The Complete Handbook of Surge Protection Devices

Chapter 1: Understanding Surge Protection Devices

Every piece of electronics you own, from a $4 microcontroller to a $40,000 CNC spindle drive, is exposed to electrical surges. A surge protection device (SPD) is a component designed to limit the duration and magnitude of a transient voltage event. It is not a power conditioners, not a battery backup, and not a magic filter. It is a one-trick device with one job: when the voltage exceeds a safe threshold, it creates a low-impedance path to shunt the excess energy away from the protected equipment and to ground. When the surge ends, it resets and continues passing normal power.

To understand why you need an SPD, you need to understand what a surge is. In an ideal AC power system, the voltage in a 120V circuit alternates sinusoidally between +170V and -170V (the RMS value is 120V, but the peak is higher). In a 230V system, the peak is about 325V. Voltage stays within a few percent of these nominal values. A surge is any event where the voltage exceeds this expected envelope, usually for a duration measured in microseconds to milliseconds. A swells that lasts for hundreds of milliseconds is considered a "swell" or "overvoltage," but a surge is specifically the short-duration transient event. Most SPDs are rated to respond to events with rise times in the nanoseconds range and durations up to a few milliseconds.

The classic surge event that everyone knows is a nearby lightning strike. A direct strike to