I2t Thermal Withsand

The thermal withstand of a cable can be determined by comparing the maximum energy at the fault, with the maximum energy the cable can absorb.  The equation for this is:

$I^{2}t\le k^2{A}^2$

I²t is proportional to the energy let-through of the protective device
       I - fault current, A
       t - fault duration, S

k²A² is proportional to the energy withstand of the cable
        k - adiabatic constant
        A - cable cross-sectional area, mm²

To better understand the theory behind the above, please see The adiabatic equation.