Relationship between limit interrupting capacity of axial magnetic field vacuum interrupters with cup type axial magnetic field contacts and their contact diameters and contact gaps is studied.Test data from synthetic circuit is used and axial magnetic flux density from 3D eddy current field calculation by FEM is introduced to obtain the relationship.It suggests that the relationship between limit interrupting capacity and contact diameter and contact gap is I=k×D2×Bz0.4

where k is a constant.The physical meaning of the relationship is that the limit interrupting capacity increases with increasing of contact diameter and decreases with increasing of contact gap.