EWRB Theory Prep – MCBs Study Questions and Correct Answers

Types of MCB trips thermal Magnetic Thermal tripping bi-metal strip, used for overload currents protection Magnetic tripping magnetic inductor, used for short circuit protection Reset procedure reset to OFF Reset to ON Arc containment includes horns carry current to chute, allow arc to jump out Arc chute lengthens distance for arc to travel Oil suppresses arc Splitter plates split the arc Arc control factors reduced horns width minimize arc x section More plates split arc into multiple smaller voltages Maximize arc chute increases travel distance for arc Four key parts of mcbs A. Trip operating unit B. Control contacts unit C. Arcing ext. Unit D. Frame unit Describe (A. Trip operating unit ) component function bi-metal/mag. Provides mechanism for detection of fault current and opens contacts Describe (B. Control contacts unit ) component function contacts allows the supply current to flow during normal running when MCB is closed Describe (C. Arcing ext. Unit ) component function all inclusive factors - horns, chamber chutes, oils, splitter plates, used to control and extinguish fault arc Describe (D. Frame unit ) component function molded case exterior provides a rigid, mechanically strong, insulated housing for MCB parts Low Voltage MCB rating 600V Circuit breaker contacts cannot be made of ____ iron. Heat breakdown & low CCA