Normal force reduction: a variable activation energy process? [electrical contact springs]

An experimental and theoretical analysis has been carried out into normal force reduction in electrical contact springs. This is a phenomenon whereby the force produced by the contact spring reduces with time. A series of ten elevated temperature heat age tests were carried out for time periods of up to three months. Over 6500 normal force reduction measurements have been made. Examination of the data revealed a high correlation for the normal force to exhibit a logarithmic relationship with time. Theoretical analysis showed that this could he explained by assuming that the phenomenon of normal force reduction is thermally activated with an activation energy that is variable and related to the amount of stored energy in the contact spring. A theoretical model for the process has been proposed, and the values of the parameters calculated for the 10 temperatures for which data have been collected. This model can be used to predict end-of-life normal forces for electrical contacts, provided the model parameters are determined experimentally at the required temperature. In addition, it is possible to obtain confidence levels associated with these predictions