the number of times the UPS switches and draws on its battery. Each time the standby UPS
switches to its battery backup, it shortens the battery’s life. Most standby UPS units use an
operating range of 103 volts AC to 132 volts AC, which means that whenever the power
sags below 103 volts or surges above 132 volts the unit goes “on-battery.”
Line-Interactive UPS A line-interactive UPS unit is especially well-suited to environ-
ments where there are few brownouts and blackouts but where surges, spikes, or sags are
common. When the power supply is available, the line-interactive UPS provides line con-
ditioning and produces a steady level of output voltage from a fluctuating input voltage
level. This type of UPS also provides good protection from EMI, RFI, and other forms of
line noise. As shown in Figure 23-5, this type of UPS adds line conditioning to the battery
backup capability of the UPS.
In terms of providing battery backup, a line-interactive UPS works just like a standby
power supply and is able to switch to its battery backup faster than any connected equip-
ment can detect the power loss. Remember that the PC’s power supply has a small
amount of reserve power on which it can draw when the power suddenly drops, and
most UPS units switch faster than this reserve can be exhausted. The benefit of a line-in-
teractive UPS over a standard SPS is that because it also filters the incoming power, it can
reduce the amount of times the UPS switches to its battery.
Use the same criteria used for an SPS unit to judge a line-interactive UPS. However,
remember that a blackout is normally preceded by a swarm of surges and spikes before
the power is lost completely. Before purchasing a line-interactive UPS, investigate how
muchover-voltagetheunitcanwithstandbeforeitneedstoswitchtoitsbatterybecausea
UPS draws on its battery to neutralize over-voltage events.
On-line UPS To many, an on-line UPS is the true uninterruptible power supply. This
type of UPS provides all of the services of a surge suppressor, a line conditioner, and a
battery backup in a single package. On-line UPS units provide the best protection of all of
the UPS technologies but they also cost more than the other technologies.
Chapter 23: Electrical Power Issues^611
Figure 23-5. A line-interactive UPS monitors the incoming power and conditions it before
passing it on