Page 178 - Power Quality in Electrical Systems

160 Chapter Eleven

Personal computers are designed to withstand line voltage sags and
surges in accordance with the CBEMA curve of Figure 11.1 or better.
Actual test results of sensitivity are given later in this chapter. The
response of an older PC to a four-cycle interruption of line voltage is
shown in Figure 11.5 [11.5]. The inrush current to recharge the filter
capacitor upon voltage restoration is about 300 percent of normal
current.
Modes of malfunction
The modes of personal computer malfunction under line voltage sag
occur as the DC filter capacitor voltage of the power supply declines with
time. The ensuing software malfunctions include the following [11.2]:

■ Lockup, interruption, or corruption of read/write operations (blue screen)
■ Blocking of the operating system, lack of response to any command
from the keyboard (frozen screen)

Hardware malfunction is identified by automatic restarting/rebooting,
or a permanent black screen, making a manual restart necessary.
Sensitivity to voltage sags and interruptions
Voltage sag tests on personal computers show that their sensitivity
to voltage disturbances follows a rectangular curve, as shown in

V
1

I
2

Ch 1 200 V 8 Ch 2 200 mv M20.0 ms Aux/ 1.40 V
v
Figure 11.5 Voltage and current waveforms to a computer during
and following a momentary outage [11.5].
[© 1998, IEEE, reprinted with permission]