Prevention of Bridging Failure in Mercury Switches

Service failures of sealed mercury switches in relays may occur by "sticking" or by "bridging," Analyses by scanning-electron-microscope microprobe and by X-ray diffraction revealed that bridging failures in certain make,before-break-type switches can be due to the accumulation of particles of NiHg4in the mercury which, because of increased viscosity, may form a quasi-steady bridge between the movable and fixed contacts. Doping of the mercury with copper and tin was proposed as a solution to the bridging problem based on the analysis of failed switches and the knowledge gained from previous studies of liquid-mercury/solid-metal interactions [1]. The results of an accelerated laboratory test and a field trial comparing the electrical characteristics of undoped and doped switches confirm that the failure mechanism is thermally activated. The undoped switches were prone to bridging failure. Prevention of bridging failure in doped switches is attributed to the preferential formation Of a tin-bearing layer which inhibits the dissolution of nickel and the subsequent formation of a viscous slurry of NiHg4in mercury.