Experimental and theoretical studies are made of small snap-action switches designed for use in thermostatic controls operating on a.c. at 240 volts r.m.s., 50Hz. The performance of the silver cadmium oxide contacts (Ag. CdO, 85/15%) is evaluated over a range of currents from 1 to 10 amps, for make and break operations. The arc at break is found to be the predominant factor contributing to the erosion of the contacts for the range of currents used. Tests using a high speed camera show that the energy dissipated in the arc between the contacts can be evaluated from equations describing the arc in terms of its voltage, current and length as functions of time. Subsequently work is carried out to develop the relation between arc energy and contact erosion, with specific regard to the distribution of energy between the two contact surfaces and the arc column. This is related to the power dissipation in the two electrode fall regions, and the resulting direction of net material transfer is thought to be influenced by the length achieved by the arc before extinction. Erosion is generally in the form of anodic loss and cathodic gain and the reasons for the directional bias in this type of switch are suggested. Ways of reducing the amount of erosion per operation by changing the switch opening characteristic are discussed and supported with experimental results.

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