An electrical appliance is designed to run at a specified voltage. The vast majority of the time, the voltage that the electrical utility delivers to our houses is constant, but it can vary under certain unusual conditions.
Voltage Surges
These can happen because of transmission equipment malfunction, lightning strikes, or accidental closing of transmission circuits due to weather, animal activity or other unusual events.
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Protection Against Voltage Surges
Your appliances have many layers of protection against voltage surges. First, there are the fuses and/or circuit breakers in your house--they will "blow" and interrupt a circuit that becomes overloaded. Second, your larger appliances will often have an internal fuse. Third, solid-state electronics often have a surge protector (your laptop's power supply has one). Fourth, the electrical utility has its own protections against power surges.
Low Voltage
Low voltage can occur during period of heavy electrical demand (such as during heat waves, when air conditioners are all running). The electric utility slightly decreases the amount of voltage being fed into the system to compensate for the high demand.
Effects of Low Voltage
Low voltage is not a problem for most appliances. Older appliances that use motors, such as refrigerators and dryers, may be in danger of having their motors overheat. Newer appliances that use electric motors have circuits that sense the motors' temperature and shut down the appliance when it overheats.
Ensuring Proper Voltage
If you have appliances that absolutely must receive the proper voltage at all times (such as sensitive electronics), then you should obtain a "dedicated" power supply, which will feed the same voltage to those appliances regardless of fluctuations in the voltage delivered to the house.