The expansion stage starts when the pressure relief doors, located at the top of the switchgear, begin to open. Gas flow is characterized by wave motion. Expansion stage typically lasts 5 to 10 milliseconds. Emission stage starts when the pressure relief doors have opened and ends when the gas in the cell reaches arc temperature. This typically lasts 50 to 100 milliseconds.
Thermal stage lasts until the arc is extinguished and combusting materials are consumed.
This lasts until the arc is extinguished and every thing has cooled down to room temperature. Such thermal energy heat melts and vaporizes parts of the equipment and causes the greatest damage to the switchgear. In accommodating arc-resistant types of switchgear in an electrical room, consideration should be given to adequate room ventilation and to clearance above the switchgear which should be at least 36 inches. Metal-clad switchgear is often accommodated in an outdoor house or power control room and if arc resistant switchgear is used, it's important to ensure arc resistant tests have been conducted on not only the switchgear itself but the combination switchgear and weatherproof enclosure to ensure complete safety and compliance with the applicable standards. Service conditions in applying Metal-clad Switchgear Its important to know the specific service conditions under which the switchgear will operate.