Explosion-proof electrical structure design features

**Explosion-Proof Electrical Structure Design Features** The design of explosion-proof electrical equipment involves multiple critical aspects, starting with the housing structure. Portable and intrinsically safe instruments can be made from either plastic or metal. When using plastic casings, it is essential to ensure that they do not accumulate dangerous static electricity during normal operation. Therefore, the surface resistance must not exceed 1×10⁹ Ω. Additionally, plastic housings must undergo impact and thermal stability tests to ensure their durability and safety. For electrical devices that may generate sparks, arcs, or dangerous temperatures during normal operation—especially Class I equipment with power over 250W or current over 5A—a junction box should be used to connect cables or wires to the main body. These junction boxes are specifically designed for safe cable connections. In modern applications, flameproof wiring methods are widely used for Class I explosion-proof equipment. The inner walls of these junction boxes are typically coated with arc-resistant paint, while the explosion-proof joints between the junction box and the main cavity should be treated with anti-rust oil or similar materials to prevent corrosion. Moreover, all explosion-proof electrical devices must have a permanent Ex explosion-proof mark and an MA safety certification in a visible location on the outer shell. To address issues such as unauthorized motor operation in underground coal mines, the QF40 explosion-proof starter was developed. It has a rated voltage of 380V and a rated current of 40A. The casing is made of 3mm thick steel plate, with flameproof joint surfaces and dimensions that comply with GB3836.2 standards. The enclosure measures 437mm × 308mm and has an IP54 protection rating. A nameplate is mounted directly in front, featuring the Ex logo in the upper right corner, along with the explosion-proof marking dI (150°C). It also includes details such as the model number, rated voltage, current, explosion-proof certification number, safety sign, factory number, and manufacturing date. **Circuit Design** The circuit design of explosion-proof electrical devices must fulfill the required electrical functions and ensure the correctness of the electrical principles. The electrical clearances, creepage distances, and insulation parameters must meet the requirements of GB3836. For example, in the development of the QF40 explosion-proof starter, the creepage distance at the terminal was set to no less than 16mm, and the electrical clearance was no less than 10mm. Similarly, the creepage distance between internal components and the assembly arrangement was also set to no less than 16mm, and the electrical clearance to no less than 10mm. The explosion-proof wiring compartment contains an internal grounding bolt, and the enclosure itself has an external grounding bolt for enhanced safety. For intrinsically safe devices like the steel string frequency meter, the maximum operating current and voltage under both normal and fault conditions must not exceed the design limits. Its creepage distance is 3mm, and when insulated with a coating, it is reduced to 1mm. The clearance remains at 3mm. **Power Supply Design** Intrinsically safe devices often use dry batteries or rechargeable batteries as independent power sources. Both types operate as resistive circuits, and their safety parameters can be determined based on the minimum ignition current curve of a resistive circuit. By determining the battery's maximum voltage and finding the corresponding minimum ignition current, then dividing by a safety factor (usually 2), the maximum allowable current for the battery can be calculated. The most severe discharge condition is a direct short circuit, so the intrinsic safety performance of the power supply must be tested against the battery’s maximum short-circuit current. If this exceeds the design limit, a current-limiting resistor must be added in series. The battery and resistor are usually sealed together using materials like epoxy resin, silicone rubber, or industrial paraffin to form an intrinsically safe unit. For instance, we designed a specialized intrinsic safety battery pack for steel string frequency measurement, consisting of four 5# rechargeable batteries connected in series. The current-limiting resistor is made of Constantin wire with a power rating of 5.2W. The entire battery compartment is filled with epoxy resin to ensure secure potting. Many intrinsically safe devices used in underground coal mines are powered by rectified power supplies from the grid. Since the input circuit is connected to the power grid, it is designed to be both flameproof and intrinsically safe. When the main power is turned off, the wiring is completed within the explosion-proof junction box to establish the electrical connection safely.

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