Off-Line UPSs
These UPSs
consist of a basic battery/power conversion circuit and a switch
that senses irregularities in the electric utility. The computer is
usually connected directly to the utility that serves as the primary
power source, and power protection is available only when line
voltage dips to the point of creating an outage. Some off-line UPSs
do include surge suppression circuits, and some possess optional
built-in power line conditioners to increase the level of protection
they offer.
In the case of power surges, an off-line UPS passes the surge
voltage to the protected system until it hits a predetermined level,
around 115% of the input voltage. At the surge limit value, the unit
then goes to battery. With high-voltage spikes and switching
transients, they give reasonably good coverage, but not the total
isolation needed for complete input protection. For power sags,
electrical line noise and brownouts, off-line UPSs protect only when
the battery is delivering power to the protected system. A similar
limitation exists in the case of frequency variation.
An off-line UPS protects only if the inverter is operating and on
batter y. If the input frequency varies outside the device’s range,
the unit is forced to go to battery to regulate the output to the
computer. In very unstable conditions, this may drain the battery,
making it unavailable during a blackout.
Standby UPSs are cost effective solutions particularly suitable for
single-user PCs and peripheral equipment that requires basic power
protection.
Off-Line UPSs
