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Electrical events, such as an overcurrent or overload, can seriously damage electrical circuits or power lines. To avoid expensive replacements of critical parts, electric switch circuit breakers can be installed. These mechanically interrupt the current flow or surge by moving a plunger as soon as a critical current is reached. In contrast to a fuse, which has to be replaced after it has been activated to protect the surrounding electrical components, a circuit breaker can be reset.

The main purpose of this tutorial model is to explore the working principle and some possible solutions for modeling one class of circuit breaker: magnetic power switches. This is an electromechanical device in which iron plungers are moved by means of the magnetic attraction exerted by current flowing in coils surrounding it. Turning off the driving current resets the switch to its initial state.

The model simulates rigid body dynamics under the influence of magnetic forces, induced currents, and spring/constraint arrangements that keep the plunger in its equilibrium position. A copper coil is placed on the central leg of a lower E-core, which is kept fixed. As current flows in the coil, an attractive force is exerted on the upper E-core (the moving plunger), which is held in place by a prestressed spring. When the force reaches a threshold value, the plunger moves toward the lower E-core, closing the air gap. The model illustrates how to properly simulate the movement and the closing time, which depends on the spring stiffness.