Brownouts are usually sustained sags that can last hours during times of peak demand (extreme cold or heat events). The lower voltages will make motors run hotter and can cause electronics to malfunction or freeze.
Momentary sags are caused by faults on neighboring grids where line protective devices interrupt the flow of power briefly (which causes a momentary total outage on the affected line obviously) and when reclosure occurs, devices which employ refrigeration will draw locked rotor amps and momentarily increase demand by as much as 500% or more. This may cause the normal 120V reading at the socket to dip to under 100V or in severe cases < 85V very briefly. Incandescent lights will noticeably flicker and PC's may reboot. If a UPS is attached it will transfer to battery power. Most UPS' have a relay that engages and this is audible as a click. A buzzing may also be heard from the inverter while it's running on batteries.
Laser printers on long runs of 14AWG - typical 15 ampere circuits - can cause a similar condition due to the high inrush current of the toner fuser heater elements which are rapidly switched on and off to maintain temperature.
A zero transfer UPS provides outstanding isolation by use of a ferroresonant transformer, motor-generator, or online inverter. These are not found in home / small office units! Most small UPS' feature a 3-5mS transfer time which is quite fast and well within the hold time of all PC power supplies. Medical and time domain sensitive equipment often require zero break - zero transfer power, however and that's what this equipment offers. If a PC reboots when the UPS goes on battery the UPS should be checked by a technician familiar with its operation. If it's operating normally, the PC power supply has a problem and should be investigated promptly.
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