Power surges occur when the flow of electricity is interrupted, then started again, or when something sends electricity flowing back into the system.
Surges can range from five or ten volts when you turn on your hair dryer to thousands of volts if lightning strikes a transformer.
Internal power surges
More than half of household power surges are internal. These happen dozens of times of day, usually when devices with motors start up or shut off, diverting electricity to and from other appliances.
Refrigerators and air conditioners are the biggest culprits, but smaller devices like hair dryers and power tools can also cause problems.
External power surges
An external power surge, stemming from outside your home, is most commonly caused by a tree limb touching a power line, lightning striking utility equipment or a small animal getting into a transformer.
Surges can also occur when the power comes back on after an outage, and can even come into your home through telephone and cable TV lines.
Why worry about power surges?
Your home is filled with items susceptible to power surges. Anything containing a microprocessor is especially vulnerable - the tiny digital components are so sensitive that even a 10-volt fluctuation can disrupt proper functioning.
Microprocessors are found in hundreds of consumer items, including TVs, cordless phones, computers, microwaves, and even seemingly "low-tech" large appliances like dishwashers, washing machines and refrigerators.
Large power surges, as with a lightning strike, can cause instantaneous damage, "frying" circuits and melting plastic and metal parts. Fortunately, these types of power surges are rare.
Low-level power surges won't melt parts or blow fuses, but they can cause "electronic rust," gradually degrading internal circuitry until it ultimately fails.
Small surges won't leave any outward evidence, so you may not even be aware they're happening - even though they may occur dozens or even hundreds of times each day.