Final answer:
An Active Ethernet switch will turn off the DC voltage on a port if it does not detect a PoE device to prevent damage and save energy.
Step-by-step explanation:
When an Active Ethernet switch fails to sense a Power over Ethernet (PoE) device on a port, the switch will turn off the direct current (DC) voltage to that port. This safety feature is designed to prevent the possibility of electrical damage to devices that do not support PoE or to the Ethernet cables themselves. The switch continuously checks for the electrical signature of a PoE-compatible device before providing power, and if a device is disconnected or if the switch does not detect a compatible device, it will immediately remove power from the port to protect against potential hazards or wasted energy.