FAA Powerplant Written Practice Test

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

1. AC motor circuits

2. LED lighting systems

3. DC motor circuits

4. Avionics power distribution

The correct answer is: DC motor circuits

The correct choice focuses on the appropriate application of de-rated switches in the context of electrical circuits. When dealing with DC motor circuits, it's crucial to select a switch that can handle potential inrush current that may occur when the motor is starting. DC motors typically draw a higher current at startup compared to their running current, which can lead to overheating and damage if a switch is not adequately rated. De-rating a switch involves selecting a switch with a nominal current rating lower than the expected maximum load to ensure reliable operation under varying conditions. In the case of DC motor circuits, where the characteristics of direct current can lead to arcing at the contacts when switching off, using a de-rated switch can provide additional safety and longevity. Thus, de-rating helps to ensure that the switch will function properly without failing, especially in high-power applications like motor circuits. The other applications, such as AC motor circuits and avionics power distribution, often require switches that can handle higher nominal ratings or have different design considerations due to the nature of AC versus DC power systems. LED lighting systems typically operate at much lower currents and are less likely to require de-rating. Consequently, applying a switch that is de-rated for a DC motor circuit may enhance its performance and durability, making it the