Electrical explosion protection and zoning in explosive environments – let’s clarify this this time!

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1. Explosive gases, liquid vapors, and dust: As is well known, flammable and explosive gases, liquids, and dust can undergo violent combustion and explosion when mixed with air (an oxidizer) at certain concentrations, under specific temperatures or upon exposure to an ignition source. Therefore, in areas with explosion hazards, all electrical equipment must be explosion-proof. Explosive substances: explosive gases, flammable liquids and combustible liquids with a flash point equal to or below the ambient temperature, explosive dusts, or flammable fibers, etc. Explosive mixture: Under atmospheric conditions, a mixture of flammable gases, vapors, mists, dusts, or fibers with air such that, once ignited, the fire will spread rapidly throughout the entire mixture. 2 What is electrical explosion protection? Electrical explosion protection involves placing the components of equipment that generate arcs or sparks during normal operation inside an explosion-proof enclosure, or employing other explosion-proof methods such as encapsulation, filling with sand, filling with oil, or maintaining positive pressure to achieve the purpose of preventing explosions. 3 The explosion limit is generally considered to be the concentration or temperature range that must be present for a substance to explode. Depending on the state of the substance and various requirements, the explosion limit is usually divided into an explosion concentration limit and an explosion temperature limit. Explosive concentration limit. It is the highest or lowest concentration range of explosive substances, commonly referred to as the explosion limit. The highest concentration at which an explosion can occur is called the upper explosive limit, while the lowest concentration at which an explosion can occur is called the lower explosive limit; this range of temperatures is known as the explosive temperature range. Since the vapor concentration of a liquid changes with temperature, in addition to an explosive concentration limit, a liquid also has an explosive temperature limit. The explosion temperature limit refers to the temperature range at which the concentration of vapor evaporated from a flammable liquid upon heating equals its explosive concentration limit. The lower explosion temperature refers to the temperature at which a liquid vaporizes to a vapor concentration equal to the lower explosion limit concentration. The lower explosion temperature limit of a liquid is its flash point. The upper explosion temperature refers to the temperature at which the liquid vaporizes to a vapor concentration equal to the upper explosion limit. 4 Classification of explosive substances Explosive substances can be classified into three categories. Category I: Mine methane ； Class II: Explosive gas mixtures (including vapors and mists) ； Class III: Explosive dusts (including fibers). 5 Principles for classifying explosive mixtures The danger level of an explosive mixture is determined by its explosion limit, detonation propagation capability, ignition temperature, and minimum ignition current. Various explosive mixtures are classified according to the maximum test safety gap and minimum ignition current, and grouped by their ignition temperature. It is mainly to install the appropriate electrical equipment in order to achieve safe production. 6 Classification of explosive gas mixtures (1) Classified by the maximum test safety gap (MESG). The maximum test safety gap is the greatest gap between the two parts of the shell cavity – namely, the interior of the shell and the exterior – under standard test conditions, such that after the mixture of the gas or vapor under test at all concentrations inside the shell together with air is ignited, the explosive gas mixture outside the shell cannot be ignited across a joint length of 25 mm. Explosive gas mixtures are classified into three categories: IIA, IIB, and IIC, based on the size of the maximum allowable test safety gap. The safety gap for Class IIA is the largest, resulting in the lowest hazard level; whereas the safety gap for Class IIC is the smallest, leading to the highest hazard level. (2) Classified by minimum ignition current (MIC). The minimum ignition current is the smallest current capable of igniting the most easily ignitable mixture, determined by conducting 3,000 spark tests on a DC circuit composed of air-core inductors using an IEC-standard spark generator under test conditions of 20–40°C temperature, 24V voltage, and 95 mH inductance. Class II explosive gas mixtures are divided into three categories: IIA, IIB, and IIC, based on the value of their minimum ignition current; the lower the minimum ignition current, the greater the hazard. (3) Grouped by ignition temperature. For explosive mixtures, the lowest temperature at which they can be ignited without the use of an open flame is known as the ignition temperature. The lower the ignition temperature of a substance, the easier it is to ignite. Explosive gas mixtures are classified into six groups: T1, T2, T3, T4, T5, and T6, based on their ignition temperatures. T6 has the highest ignition temperature, while T1 has the lowest ignition temperature. 