The repulsion or blow-off force between closed contacts carrying current

Experimental data for the repulsion or blow-off force F
_B
between closed contacts carrying current were compiled in order to derive an empirical relationship for this force. The data cover a period of 65 years, and includes data from contacts in both air and in vacuum with currents ranging from 1-200 kA peak. Plotting the blow-off force vs. the current reveals a simple relationship independent of the contact material, F
_B
= k· I
²
, where i is the peak current and k is a constant. This relationship provides a quick estimate of the blow-off force in practical designs for overload and fault currents. It can help guide the selection of the proper contact force to prevent welding, and is very useful in the development of theories calculating the welding current for closed contacts carrying current.