Prediction of Voltage Surges when Switching Contacts Interrupt Inductive Loads

When contacts interrupt the current path to an inductive load, the resulting voltage surge produces effects that can be detrimental to performance of the system. The high and often oscillatory voltages can cause both radiation interference and voltage breakdown in the wiring, in associated components, or in the contact gaps themselves where the resulting arcs lead to contact erosion. Some general quantitative relationships between these voltages and the important parameters of the contact and of the local circuit are derived and discussed. The resulting equations are then applied specifically to the case where the load is an unprotected relay coil, with discussion of peak voltages, time to reach peak, and energy content that is available to be dissipated outside the coil terminals as, for example, in contact arcs. Experimental confirmation is also demonstrated.