How can I calculate the forces on non-braced cables during a fault? The cables are freestanding and rub against several undesireable places. I am concerned the insulation will be compromised, the screening exposed and thus generate additional spectaculor results. It is a 5KV cable.

I don't know the equations that apply for determining the force over time in faulted three phase circuits.

The DC equation is pretty simple, this equation also applies for the 'instantaneous' force between two conductors at a given moment in time if the current is changing (note): the force between two current carrying conductors is proportional to the current in one times the current in the other, and inversely proportional to the distance between them. (note: this is only strictly true for changing currents if you ignore the speed of light )

See http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/wirfor.html for the equation and a calculator.

After you've calculated the force between the conductors, you will also need to consider the geometry of the situation. Even if the conductor forces are not all that high per unit length, it may be that the force on a length of cable creates a concentrated load at a single point on the cable where it hits a support.

-Jon