Materials for electrical contacts

The principal types of failure in light- and medium-duty electrical contacts are analysed in detail and an account is given of the characteristics of the commonly used contact materials. The effects on contact resistance of the size, shape, surface finish and closing pressure of light-duty contacts are discussed; an account is given of the resistance of contact materials to the formation of tarnish films; and attention is directed to the effects of dust and grease films in causing failure. In medium-duty contacts, when arcing occurs, the most serious cause of failure is the action known as ¿material transfer¿ which takes place when direct currents are interrupted. As a result of transfer the contact gap may close and the contacts finally interlock. The factors which influence material transfer are discussed, and curves are reproduced to show the limiting current and the rate of build-up for a number of common contact materials under given test conditions. A second cause of failure is the welding-together of the contacts by the current surge at make. The inherent tendency of a number of contact materials to weld together has been measured. In sliding contacts, excessive wear is a frequent cause of troubles. A method of testing the wear of unlubricated surfaces is described, and the results of measurements on a number of combinations are tabulated.