surge protective device (SPD)

Surge protective devices (SPD) are used in lightning protection systems to discharge lightning currents. They provide lightning equipotential bonding for live lines and discharge lightning currents to ground.

Since surge protection devices discharge all lightning currents from all lines, corresponding devices must be able to discharge surge currents of 50 kA and more to ground. Surge protection devices are divided into three categories in EN 61643. The first category, SPD Type 1, is concerned with the introduction of lines into the outer lightning protection zone. This type dissipates lightning, but can result in an overvoltage that can damage sensitive electronic equipment. InSPD Type 2, the remaining overvoltage is reduced so that damage to electronic equipment can occur only in exceptional cases. SPD Type 3 provides further reduction of the overvoltage to a voltage level that precludes damage to electronic equipment.