Explain the operating principle of an uninterruptible power supply (UPS) and its role in maintaining power to critical building systems during outages.

An Uninterruptible Power Supply (UPS) is a device that provides backup power to electrical equipment when the normal power source fails. The operating principle of a UPS involves storing energy in batteries and then using that energy to provide power to connected equipment during a power outage. In normal operation, the UPS continuously charges its batteries while also allowing the incoming AC power to pass through to the connected equipment. There are different types of UPS systems, but a common type is the "online" UPS. An online UPS continuously converts the incoming AC power to DC power to charge the batteries, and then inverts the DC power back to AC power to supply the connected equipment. This means that the connected equipment is always running off the UPS's batteries, providing a seamless transfer to battery power during an outage. When a power outage occurs, the UPS immediately switches to battery power, providing uninterrupted power to the connected equipment. The batteries provide power for a limited time, depending on the size of the UPS and the load it is supporting. The primary role of a UPS is to maintain power to critical building systems during outages, preventing data loss, equipment damage, and disruptions to building operations. Critical systems commonly supported by UPS include computer servers, network equipment, security systems, emergency lighting, and critical HVAC controls. By providing uninterrupted power, the UPS allows these systems to continue operating normally during power outages, ensuring business continuity and safety.