FAA Powerplant Written Practice Test

For which application should a switch that is de-rated from its nominal current rating be chosen when installing a conventional aircraft electrical switch?

1. AC motor circuits

2. DC motor circuits

3. Lighting circuits

4. Communication systems

The correct answer is: DC motor circuits

A switch that is de-rated from its nominal current rating is best suited for DC motor circuits because DC motors typically draw higher inrush or starting currents compared to their normal operating current. When a switch is rated for a specific current, using it in a DC application, particularly with motors, necessitates careful attention to these inrush currents. De-rating the switch can help ensure that it can handle the higher currents encountered during startup without risk of failure. In contrast, while AC motor circuits do also have starting currents, those currents are managed differently due to the alternating nature of AC. Lighting circuits usually have steady-state load conditions, which means they are less prone to large current spikes, while communication systems operate on very low current levels, thus not requiring the same level of attention to switching capacity that a de-rated switch for DC motors provides. Therefore, the application that benefits most from a de-rated switch is the DC motor circuits, where the potential for higher inrush current is significant.