How does a SPD work? The design goal is to divert as much of the transient disturbance away from the load as possible. This is accomplished by shunting the energy to ground through a low impedance path such as a surge suppressor. Metal Oxide Varistors (MOVs) are the most reliable and proven technology to reduce transient voltages. Under normal conditions the MOV is a high impedance component. When subjected to a voltage surge the MOV will quickly become a low impedance path to divert surges away from loads. The MOV reaction time is nanoseconds – 1000 times faster than the incoming surge. In AC power applications, over 95% of SPDs use Metal Oxide Varistors (MOVs) because of their high-energy capability and reliable clamping performance. Silicon Avalanche Diodes (SADs) are frequently used in data line or communication surge protectors. SADs are not recommended for use in high exposure AC applications due to their limited energy capabilities. GDT devices are capable of handling large amounts of transient energy, but require high line impedance to ensure the GDT will turn off after the transient event has passed. For this reason, GDT devices are typically used in communication circuits and not AC power circuits.