For first-time uninterruptible power supply (UPS) buyers, the choices can be intimidating, but there are essentially three UPS topologies, each appropriate for different applications: on-line, off-line, and line-active UPSs.

On-line UPSs use batteries to deliver power to an attached device. Batteries continually charge during normal source power. Since a battery stores dc power, an inverter in the UPS converts dc to ac. Because the inverter is in constant use, system operators are aware of UPS performance and status.

Utility records indicate 98% of blackouts last no longer than two minutes, and most on-line UPSs provide 5 to 10 min of battery backup, sufficient time to permit mission-critical equipment to operate uninterrupted. Situations requiring additional time to conduct orderly shutdowns are accommodated with additional battery capacity or standby generators.

Off-line (stand-by) UPSs must be turned on when normal source power becomes unavailable. Turning the UPS on usually is performed automatically by a monitoring circuit and relays. Unfortunately, inverter/converters most likely fail when first turned on, thus the availability of off-line UPSs cannot be monitored. Nevertheless, off-line UPSs offer cost-effective solutions for non-mission-critical applications.

Line-interactive UPSs borrow from on- and off-line topologies. During normal operation the inverter filters line current to the load and converts a dc trickle to keep batteries at full charge. When normal source power is removed, the UPSs transfer switch shifts from utility to battery output and the inverter reverses operation, converting the battery's dc to ac. Line-interactive UPSs are especially applicable in areas where power outages are rare, but power fluctuations are frequent.

Global UPS designs use the above topologies to provide wide ac-input voltage (85 to 280 V) capability to allow operation in Japan (100-V service) to Australia (250-V service), a frequency range of 45 to 450 Hz for use with domestic and international utilities, military power sources, and engine generators, and a 28 V dc input that allows operation from vehicle or dc engine generators. Global UPSs are complex and require a careful evaluation to ensure the additional complexity and cost is justified.

Today, many UPSs protect entire networks of equipment. Network-wide power protection can be represented in a simple shutdown interface, but complex networks require network communications to inform connected devices of source power condition and status.

Using Simple Network Management Protocal UPSs communication information including power quality, battery status, usage loads, power-related events, environmental temperatures, and UPS self-diagnostic results to other network devices. Each network device uses UPS provided information to initiate appropriate actions, including orderly shutdowns. Additionally, intelligent UPSs can monitor other vital network protection systems such as water detection, smoke and fire protection devices, and access and security controls.

Intelligent UPSs can assist in load management and power factor correction.

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