The "selective" deposition of particles on electric contact and their effects on contact failure

As it is known, dust contamination may cause serious contact failure. Deposition of dust and adhesion of particles are affected by the electric charges carried by the dust particles. In an external electric field, the deposition of particles is greatly enhanced. By using the Milliken method, the electric charges carried by particles are measured. It is found that particles of different materials may carry different electric charges. Therefore, dust particles are not equally deposited when in the presence of an electric field. Inspection on some failed contact surfaces shows that gypsum, organic materials, mica and titanium oxide etc. that are present in less amounts in the dust but contribute more to the contact failure. They seem to be "selectively" deposited on the surface and caused contact problem. Soft materials such as gypsum can be deformed and squeezed at the contacts and provide SO₄^2- ions in the water film to cause corrosion at the local region. Organic material may act as adhesives to stick with other particles. Thus, various sizes of particles can grow during the operation. Silicon compounds are in the majority of dust, even mica is hard to be distinguished and some of them possess less charge, however they are still commonly found at the failed contacts. Particles may attach water-soluble salts that carry less electric charges, but they frequently appeared in hand set telecommunication, and cause serious corrosion.