minimum ignition current of various flammable gas (or vapor) mixtures with air to that of the methane-air mixture. 13. Into how many temperature groups are Class II explosion-proof electrical equipment divided? What is the symbol? 　　Answer: According to the **standard GB3836—83**, Class II explosion-proof electrical equipment is divided into 6 groups based on its maximum surface temperature, denoted as T1~T6. The grouping criteria are as follows: Temperature group Maximum allowable surface temperature/°C Temperature group Maximum allowable surface temperature/°C T1T2T3 450 200 200 T4T5T6 135 100 85 T1–T6 correspond to the ignition temperature groups for explosive gas mixtures. 14. How to choose explosion-proof instruments? 　　Answer: Generally speaking, the choice can be made based on the following two points. 　　①The explosion-proof type of the instrument should be selected based on the hazardous area where it is installed and used: Zone 0 – only IA type or S type (referring to the S type designed specifically for Zone 0) can be used ； 　　Zone I – possibly other types except n-type ； 　　Zone 2 – all explosion-proof types are available ； 　　②Based on the deflagration level and ignition temperature category of the combustible gases and vapors that may be present, select the explosion-proof rating and the highest allowable surface temperature category for the instrument. Refer to Table 8-3, which is compiled based on GB3836-1983. 　　Table 8-3 Examples of Classification Levels for the Propagation of Flammable Gases and Vapors and Ignition Temperature Groups
Group Level: T1 (T > 450°C), T2 (450°C ≥ T > 300°C), T3 (300°C ≥ T > 200°C), T4 (200°C ≥ T < 135°C), T5 (135°C ≥ T < 100°C), T6 (100°C ≥ T < 85°C)
II A: Methane, ethane, propane, styrene, benzene, toluene, xylene, trimethylbenzene, naphthalene, carbon monoxide, phenol, cresol, propyl alcohol, methyl acetate, acetic acid, chloroethane, chlorobenzene, ammonia, acetonitrile, aniline, cyclopentane, propylene, ethylbenzene, isopropylbenzene, methanol, ethanol, propanol, butanol, methyl formate, ethyl formate, ethyl acetate, methyl methacrylate, vinyl acetate, dichloroethane, vinyl chloride, methylamine, dimethylamine, ethane, heptane, octane, nonane, decane, cyclohexane, turpentine, naphtha, petroleum, gasoline, fuel oil, kerosene, diesel, pentanol, hexanol, cyclohexanol, acetaldehyde, trimethylamine, ethyl nitrite
II B: Acetylene, cyclopropane, propylene wax, hydrogen cyanide, household gas, ethylene, butadiene, ethylene oxide, propylene oxide, methyl acrylate, ethyl acrylate, dimethyl furan, butyraldehyde, acrolein, furan, hydrogen sulfide, ethyl methyl ether, diethyl ether, diglyme, tetrafluoroethylene
II C: Chlorine, water gas, acetylene, carbon disulfide, ethyl nitrate
Note: a. The propagation classification level for flammable gases and vapors is also the explosion protection classification level for electrical equipment; both are divided into three grades: IIA, IIB, and IIC. 　　b. The ignition temperature categories for flammable gases and vapors correspond one-to-one with the highest surface temperature categories of electrical equipment; for example, gases in category T4 have an ignition temperature of 200°C ≥ T > 135°C. 15. What components make up the explosion-proof marking in our country? Explain their meanings separately? 　　Answer: An explosion-proof mark generally consists of the following 5 parts: ① The explosion-proof mark EX – indicates that the device is an explosion-proof electrical device ； 　　②Explosion-proof structure type – indicates the measures taken to prevent explosions in this equipment; for example, d denotes flameproof type, while p denotes positive pressure type ； i refers to intrinsically safe types, etc ； 　　③Classification of explosion-proof equipment—divided into two main categories: I refers to electrical equipment used in coal mines, while II refers to electrical equipment used in factories ； 　　④Explosion protection rating – divided into levels A, B, and C, indicating the strength of its explosion resistance ； 　　⑤Temperature groups — divided into six groups from T1 to T6, indicating the allowable maximum surface temperature of the equipment. 16. The explosion protection rating of a instrument is EXdIIBT4. Please explain what this means. 　　Answer: EX – General explosion-proof mark ； 　　d——Structural type, flameproof type ； 　　II——Category, for use in factories ； 　　B – Explosion protection rating, Class B. T4 – Temperature group, Group T4; maximum surface temperature ≤ 135°C. 17. The explosion protection rating of a polyester power distribution box is EXedIICT4; please explain what this means. 　　