What is a surge? A surge is as a brief increase in the voltage on an AC power, telephone/modem, network or coax line. Surges can last anywhere from a few milliseconds (or 1/1000ths of a second) up to several seconds in duration. The intensity can be anywhere from several dozen volts up to several thousand volts in strength. The term "spike" is often used in addition to, or in place of, the term "surge". A spike is a type of surge that is shorter and more intense. However, since the causes, effects, and solutions are identical, the term "surge" will be used exclusively in this course.