A surge occurs when the power line voltage goes higher than nominal and lasts longer than 10 milliseconds. There are three main forms of power interference: voltage dips, electromagnetic interference, and surges. Surges can occur on your electric, phone or even cable TV lines. Every piece of electrical equipment in your home is designed to operate at a specified nominal voltage such as 120 Volts AC. Most equipment is designed to handle minor variations in their standard nominal operating voltage; however, even the smallest power surges can be very damaging to nearly all equipment.