Answer: EX – General explosion-proof mark ； 　　ed — structural type; e: increased safety type, d: flameproof type ； 　　II——Category, for use in factories ； 　　C – Explosion protection rating, Class C. T4 – Temperature group, Group T4; maximum surface temperature ≤ 135°C. 18. The explosion protection rating of an imported gas chromatograph is EEXdpsIIB+H2T4. Please explain what this means. 　　Answer: EEX – the general explosion-proof marking for the European Community ； 　　dps — This instrument adopts three types of explosion protection measures: flameproofing, positive pressure, and special protection ； 　　II——Electrical equipment for factories ； 　　B – Explosion protection rating, Class B. + H2 – Also suitable for H2 environments (Class B explosion protection is not appropriate for H2; however, this instrument can be used in H2 environments thanks to various explosion protection measures) ； 　　T4 — Maximum surface temperature rise ≤ 135°C. 19. The explosion protection mark of a Japanese-made instrument is JISia3Ng4; please explain what this means. 　　Answer: JIS — the code for Japanese Industrial Standards ； 　　 ia — intrinsically safe explosion-proof, class ia ； 　　 3n —— The explosion protection rating is 3n, protecting against all Class 3 explosive gases, which is equivalent to IIC in China ； 　　 G4 — temperature category, equivalent to T4 in China. 　　It should be noted that JISia3nG4 was the explosion-proof marking originally used in Japan; however, Japan now adopts IEC standards, and the aforementioned explosion-proof marking is now denoted as JISEXiaIICT4. 20. An imported instrument has an explosion protection rating of Class 1, Division 1, Groups B, C, D, T4A. Please explain what this means. 　　Answer: Class 1—Level 1, areas with flammable gases or vapors ； 　　Division 1—Area 1: Areas where an explosion or fire may occur or could be generated ； 　　Groups B, C, D — Suitable for areas where hazardous gases are present in Groups B, C, and D ； 　　T4A—Maximum surface temperature ≤120°C. 　　The aforementioned explosion-proof rating corresponds to China’s E*IA, IIB, IICT4, and can be used in Zone 0 and Zone 1 hazardous areas. It should be noted, however, that this table cannot be used in acetylene environments. 21. The explosion protection mark of an imported instrument is as follows; please explain its meaning. 　　UL/FM/CSA Class1, Group B, C, D, T5 　　Class3, Group E, F, T5 　　CENELEC EEXedIICT5 　　Answer: UL – Underwriters Laboratories ； 　　 FM—United Research Association of American Factories ； 　　 CSA — Canadian Standards Association ； 　　CENELEC — The European Committee for Electrotechnical Standardization. 　　This indicates that the explosion-proof performance of this instrument has been certified by UL, FM, CSA, and CENELEC tests. 　　Class 1, Groups B, C, D, T5 — suitable for flammable gases in Groups B, C, and D as specified by NEC, with a surface temperature ≤ 100°C. 　　Class 2, Groups E, F, T5 are suitable for the dusts in Groups E and F as specified by NEC, with a surface temperature of ≤100°C. 　　EEXedIICT5 — meets EN standards (EN is the code for European Community standards; its explosion protection requirements are equivalent to those of IEC, etc.). It is an explosion-proof instrument for use in factories, available in increased safety and flameproof types, with an explosion protection rating of Class C and a maximum surface temperature of ≤100°C. 22. Is there no limit on the transmission distance of the transmitter’s output signal? 　　Answer: The capacitive transmitters produced today are powered by 24VDC. Based on a maximum output of 22.5mA from the instrument and a minimum operating voltage of 10.5VDC, their load resistance is 600Ω. For flameproof transmitters, as long as the sum of the resistance of the wire assembly and the resistance of the equipment connected to the transmitter does not exceed 600Ω, there is no limit on the transmission distance of the wires. However, for intrinsically safe transmitters, the length of the wires is specified. Because Intrinsic Safety is a system, not just an Intrinsic Safety instrument; it also includes associated equipment and external wiring. The associated device is usually a safety barrier, which has requirements regarding the length of the wires; each safety barrier specifies the maximum allowable inductance and maximum allowable capacitance. If the distributed capacitance and inductance of the wires, along with the unprotected capacitance and inductance of the transmitter, exceed the specified limits, the instrumentation system is no longer intrinsically safe. 23. What are the requirements for installing instruments in explosive hazard areas? 　　