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Adverse Power Conditions

There are two adverse AC power conditions that can damage or adversely affect a computer: overvoltage and undervoltage. Overvoltage occurs when the output voltage from the wall outlet (the AC voltage) is over the rated amount. Normally, the output of a wall outlet is 110 to 130 volts AC. When the voltage rises above 130 volts, an overvoltage condition exists. The power supply takes the AC voltage and converts it to DC. An overvoltage condition is harmful to the components because too much DC voltage destroys electronic circuits. An overvoltage condition can be a surge or a spike.

When the voltage falls below 110 volts AC, an undervoltage condition exists. If the voltage is too low, a computer power supply cannot provide enough power to all the components. Under these conditions, the power supply draws too much current, causing it to overheat, weakening or damaging the components. An undervoltage condition is known as a brownout or sag. Table 4.7 explains these power terms.

Table 4.7. Adverse power conditions

Major type





A spike lasts 1 to 2 nanoseconds. A nanosecond is one-billionth of a second. A spike is harder to guard against than a surge because it has such short duration and high intensity.


A surge lasts longer (3 or more nanoseconds) than a spike. Also called transient voltage. Causes of surges include lightning, poorly regulated electricity, faulty wiring, and devices that turn on periodically, such as elevators, air conditioners, and refrigerators.



In a brownout, power circuits become overloaded. Occasionally, an electric company intentionally causes a brownout to reduce the power drawn by customers during peak periods.


A sag occurs when the voltage from the wall outlet drops momentarily.


A blackout is a total loss of power.

Electric companies offer surge protection for homes. Frequently, there are two choices. A basic package protects large appliances, such as refrigerators, air conditioners, washers, and dryers. It allows no more than 800 volts to enter the electrical system. A premium package protects more sensitive devices (TVs, stereos, and computers) and reduces the amount of voltage allowed to 323 volts or less. Some suppressors handle surges up to 20,000 volts. The exterior surge arrestor does not protect against voltage increases that originate inside the building, such as those caused by faulty wiring.

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