Question

Material used for making a fuse wire must have :

• A. high melting point
• B. low melting point
• C. high resistance
• D. none of these

Hint:

A fuse wire is a safety device that {melts} to break a circuit if the current gets too high. For it to melt easily at a specific overcurrent: 1. It must have a {low melting point}. 2. It should also have relatively {high resistance} (compared to copper wires) so that it heats up significantly with the overcurrent (\(Heat = I^2Rt\)). The low melting point is the most defining characteristic for its protective action.

Answer 1

The Correct Option is B

Concept: A fuse wire is a safety device used in electrical circuits to protect appliances from damage due to excessive current (overcurrent conditions like short circuits or overloads). Its operation relies on the heating effect of current. Step 1: Understanding the function of a fuse wire When an excessively high current flows through a circuit, the fuse wire is designed to heat up rapidly and melt. This melting breaks the circuit, stopping the flow of current and preventing damage to the connected appliances or wiring. Step 2: Desired properties of a fuse wire material For a fuse wire to function effectively as a safety device, it must have: (A) Low Melting Point: This is crucial. The wire must melt at a temperature that is reached when a dangerous level of current flows, but before the circuit wires or appliances get dangerously hot. If the melting point is too high, the fuse won't blow (melt) quickly enough to protect the circuit. (B) High Resistivity (or Resistance for a given length/thickness): The heating effect is given by \(P = I^2R\). For a given current (\(I\)), a higher resistance (\(R\)) will lead to more heat being generated (\(H = I^2Rt\)). This helps the fuse wire heat up and melt quickly when an overcurrent occurs. So, higher resistivity compared to the main circuit wires is desirable. (C) Low cost and resistance to oxidation/corrosion under normal conditions are also practical considerations. Comparing the primary safety characteristics, the low melting point is the most critical for the fuse to act as a protective break. High resistance contributes to the heating, but without a low melting point, it won't melt appropriately. Step 3: Analyzing the options
(1) high melting point: Incorrect. This would prevent the fuse from melting and breaking the circuit during an overcurrent.
(2) low melting point: Correct. This ensures the fuse wire melts and breaks the circuit before excessive current can cause significant damage.
(3) high resistance: This property is also desirable for a fuse wire (relative to the circuit wires) because it helps the wire to heat up quickly (\(H \propto R\)). Materials like tin-lead alloy are used, which have higher resistivity than copper (used for circuit wires) and a low melting point. While important, the "low melting point" is the defining characteristic for its safety function. If both were equally weighted as essential properties, the question would be ambiguous. However, the primary purpose is to melt.
(4) none of these: Incorrect. Considering the primary function, "low melting point" is the most critical characteristic. Option (3) "high resistance" is also a desirable property, but the melting is the key action. Typically, questions prioritize the low melting point.