Each type of electrical field stress changing due to sitting of an external body on wire can to increase air ionization and breakdown probability. Amount of electrical field strees is related to dimention and shape of added subject to system.
Normally we have treated ‘macroscopic’ fields acting between conducting electrodes with dimensions suitable to insulate high voltages by controlling the maximum electrical field strength by large curvatures of the electrodes.
In actual insulation systems the real surface of any conductor may not be really plane or shaped as assumed by macroscopic dimensions, or the real homogeneous insulation material may be contaminated by particles of a more or less conducting nature. Although a real surface roughness of an electrode, or the real shape of particles within the insulating material, may be very complex, the local distortion of the electrical field which can be assumed to be ‘microscopic’ in dimensions can easily lead to partial discharges or even to a breakdown of the whole insulation system.