The failure mechanism of low-voltage electrical relays

The mechanism of increase in the resistance of low-voltage electrical contacts with the number of on-off switch cycles is investigated. Experiments are conducted at room temperature on Au and a number of base metals (Ni, Fe, Ti and Sb) in mixtures of air and organic vapors (benzene, toluene and xylene). The increase in contact resistance is found to be primarily due to carbon formation on the contact surfaces. It is proposed that molecules of the organic vapor first adsorb on the contact surfaces, possibly forming a monolayer. Then, as a spark strikes, carbon is produced by pyrolysis of that layer. Subsequently, more vapor adsorbs on carbon and the process repeats. When a sufficient amount of carbon builds up on the electrodes the contact resistance will be very high.<>