Analysis of overtravel in induction disc overcurrent relays

• Experimental results show that the equivalent overtravel time (Δt) is constant.
• Dynamic equations are analyzed, to show the root of this behavior.
• Explanation can be based on a differential equation of 1st-order or 2nd-order.
• 1st-order differential equation is very simple but it is not rigorously valid.
• 2nd-order differential equation is preferable for the sake of accuracy.

This article analyzes the overtravel in induction disc overcurrent relays. Based on experimental results, obtained from tests on three relays from different manufacturers, it is verified that the equivalent overtravel time can be considered constant for a wide range of settings in the device. Dynamic equations of the device are analyzed, in order to find the root of this behavior. This analysis was performed in two ways: by assuming a first-order differential equation, and by assuming a second-order differential equation. Both methods allow to demonstrate the fact of a constant value for the equivalent overtravel time. The method based on a first-order differential equation is very simple, and this fact is attractive from a pedagogical perspective; however, it is based on assumptions which are not rigorously valid. Thus, the method based on a second-order differential equation should be preferred for the sake of accuracy.