Electrical surges and spikes cause damage and premature failure of machines, components, and power-distribution systems as well as disrupt signals in industrial control systems. External lightning strikes and disruptions in utility power can send high energy transients into a facility. Internally, the switching of lighting circuits, contactors, and other devices can cause fatigue of components or systems over time. Proper mitigation of these anomalies is imperative to maintaining a robust and reliable power system. UL 1449 sets safety and performance standards for surge protective devices (SPDs), which are identified by Type 1, 2, 3, or 4 depending on where the SPD is incorporated within the power distribution system.