UL and IEEE define three types of SPDs, and they are either type 1, 2, 3, or A, B, C to help categorize where the SPD can be installed. Type 1 SPDs are typically mounted on the line side of the main service entrance. Type 2 SPDs are installed inside a facility and protect the load side of the main service entrance and branch panels. Type 3 SPDs should be installed as close to the equipment they are trying to protect as possible. In contrast, IEC defines two classes of SPDs to identify how much protection they provide. Class 1 SPDs are primarily used for protection from surges due to lightning strikes and are characterized by a 10/350 µs current waveform. Class 2 SPDs are designed for protection from indirect lightning strikes and characterized by the 8/20 µs current waveform.