When does an uninterruptible power supply (UPS) draw energy from its alternative source?

An uninterruptible power supply (UPS) is designed to provide emergency power to connected devices when the main utility power fails or falls outside of acceptable parameters, often referred to as "out of tolerance." This means that if the voltage or frequency supplied by the utility power source falls below or exceeds the set thresholds, the UPS will switch to its alternative energy source, typically its internal battery or a backup generator. This automatic response ensures that the connected equipment continues to receive stable and reliable power without experiencing interruption, which is crucial for sensitive electronic devices that could be damaged or lose data during power disturbances.

The other options do not accurately represent the primary scenarios under which a UPS would activate its alternative energy source. For instance, if the utility input is within tolerance, or during maintenance mode, the UPS operates normally without needing to draw energy from its backup source. Similarly, an increase in power demand does not trigger a switch to the alternative source unless it coincides with a failure or out-of-tolerance condition of the utility power.