The most visible and destructive cause of surge damage is lightning.

However, while lightning is the most destructive, it is not the most common source of surges. In many areas, 80 to 90 percent of power disturbances come from other sources.

Utility System: The utility distribution system interconnects many different types of loads that are difficult to control. In an effort to control the various loads, the utility relies on load switching. Feeder and capacitor switching, combined with momentary short circuits and contact re closures, have become a hazard for computers and other sensitive equipment.

Inductive Loads: A frequent power disturbance seen in building wiring systems today is transient voltage associated with inductive loads. These disturbances are a result of turning heavy electrical equipment on and off in the vicinity of a sensitive device. The resultant voltage surge can migrate through the electrical system causing damage to equipment down line.

Electro-Magnetic Interference and Radio Frequency Interference: The proliferation of electronic equipment in the workplace has caused a dramatic increase in EMI/RFI. This increased interference can cause data errors and software malfunctions.

Lightning: Lightning is the most visible and poten-tially destructive cause of high energy surges. Lightning has the capability to generate surges of extreme magnitude. A direct lightning strike is not necessary for system damage. As an example, a lightning discharge three kilometres from exposed overhead electrical lines has the potential to produce a 20 kA surge and cause electronic equipment failure.