Sunday, 12 August 2018

fuses - What is melting integral I^2t


I am going to buy a fuse, but I am confused about what the purpose of I^2t is can someone please explain to me what it means.



Answer




There are two extremes for specifying the current carrying capability of a conductor.


a) Over a long time, so all heat is lost to the environment. The current will be given for some permitted temperature rise. Typically this is the temperature rating of the insulation.


b) Over a very short time, so short that no heat is lost to the environment. This is the so-called adiabatic case.


As the power generated in a conductor is \$I^2R\$, the heat energy deposited will be \$I^2Rt\$. For any given conductor, R will be constant, so the energy to some temperature, for the short time case is usually given as \$I^2t\$.


For a fuse, typical temperatures will be the 'guaranteed to still work', and 'guaranteed to break' or melting temperature.


When you're protecting something else that has a quoted survival \$I^2t\$, for instance a rectifier diode, you would want the rating of the diode to exceed the 'guaranteed to break' rating of the fuse, also known as the 'let through' energy.


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