Question

Mixture Circuit Breaker (MCB) works on the heating effect of current.
A. True
B. False

Answer 1

Hint: When there is overload of current due to excessive current flow, the MCB trips off the current flow automatically. MCB has a bimetallic strip which displaces from its position when there is excessive flow of current through it. Recall what causes the increase in the length of the bimetallic strip.

Complete step by step solution:
As we know, when there is overload of current due to excessive current flow, the MCB trips off the current flow automatically. The tripping of MCB is due to deflection of bimetallic strips in the MCB. The MCB works on two principles: the thermal effect of over current and electromagnetic effect of over current.

In the thermal operation of MCB, the bimetallic strip is heated due to over current and thus it deflects by bending. The bending of the strip releases a mechanical latch. Since the mechanical latch is connected to the operating mechanism, the release of mechanical latch causes the opening of the circuit. As we know, the open circuit prevents the flow of current.We know that the heating of the current carrying element is proportional to the square of current passing through it.
\[Q = {I^2}Rt\]
Here, Q is the heat generated, I is the current, R is the resistance and t is the time.

Thus, if the over current flows through the bimetallic strip, the heat is generated. According to the expression for the linear expansion as a function of temperature, the increase in the temperature causes an increase in the length of the bimetallic strip. Thus, due to increase in the length of the bimetallic strip, the position of latch on the bimetallic strip is displaced which results in the release of mechanical latch. Thus, the given statement is true.

So, the correct answer is option A.

Note: The MCB also works on the electromagnetic effect of over current flow. The fuse also works on the heating effect of over current through the fuse wire. But the MCB is preferred over fuse since it is quite sensitive to the over current flow. To answer this question, students can visualize the cross-section of MCB to understand the position of mechanical latch and bimetallic strip.