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■    A sag is an inverted spike. Sags occur when power levels drop below normal and rise back to normal within a brief period of time (usually less than one second). It is doubtful that you would be aware of sags (you might see a light flicker off and then on), although your computer might reboot.


■    A brownout, on the other hand, occurs when power drops below normal levels for several seconds or longer. In other words, a brownout is an inverted surge. The lights in the room will dim for a short period of time and then come back to full brightness.


■    A blackout is a total loss of power for several seconds, several minutes, or several hours.


Any one of these problems will cause your computers and other network devices to malfunction. Figure 6.2 contrasts these power problems.


FIGURE 6.2 Comparing power underage problems


Voltage


Voltage


Voltage


To ward off power underage problems, you need only one device: a UPS, which allows network devices to continue to function even in the complete absence of power. Some are intelligent and can shut down your computer in the case of a blackout.

Power Quality Problems


Power quality problems generally indicate that stray frequencies have entered the power supply through the power cord. Stray frequencies can cause strange problems (such as intermittent reboots or hangs) and can damage a device’s power supply. You can detect problems in power quality only with an oscilloscope. If you’re having power quality problems, you need either a UPS or a line conditioner.

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