Performance Characteristics of Snap-Action Switches with Silver-Cadmium-Oxide Contacts

Detailed studies have been made of small snap-action switches designed for use in thermostatic controls operating on ac at 240-V rms, 50 Hz. The predominant factor contributing to the erosion of the 4~mm diameter silver-cadmium-oxide contacts, i.e., the break arc, has been defined in terms of the nonuniform distribution of the arc energy between the contacts for the range of currents used (1-10 A). The amount of material eroded from either contact at any one opening was shown to depend on the ignition point of the arc with respect to an ac zero. This was thought to be due to the relative magnitude of the anode and cathode fall voltages being different for arcs of differing duration and lengths. This affects the amount of energy dissipated at the contact surfaces, hence a directional bias to the erosion ensues---shorter duration arcs (<3 ms) producing transfer from cathode to anode and longer ones (>5 ms) transfer from anode to cathode. It is suggested that this effect may be used in switch design to reduce the erosion rate of the contacts.