Answer: ① Instruments, electrical equipment, and installation materials such as junction boxes, distribution boxes, and terminal boxes used in explosive hazard areas must possess explosion-proof certification issued by the authorized authorities of the country. Before installation, it is necessary to check whether their specifications and models meet the design requirements, and they should be free of any damage or cracks on the outside. 　　②In areas with explosion hazards, positive-pressure explosion-proof instrument boxes can also be installed, containing non-explosion-proof instruments and other electrical equipment. The ventilation ducts of these instrument boxes must remain unobstructed; before power is supplied, gas in an amount more than 5 times the volume of the box should be introduced to perform purging. 　　③The instrument wiring in Zone 1 of explosive hazard areas must be designed in such a way that, in the event of accidents such as grounding, short circuits, or wire breaks, no ignition source will be created. Therefore, cables and wires must be laid in tubes; pressure-resistant and explosion-proof metal tubes should be used. Cylindrical pipe threads shall be employed for the connections between the wire protection tubes, as well as between those tubes and junction boxes, distribution boxes, and pull boxes. The effective thread engagement length should be 5 to 6 turns or more. Explosion-proof flexible connection pipes should be used when flexible connections are required. 　　In Zone 2, instrument wiring should generally also be enclosed in tubes, but only to protect the insulation of the cables and wires from damage. 　　④When raceways, cable trenches, and protective pipes pass through the boundary between areas with different levels of explosion hazard, sealing measures should be taken to prevent explosive gases from spreading from one hazardous area to another. 　　⑤When connecting the protective tube to field instruments, sensing elements, electrical equipment, instrument boxes, distribution boxes, junction boxes, pull boxs, etc., flameproof sealing fittings shall be installed within 0.45 m of the connection point; for protective tubes larger than 2 inches, a sealing fitting shall be provided every 15 m. 24. What are the requirements when installing intrinsically safe instruments? 　　Answer: ① Intrinsically safe instruments of different series, as well as related devices such as safety barriers, should not be used interchangeably arbitrarily; they must be evaluated by the relevant authorities to confirm that their technical specifications are compatible before they can be replaced with one another. 　　②Intrinsic safety-related devices such as safety barriers, current isolators, and buffer amplifiers should be installed on the safe side and properly grounded. 　　③To prevent hazards caused by accidental contact, electrostatic induction, and electromagnetic induction between the wiring of the intrinsically safe system and that of the intrinsically safe shutdown circuits as well as ordinary circuits, wiring through pipes should be used. Intrinsic safety circuits and non-intrinsic safety circuits should not share the same cable or protective conduit. Intrinsic safety circuits from two or more different systems shall not share the same cable (except when the conductors are individually shielded) nor the same protective conduit (except when shielded wires are used). 　　④When intrinsically safe circuits and non-intrinsically safe circuits are laid in the same trunking or cable trench, they should be separated by a grounded metal plate or insulated barrier; otherwise, they should be arranged separately with a distance of more than 50 mm between them, and each should be fixed individually. 　　⑤The terminal blocks for intrinsically safe and non-intrinsically safe circuits within the dashboard should be separated from each other by a distance of more than 50 mm; otherwise, they should be isolated using insulating sheets. The two types of circuits should be laid separately and secured firmly. 　　⑥The length of the intrinsically safe circuit should be such that its distributed capacitance and distributed inductance do not exceed the maximum allowable values specified by the instrument manufacturer. 　　⑦Wiring in intrinsically safe systems should generally be marked with blue. 　　⑧Intrinsic safety circuits should generally not be grounded, but they are grounded when it is necessary to establish a signal grounding reference point. This grounding point should be the single grounding point for all the grounding conductors of the intrinsic safety instrument system, and it must be separate from the power supply grounding system.