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ITFrequently Asked Questions · UPS
What does a UPS protect against?
Depending on its type, a UPS protects your loads from: loss of power (power outage), overvoltage (short- or long-term), frequency variation, switching transients (momentary interruption), harmonic distortion, and noise (electromagnetic interference).
Grid problems are not only outages. More common are momentary voltage dips when a large load starts, short spikes from switching or lightning, and electrical noise from motors and power supplies. A computer can restart even from a voltage dip lasting a fraction of a second, without the power actually going off.
How many of these are covered depends on the topology. An offline UPS mainly covers outages and large deviations by switching to battery. A line-interactive UPS also corrects sustained under- and overvoltage through AVR, without using the battery. An online UPS fully isolates the load, since it continuously generates a new voltage at a stable frequency; it is the only one of the three topologies that protects continuously against frequency variations and harmonics, which matters when power comes from a generator.
No UPS replaces lightning protection or proper earthing. In practice, choose the topology according to how sensitive the load is and how "dirty" the power is at your site: offline for a simple home computer, line-interactive for an office, online for servers and medical or industrial equipment.
