Tag: physics

In MCB, whenever continuous current flows, the bimetallic strips is heated and deflect by bending. This deflection of bimetallic strips releases the mechanical latch. It causes to open the miniature circuit breakers contact and the MCB turns of thereby stopping he current to flow in the circuit. Hence $A$ is the wrong statement. 

Fuse wire have high value of resistance so when high magnitude of current flows through it then due to melting point of it it melts by excessive heating. Fuse is used to protect the circuit from any accidental heavy flow of current.

The work of fuse wire to protect electric appliances if some particular amount of current that should not be passed through is passed accidentally.

 Electric fuse wire is a wire which allows only upto a limit of current through it , if a particular amount of value of current is exceeded then it melts down.

Fuse is a device used to save the electric current from overheating due to the sudden increase in the current in the circuit.
It protects the device and the user when the electric current is higher than the normal needed by a circuit, it blows up breaking the circuit.
Hence protecting the circuit from an a power overload.

A fuse is a type of low resistance resistor that acts as a sacrificial device to provide over current protection, of either the load or source circuit. It just controls the amount of current flowing.
It neither increase the resistance of the circuit nor it allows high current. It doesn't allow high current to pass but that doesn't mean, it protect appliances from heating.

A fuse wire may possibly melt when :
(i) the earth wire is broken
(ii) the fuse is fixed along the neutral wire instead of the live wire.
(iii) there is a short circuit in the electrical circuit.

A fuse wire must be made of a material having less specific heat and low melting point. When the current in electric circuit exceeds a safe, limit, the low value of specific heat results in more increase in temperature and the low value of melting point causes it to melt. It thus saves the electrical appliances from getting damaged.

Fuse are used to protect electric appliances at home from large magnitude of current. they are made up of very high resistance. fuse  blows up (Melts) due to overheating when the current through it exceeds the rated current.

When the circuit is switched on, the temperature of the fuse will increase up to a certain value (below the melting point) and then remains constant At this equilibrium stage, the heat lost by the radiation per second balances the heat liberated per second. That is
$(2\pi rI)H=I^2R$
where $H$ is the rate of loss of heat per unit area, $I$ is the current and $R$ the resistance of the fuse wire.
But $\displaystyle R = \frac {\rho l}{\pi r^2}$

$\displaystyle \therefore H(2\pi rl) = \frac {I^2 \rho l}{\pi r^2}$ or $\displaystyle r^3 = \frac {\rho I _2}{2\pi ^{2}H}$

For a given material of the fuse wire, $\rho$ and $H$ are constants.

$r^3 \propto I^2$

If $r _1$ and $r _2$ are the radii of two fuse wires of the same material which blow $u$ at currents $I _1$ and $I _2$,
respectively, then

$\displaystyle \left ( \frac {r _2}{r _1} \right)^3 = \left ( \frac {I _2}{I _1} \right),$ i.e., $\displaystyle r _2 = r _1 \left ( \frac {I _2}{I _1} \right)^{\displaystyle \frac {2}{3}}$

Given: $I _1 = 5A, r _1 = 0.1 mm, I _2 = 40 A$,

$\displaystyle r _2 = 0.1 \left ( \frac {40}{5}\right)^{\displaystyle \frac {2}{3}} = 0.1 (4) = 0.4mm$