Question

Which of the following statements are true in case of electric fuse:
(A) It is generally made of materials having low melting point.
(B) It is made of materials having high conductivity.
(C) It has inverse time-current characteristics.
(D) It is inserted in series with the circuit to be protected.

Choose the correct answer from the options given below:

• A. (A), (B) and (C) only.
• B. (A), (B) and (D) only.
• C. (A), (C) and (D) only.
• D. (A), (B), (C) and (D).

Hint:

Think about what a fuse must do both during normal load and during a fault.

Answer 1

The Correct Option is D

Step 1: Go through what an electric fuse must do. It has to allow the normal rated current to flow with negligible heating, but melt and break the circuit quickly once current rises above a safe limit during a fault.
Step 2: To melt quickly once overloaded, the fuse element must be made of a metal or alloy with a low melting point, such as tin-lead alloy, zinc or silver, so a small rise in current is enough to raise its temperature past its melting point. This makes statement (A) correct.
Step 3: To carry the normal rated current without excessive heating and without needing a very thick wire, the material must also have reasonably high electrical conductivity. High conductivity keeps the resistance of the element low enough that everyday load current does not overheat it, while its thin cross section still lets fault current push it past melting quickly. So statement (B) is also correct.
Step 4: A fuse does not take the same time to blow for every level of overcurrent. A small overload takes a long time to melt the element, while a heavy fault current melts it almost instantly. This behaviour, where operating time goes down as current goes up, is called an inverse time-current characteristic, and it is a standard property of fuses, so statement (C) is correct.
Step 5: A fuse only works if it can interrupt the current path when it melts, and that is only possible if it is connected in series with the circuit or equipment it protects, never in parallel. So statement (D) is correct.
Step 6: All four statements hold up, so the option that lists (A), (B), (C) and (D) together is correct.