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Thermal short time

 

If a conductor which is rated to carry full load current Iload continuously is rated to carry a maximum fault current Ilimit for a time tlimit, then a lower fault current Ifault can be carried for a longer time tfault according to:
( Ilimit - Iload )2 tlimit = ( Ifault - Iload )2 tfault

Rearranging for Ifault and tfault:
Ifault = ( Ilimit - Iload ) ( tlimit / tfault )½ + Iload
tfault = tlimit ( Ilimit - Iload )2 / ( Ifault - Iload )2

If Iload is small compared with Ilimit and Ifault, then:
Ilimit2 tlimit
» Ifault2 tfault
Ifault
» Ilimit ( tlimit / tfault )½
tfault
» tlimit
( Ilimit / Ifault )2

Note that if the current Ifault is reduced by a factor of two, then the time tfault is increased by a factor of four.