Temperature rise behind fixed polarity Ag-W contacts opening on an half cycle of high current and its relationship to contact erosion

The work presents the state of knowledge about thermal-erosion processes in contacts of low-voltage switching devices for power engineering on switching currents under short-circuit conditions. The graphical models of the short arc and the distribution of arc power introduced into contacts are shown. The method for measurements of a contact temperature during an electric discharge has been elaborated. The obtained test results are presented, i.e. changes of a contact temperature as a function of arc parameters such as current, energy, and integral ∫idt. The tests have shown that a "break point" exists on the curve expressing the relationship between a temperature rise and arc parameters in the range of high currents. The location of this point is dependent on a diameter of contacts and a value of current, and is associated with thermal energy delivered to electrodes. It has been observed that for each diameter of contacts there exists such value of an energetic quantity of arc (current, ∫idt, energy), at which diameters of arc roots are the same as a contact diameter. Above this value, the shape of a curve is changed. The obtained results explain and confirm the discontinuity of a curve expressing a contact arc erosion as a function of current, which was observed earlier by the other research workers.