7 Classification of explosive dust mixtures: Explosive dust mixtures are classified into two grades, IIA and IIB, based on the properties of the dust (conductive or non-conductive) and its ignition temperature, and further into three groups: T11, T12, and T13. 8 Classification of explosion hazard zones in workplaces: Explosion-hazardous environments are classified into explosive gas environments and flammable dust environments, depending on the physical state of the substances present in the area. An explosive gas environment is an environment in which, under atmospheric conditions, a mixture of flammable gases, vapors, or mists and air, once ignited, causes the combustion to spread to all unburned portions of the mixture. A combustible dust environment refers to an environment in which, under atmospheric conditions, a mixture of dust or fibrous combustible materials with air ignites, and the combustion spreads to the entire unburned mixture. Table of explosion hazard environment categories and zone levels: http://www.isofans.com/data/attachment/forum/201610/23/085831qf9zf9z1jkg149mr.jpg 9 Principles of electrical explosion protection: Electrical equipment used in environments with explosion hazards must take necessary measures, both in terms of explosion protection for the equipment itself and in terms of ensuring safe operation, in order to prevent and reduce factors that could lead to explosions. There are two reasons why electrical equipment can ignite explosive materials: first, sparks and arcs generated by the electrical equipment; second, heating of the surface of the electrical equipment (i.e., the surface in contact with the explosive materials). 10 Electrical explosion protection measures 1. Stay away from areas with explosion hazards. Electrical equipment and wiring that produce sparks, arcs, and hazardous temperatures during normal operation should be installed in environments where the risk of explosion is low or nonexistent. 2. Use explosion-proof electrical equipment. The number of explosion-proof electrical devices should be reduced, provided that process production and safety requirements are met. 3. Electrical grounding. Even in areas where, according to the relevant technical regulations on grounding design for electrical equipment, grounding is generally not required, grounding must still be implemented within explosive hazard zones. The metal enclosures of electrical equipment shall be reliably grounded. 4. Install leakage fire alarm and emergency power-off devices. Take appropriate remedial actions or automatically cut off the power to the explosive area before electrical equipment fails. 5. Use explosion-proof electrical equipment safely. That is, correctly classifying the categories of explosive hazardous environments, properly selecting and installing explosion-proof electrical equipment, and correctly maintaining and servicing such equipment. 6. Spark-proof floor. Class A factories that emit flammable gases or vapors heavier than air, as well as Class B factories with a risk of explosion due to dust or fibers, should use non-sparking floors. 11 Models of explosion-proof electrical equipment 1. Flameproof type (d): All components of the equipment that could ignite explosive gas mixtures are enclosed within a housing. This housing is capable of withstanding internal explosions caused by combustible mixtures that penetrate through any joints or structural gaps in the housing, without being damaged, and it also prevents ignition of an explosive environment outside resulting from one or more gases or vapors. This type of equipment is suitable for Zone 1 and Zone 2 hazardous environments. 2. Enhanced safety type (e): Additional measures are taken for electrical equipment that does not generate arcs or sparks under normal operating conditions, in order to improve its safety level and prevent the occurrence of dangerous temperatures, arcs, and sparks in such equipment. It does not include devices that generate sparks or arcs under normal operating conditions. This type of equipment is mainly used in Zone 2 hazardous environments, and some types can be used in Zone 1. 3. Intrinsically safe type (i): All circuits inside the device are such that any electric spark or thermal effect generated under standard operating conditions (including normal operation or specified fault conditions) cannot ignite the specified explosive gas environment in an intrinsically safe circuit. This explosion-proof type is divided into two grades: ia and ib. This type of device can only be used in low-voltage equipment; ia is suitable for hazardous areas 0, 1, and 2, while ib is suitable for hazardous areas 1 and 2. 