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1. How are explosive hazardous areas classified in our country? Answer: Our country uses a classification method that is equivalent to that of IEC for 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 – depending 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 adopt two different approaches to 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 (refer to 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 ***, and it is based on the definitions set by 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, while the English term for \"Division\" in NEC is Division. 3. How many types of explosion-proof structural designs are there for explosion-proof electrical equipment in our country? List their names and markings. Answer: According to **standard GB 3836—83**, there are 8 types of explosion-proof construction forms 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 Intrinsically safe type i Spark-free type n Positive pressure type p Special type s 4. What is a flame-proof instrument? What are its characteristics? Answer: Flame-proofing, also known as pressure-resistant explosion protection, involves enclosing the components of an instrument that could ignite explosive substances within a robust enclosure capable of withstanding the explosive pressure from any internal explosive mixtures, and preventing the spread of such explosions 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 elements and the conversion elements of the transmitter, and the wire connections. Apart from being relatively bulky, flameproof instruments are simple in other aspects and do not require associated devices such as safety barriers. However, before opening the table cover, the power must be turned off first; otherwise, if sparks are generated, they will be exposed to the atmosphere, posing a danger. 5. What are intrinsically safe instruments? What are their characteristics? Answer: Intrinsically safe instruments are also known as spark-safe instruments. Its feature is that, whether under normal conditions or in case of a fault, the sparks generated by the circuits and systems, as well as the temperatures reached, will not ignite explosive mixtures. Its explosion protection is achieved through the following measures: ① The instrument circuit is composed of new types of integrated circuit components, allowing it to operate at lower operating voltages and currents; ② Safety barriers are used 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 ones; ③ The connection wires of the instrument should not create excessive distributed inductance and 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. There are two types of intrinsically safe instruments: ia and ib. Please explain the differences 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 is not possible for an explosive gas mixture to ignite. 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 only suitable 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 (type P) instrument? Answer: It is an instrument device that fills the instrument enclosure with clean air or inert gas under positive pressure, or continuously supplies clean air or non-flammable gas, thereby keeping the pressure of the protective gas inside the enclosure higher than that of the surrounding hazardous environment. This prevents external explosive gas mixtures from entering the enclosure and isolates the electrical components from such hazards. 8. What is an increased safety type (type e) instrument? Answer: An instrument device that does not generate sparks or dangerous temperatures that could ignite explosive mixtures under normal operating conditions, and which incorporates structural measures such as sealing to enhance its safety level, thereby preventing ignition under normal conditions as well as under specified overload conditions. 9. What is a special-type (S-type) instrument? Answer: It refers to special forms other than D, E, I, P, and N, or combinations of these forms; explosion-proof instruments that use such a structural design are called special-type instruments. 10. Into how many major categories are explosion-proof electrical equipment divided? Answer: They are divided into two major 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 are the symbols used? Answer: According to standard GB 3836—83, Class II explosion-proof electrical equipment is divided into three grades, with the symbols 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 Grades 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 terms: Maximum Test Safety Gap MESG; Minimum Ignition Current MIC; Minimum Ignition Current Ratio MICR. Answer: Maximum Test Safety Gap (MESG) – refers to the maximum gap between the joints of a housing filled with a mixture of the gas under test and air at a specified concentration; under specified test conditions, even after ignition, this gap prevents the explosive gas mixture within the housing from igniting across a joint length of 25 mm. 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 marking? Answer: According to standard GB3836—83, Class II explosion-proof electrical equipment is divided into 6 groups based on their maximum surface temperature, with the markings being T1~T6. The grouping criteria are as follows: Temperature group Maximum allowable surface temperature/°C Temperature group Maximum allowable surface temperature/°C T1T2T3 450200200 T4T5T6 13510085 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 factors. ①Select the explosion-proof type of the instrument based on the hazardous area where it is installed and used: Zone 0 – only type ia or type S (referring to type S designed specifically for Zone 0) can be used; Zone 1 – all types except type n may be used; Zone 2 – all explosion-proof types are acceptable. ② Determine the explosion-proof rating and the maximum allowable surface temperature category of the instrument according to the explosion propagation level and ignition temperature category of the flammable gases or vapors that may be present. Refer to Table 8-3, which is compiled based on GB3836-1983. Table 8-3 Examples of classification levels for the propagation of fire by flammable gases and vapors, and ignition temperature categories. Category Level: T1 (T>450°C), T2 (450°C≥T>300°C), T3 (300°C≥T>200°C), T4 (200°C≥T