A surge protective device (SPD) is an electrical device designed to protect electrical equipment and systems from voltage spikes or surges caused by lightning strikes, power grid switching, or other sources of transient voltage.

SPDs work by diverting excess voltage to the ground, thus preventing it from reaching and damaging connected devices. They typically consist of metal oxide varistors (MOVs), gas discharge tubes (GDTs), or silicon avalanche diodes (SADs) connected between the electrical conductors and ground. When a surge occurs, these components quickly conduct the excess voltage to ground, providing protection to the connected equipment.