90% of all electrical surges or transient voltage activity is generated within homes and businesses. In homes, most surges occur when motor-driven devices such as refrigerators, televisions, hair dryers, water pumps, etc. shut off. Suddenly the energy these devices were consuming is diverted elsewhere in the form of excess voltage. Surges at businesses are caused by elevators, air conditioners, vending machines, copiers, large computers, even lights turning on and off, resulting in rushes of power and transient voltages back up the line.
10% of the problem is generated outside the home or office by events such as utility grid switching, line slapping, bad wiring, etc. Surges also happen when the electric company switches power from one geographic area of the grid to another as supply and demand in the region changes. Thunderstorms and lightening are the most dramatic and destructive causes of power line problems.