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Each piece of equipment in an electrical system is a potential source of transient voltages. At the same time, it also has some degree of susceptibility to transient voltages. This paper discusses the allowable limits of transient voltages generated by each piece of equipment and the total system, and the susceptibility of each piece of equipment and the system. For satisfactory operation of a system, the allowable limits of the generated transient voltages should be coordinated with the specified susceptibility of each piece of equipment as well as the system. The coordinated protection of an electrical system is the joint responsibility of the equipment designer and the systems engineer. Techniques to measure susceptibility are also described.