A surge protective device (SPD) is designed to protect electrical systems and equipment from surge events by limiting transient voltages and diverting surge currents.

Surges can originate externally, most intensely by lightning, or internally by the switching of electrical loads. The sources of these internal surges, which account for 65% of all transients, can include loads turning on and off, relays and/or breakers operating, heating systems, motors and office equipment.

Without the appropriate SPD, transient events can harm electronic equipment and cause costly downtime. The importance of these devices in electrical protection is undeniable, but how do these devices actually work? And what components and factors are central to their performance?

How Does a SPD Work?

In the most basic sense, when a transient voltage occurs on the protected circuit, an SPD limits the transient voltage and diverts the current back to its source or ground.

To work, there must be at least one non-linear component of the SPD, which under different conditions transitions between a high and low impedance state.

At normal operating voltages, the SPDs are in a high-impedance state and do not affect the system. When a transient voltage occurs on the circuit, the SPD moves into a state of conduction (or low impedance) and diverts the surge current back to its source or ground. This limits or clamps the voltage to a safer level. After the transient is diverted, the SPD automatically resets back to its high-impedance state.