Читать книгу Arc Flash Hazard Analysis and Mitigation - J. C. Das - Страница 18

The arc formation in a cubicle may be described in four phases:

 Phase 1: Compression. The volume of air is overheated due to release of energy, and the remaining volume of air inside the cubicle heats up due to convection and radiation.

 Phase 2: Expansion. A piece of equipment may blow apart to create an opening through which superheated air begins to escape. The pressure reaches its maximum value and then decreases with the release of hot air and arc products.

 Phase 3: Emission. The arcing continues and the superheated air is forced out with almost constant overpressure.

Figure 1.2. The various stages of pressure buildup and its release for an arc in a cubicle. A: Compression, pressure rises; B: Expansion, relief of pressure; C: Emission, gases exhausted; D: Thermal, pressure equalizes (not to scale).

 Phase 4: Thermal. After the release of air, the temperature inside the switchgear nears that of an electrical arc. This lasts till the arc is quenched. All metals and insulating materials undergo erosion, may melt and expand many times, produce toxic fumes, and spray of molten metal.

Figure 1.2 shows these four phases.