A model of contact activation and the long-term behavior of R C protected contacts

The long-term arcing behavior of resistor-capacitor protected contacts is important for many telephone relay applications; and the surrounding organic vapors and the contact surface conditions (rather than the electrical circuit) are chiefly responsible for determining the arc duration, erosion, and life of these contacts. A model of this contact activation process is developed based upon rate equations using averaged variables to characterize the contact surface conditions. Two limiting cases of the general steady-state and transient solutions are discussed. Experimental transient average arc time observations corroborate the model and show a single exponential variation of the average arc time with the number of operations. Other results imply that the adsorption of organic vapors occurs over periods of time comparable to, or greater than, the open time of the contacts which was approximately 0.1 sec. Therefore, the slow adsorption limit of the model is the most appropriate for experimental analysis. In addition to the transient average arc time observations, individual and average arc times per operation are presented for contacts in the laboratory for periods up to 2 weeks. It is observed that while the arc durations per operation are quite unpredictable, they are not truly random. There is a positive autocorrelation in the arc durations for 60 operations followed by a slight negative autocorrelation out to about 150 operations. Even after 150 operations, the arc durations are still not a random sampling of the arc time distribution since the accuracy of the average arc time does not increase as n−1/2.