Predicting the Erosion of Switching Contacts That Break Inductive Loads

When a contact breaks an inductive load, large voltage surges lead to breakdowns and arcing in the contact gap, which tend to complicate the estimation of resulting contact erosion. A description is given of an empirical method for treating this form of contact wear that has been widely used in telephone-switching practice. The method relates the contact life to a "load energy factor" involving the stored inductive energy of the load and the circuit current flowing at the instant of contact break. Approximate arc energy expressions are then developed, which are found to indicate a reasonable theoretical basis for the original empirical method. These expressions are also found to indicate how contact energy and related erosion are likely to be influenced by the various properties of circuit, contact, and load.