Imagine trying to conduct business without telephones or e-mail. For most people, it's a scary thought. Today's successful businesses depend on communication, and that means reliable communication networks. In order for these networks to operate continuously, they must be able to withstand transient overvoltages caused by inductive switching or lightning strikes. Typically, engineers achieve this goal by placing surge suppressors in the power lines at the main entrance and/or point of use. This increases the amount of protection, but it doesn't prevent overvoltages from traveling through I/O lines, network lines, and telephone lines and damaging equipment (see Fig. 1). That's why it's important to install surge suppressors in all paths that carry potentially disruptive transients.