How is the arc in a medium-voltage motor starter managed after contact opening?

In a medium-voltage motor starter, managing the arc that occurs when contacts open is critical for the longevity and reliability of the equipment. The correct answer emphasizes the use of magnetic forces to elongate and cool the arc during contact separation.

When the contacts in a motor starter open, an electric arc is generated due to the high voltage present in the circuit. To mitigate the effects of this arc, magnetic fields are employed, created typically by the current flowing through the contacts. This magnetic field helps to elongate the arc, effectively allowing it to stretch out rather than remain concentrated in a small area. As the arc is elongated, it also experiences cooling effects, which help in reducing its temperature and thus minimizing damage to the contacts and the surrounding components.

This process is crucial in maintaining the operational integrity of the starter and preventing failures. In contrast, other methods such as dissipating the arc freely or using resistive elements do not effectively control the arc dynamics and can lead to greater wear and risk of failure. Containment within a chamber, while effective in some designs, does not leverage the benefits of magnetic elongation and cooling for effective arc management. Thus, the use of magnetic elongation combined with cooling is a sophisticated method to manage the arc safely and