EWRB Theory Prep – MCBs: Questions With Solutions
Types of MCB trips Right Ans – thermal magnetic
Thermal tripping Right Ans - bi-metal strip, used for overload currents
protection
Magnetic tripping Right Ans - magnetic inductor, used for short circuit
protection
Reset procedure Right Ans - reset to OFF
reset to ON
Arc containment includes Right Ans - 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 Right Ans - 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 Right Ans - A. trip operating unit
B. control contacts unit
C. arcing ext. unit
D. frame unit
Describe (A. trip operating unit ) component function Right Ans - bi-
metal/mag. provides mechanism for detection of fault current and opens
contacts
Describe (B. control contacts unit ) component function Right Ans -
contacts allows the supply current to flow during normal running when MCB
is closed
Describe (C. arcing ext. unit ) component function Right Ans - all inclusive
factors - horns, chamber chutes, oils, splitter plates, used to control and
extinguish fault arc
Types of MCB trips Right Ans – thermal magnetic
Thermal tripping Right Ans - bi-metal strip, used for overload currents
protection
Magnetic tripping Right Ans - magnetic inductor, used for short circuit
protection
Reset procedure Right Ans - reset to OFF
reset to ON
Arc containment includes Right Ans - 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 Right Ans - 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 Right Ans - A. trip operating unit
B. control contacts unit
C. arcing ext. unit
D. frame unit
Describe (A. trip operating unit ) component function Right Ans - bi-
metal/mag. provides mechanism for detection of fault current and opens
contacts
Describe (B. control contacts unit ) component function Right Ans -
contacts allows the supply current to flow during normal running when MCB
is closed
Describe (C. arcing ext. unit ) component function Right Ans - all inclusive
factors - horns, chamber chutes, oils, splitter plates, used to control and
extinguish fault arc
