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:
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.