4. Positive pressure type (p): It features a positive-pressure enclosure that maintains the pressure of the protective gas inside at a level higher than that of the explosive environment surrounding it, thereby preventing external mixtures from entering the enclosure. Devices of this type can be used in Zone 1 and Zone 2 hazardous environments depending on the protection method. 5. Oil-immersed type (O): The entire device or its components are immersed in oil (a protective fluid), thereby preventing explosive gases above the oil surface or outside the enclosure from igniting. This type of equipment is suitable for Zone 1 and Zone 2 hazardous environments. 6. Sand-filled type (q): Sand particles or other powder materials with specified properties are filled inside the enclosure, so that under specified operating conditions, neither the arc generated within the enclosure nor the high temperatures can ignite the surrounding explosive gas environment. This type of equipment is suitable for Zone 1 and Zone 2 hazardous environments. 7. Spark-free type (n): Under normal operating conditions, it cannot ignite the surrounding explosive gas environment, and faults that could lead to ignition are also highly unlikely to occur. This type of equipment is only suitable for Zone 2 hazardous environments. 8. Encapsulation type (m): Electrical components that may generate sparks, arcs, or hot spots capable of causing an explosion in an explosive gas environment are encased in an encapsulating material (compound), preventing them from igniting the surrounding explosive gas atmosphere. This type of equipment is suitable for Zone 1 and Zone 2 hazardous environments. 9. Special type (S): Refers to **explosion-proof types not included in the standards. When using electrical equipment of this type, provisional regulations must be established by the competent authorities, and it must be inspected and approved by a designated explosion-proof testing agency before it can be used as explosion-proof special-type electrical equipment. This type of equipment is developed based on actual usage and can be applied in corresponding hazardous environments. 10. Dust explosion-proof type (DIP): It prevents the ignition of combustible dust by limiting the maximum surface temperature of the enclosure and using a \"dust-tight\" or \"dust-proof\" enclosure to prevent dust from entering. Based on its explosion-proof performance, it can be used in hazardous areas 20, 21, or 22. 12 Categories of explosion-proof electrical equipment. 1. Electrical equipment for use in explosive gas environments is divided into two categories: Category I and Category II. Class I: Electrical equipment for coal mines. Class II: Electrical equipment for explosive gas environments other than coal mines. 2. Electrical equipment for flammable dust environments is divided into Type A dust-tight equipment, Type B dust-tight equipment, Type A dust-proof equipment, and Type B dust-proof equipment. 13 Explosion-proof electrical equipment: Electrical equipment intended for use in explosive gas environments of Category II is divided into six groups, T1, T2, T3, T4, T5, and T6, based on its maximum surface temperature. The selection of such equipment should be made ensuring that the maximum surface temperature of the corresponding T1–T6 group does not exceed the ignition temperature of any gases or vapors that may be present. http://www.isofans.com/data/attachment/forum/201610/23/085832fjjhlveqjuhv74vv.jpg 14 Explosion-proof marking: The elements of the explosion-proof marking for explosion-proof electrical equipment include: explosion-proof type + equipment category + temperature. Type I flameproof type: Exd I ； Class IIB flameproof, Group T3: ExdⅡBT3 ； For dust explosion-proof equipment, such as Type A equipment suitable for Zone 21, the maximum surface temperature TA is 170°C; its explosion-proof marking is: DIPA21 TA170°C or DIP A21 TA, T3 ； 15 Selection principles for explosion-proof electrical equipment 1. The explosion-proof classification of the electrical equipment should be appropriate for the explosive hazard area. 2. The explosion-proof performance of electrical equipment shall be appropriate to the hazard level of the substances in the explosive hazardous environment. 3. It should adapt to environmental conditions. 4. It shall comply with the principle of overall explosion protection, being safe and reliable, economically reasonable, and easy to use and maintain. 16 Inspection and acceptance of explosion-proof electrical equipment: The inspection and acceptance of explosion-proof electrical equipment involve a formal review and a visual inspection. 1. The formal review involves checking the industrial product production license certificates (except for explosion-proof lighting fixtures) and explosion-proof certification certificates for production inspections; these must be verifiable online. 2. Visual inspection involves checking whether there is a permanent EX (DIP) marking on the exterior of the explosion-proof electrical equipment, and then verifying that the explosion-proof certification number indicated on the nameplate matches the number on the actual explosion-proof certificate. ——END——