Long-life low-voltage contacts

Existing performance standards for low-voltage contacts are too low because they do not ensure a useful operational life. Improvement is both possible and necessary, and requires increased rigour in testing. Structures differ in the mechanical duties which they perform, but are alike in that they are required to transmit similar magnitudes of voltage and current and are susceptible to environmental attack. For a test to be appropriate, the environment and measuring circuit must be relevant to service conditions, and for it to be sufficiently rigorous and brief, the environment must be ?artificial?, so as to produce accelerated deterioration. The voltage and current employed in the measuring circuit must be limited, so that there is no tendency to obscure the effect of environmental attack. Unification is necessary in the acceptance tests specified for nominally different structures, and, correspondingly, in the specffications issued to industry. The most generally useful artificial environment is probably the ?industrial? or ?sulphur-polluted? type, and the simplest way of producing it is by the combustion of a fuel similar to those used for industrial or domestic purposes. For semi-permanent structures, comparisons have been made with a variety of test specimens and commercial components of the effects of exposure in three different environments, namely a ?natural? one, and two ?artificial industrial? ones. With low-level pollution (between ten and twenty times greater than that encountered in a severe natural industrial environment) at room temperature, and a working week of five eight-hour days, exposure times of between two and three months are required. An increase of pollution or temperature, or both, or continuous working, might enable this time to be shortened. For sliding contacts, the effect of exposure during sliding is important at even lower levels of pollution. The preservation of near-metallic contact, with reasonable contact forces in adverse environ- - ments, usually necessitates the use of coatings of precious metals such as gold, platinum, palladium or rhodium. For semi-permanent contacts, the coating requirements are determined almost entirely by considerations of environmental attack. For sliding contacts, considerations of mechanical wear are also involved. Such coatings can be applied in several ways: usually, they are deposited from solutions of metallic salts?most commonly, by electrodeposition. Various coatings of different kinds have been examined, and the results show that they are often not good enough for contact use. Thickness, adhesion and freedom from porosity are frequently inadequate. Improvements in basic plating techniques, and more stringent control of production processes, are needed. Thorough quality control in manufacture requires checks occupying, perhaps, only a few minutes. Electrographic tests are very suitable for this purpose, especially when supported by long-term exposure tests on a smaller number of samples. Printed-circuit boards can be plated successfully, provided solutions are used which do not destroy the copper-laminate bond. With sliding contacts, given good mechanical design and manufacture, precious-metal coatings can have much longer lives than are commonly supposed. These lives can be increased, at least threefold, by dry transfer lubrication using a suitable plastic. Laboratory tests, with a variety of coatings of different kinds, and also field experience, show that two-motion switches of the kind used in the Strowger system of automatic telephony can be made suitable for the transmission of low voltages, without ?wetting?. With plug-socket structures, design experience shows that the use of pre-tensioned buffered springs is worthy of more general application.