Ewrb Theory Prep – Mcbs Questions
With Correct Detailed Answers
types of MCB trips - ANSWER- thermal
magnetic
thermal tripping - ANSWER- bi-metal strip, used for overload currents protection
magnetic tripping - ANSWER- magnetic inductor, used for short circuit protection
reset procedure - ANSWER- reset to OFF
reset to ON
arc containment includes - ANSWER- 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 - ANSWER- 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 - ANSWER- A. trip operating unit
B. control contacts unit
C. arcing ext. unit
D. frame unit
describe (A. trip operating unit ) component function - ANSWER- bi-metal/mag. provides
mechanism for detection of fault current and opens contacts
describe (B. control contacts unit ) component function - ANSWER- contacts allows the
supply current to flow during normal running when MCB is closed
describe (C. arcing ext. unit ) component function - ANSWER- all inclusive factors -
horns, chamber chutes, oils, splitter plates, used to control and extinguish fault arc
describe (D. frame unit ) component function - ANSWER- molded case exterior provides
a rigid, mechanically strong, insulated housing for MCB parts
Low Voltage MCB rating - ANSWER- 600V
circuit breaker contacts cannot be made of ____ - ANSWER- iron. heat breakdown &
low CCA
