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There are two different aspects of your question.
First one is that the resistivity of all metals in between 1.5*10^-8 ohm*m and 10*10^-8 ohms*m. For a fuse wire which is about 5 cm long and has about 1 mm^2 surface area its resistance is between 0.00075 ohm and 0.005 ohm (`R=rho*l/S` , where R is resistance, `rho` is resistivity, l is wire length and S is surface area). In a physical way of speaking it means all fuse wires have about the same resistance.
Second aspect is that for a fuse to melt you want it to heat as fast as possible from the current through it. The heating power of a wire having resistance R from a current I is
`P= I^2*R` , which tells you that the higher the resistance of the wire, the faster it heats from the current and the faster it melts down.
What really confuses me is that fuses are connected in series with the circuit so there must be enough conductivity for the current for the appliance to work normally with which it is connected and we know that conductivity is inversely proportional to resisitvity.
In comparison to the appliance with which they are related, fuses must have low resistance and therefore low resistivity. This is to cause the fuse wire to melt if a current more than the safe current for the appliance starts flowing through the circuit. The melting of the wire opens the circuit, thus saving the appliance from damage due to excess current.
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