When does a transfer switch automatically connect the signal mains to emergency power?

The correct response is based on understanding the function of a transfer switch in an electrical system. A transfer switch is primarily designed to monitor the voltage of the normal power supply. When the voltage drops below a certain threshold—in this case, 85 volts—the transfer switch will automatically engage to connect the signal mains to the emergency power supply. This action ensures that critical systems remain operational during outages or significant dips in power, thereby maintaining safety and functionality.

In contrast, a transfer switch does not activate based on scenarios like a power surge, as surges usually do not trigger the switch to engage emergency power; instead, they may even cause protective mechanisms to isolate circuits to prevent damage. Additionally, the automatic connection does not occur simply when the normal power supply voltage exceeds a specific level like 120 volts; such a condition would allow for continued reliance on normal power. Also, while the activation of the emergency power supply is essential for transitioning to backup systems, it is the monitoring of the normal power's voltage level that prompts the switch to connect to the emergency source when power drops too low.