In summary, Littelfuse's SPDs are designed for equipment protection from transient overvoltage events. SPDs look to limit the overvoltage surges that can occur in sub-distribution boards where sensitive equipment is stored. These devices are offered in multiple pole versions for all types of distribution systems. SPDs reduce surge voltages to a level that the power distribution system and the equipment that is connected to the system can handle. SPDs protect the programmable logic controller in the equipment that they are building: power supplies, the step-down transformer, variable-frequency drives, and the input/output devices. SPDs are primarily rated by how much of a surge current magnitude they can handle and how well they limit the voltage when conducting that surge current. They are not one-size-fits-all devices, nor do the size of their kA rating (the surge rating) go hand-in-hand with the size of the panel. In other words, do not select an SPD with a maximum kA rating just because the panel is large.