So, for a pair of parallel conductors, if the fault current was 10kA = I1 = I2 = then force per metre of line when the lines are a metre apart is:
1.25 x 10exp-6 x 10 x 1000 x 10 x 1000/(2pi x 1) =
approximately
1.25/6 x 10exp-6 x 10exp8 = 0.2 x 10exp2 = 0.20 x 100 = 20N/m
mu is the permeability of free space or magnetic constant and is - 1.25663706 × 10-6 m kg s-2 A-2
So for a 50m span total force is 50 x 20 = 1000Newtons.
As the force pulls the conductors together it increases - F is inversely related to r - so towards mid span where the conductors are freer to sway more, the forces will be much higher. On top of this, the impulse nature of this force will set up transverse vibrations of the conductors.