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There are two major categories of transient suppressors: a) those that attenuate transients, thus preventing their propagation into the sensitive circuit; and b) those that divert transients away from sensitive loads and so limit the residual voltages. Attenuating a transient, that is, keeping it from propagating away from its source or keeping it from impinging on a sensitive load is accomplished with filters inserted in series within a circuit. The filter, generally of the low-pass type, attenuates the transient (high frequency) and allows the signal or power flow (low-frequency) to continue undisturbed. Diverting a transient can be accomplished with a voltage-clamping type device or with a “crowbar” type device. The designs of these two types, as well as their operation and application, are different enough to warrant a brief discussion of each in general terms. A more detailed description will follow later in this section. A voltage-clamping device is a component having a variable impedance depending on the current flowing through the device or on the voltage across its terminal. These devices exhibit a nonlinear impedance characteristic that is, Ohm’s law is applicable but the equation has a variable R. The variation of the impedance is monotonic; in other words, it does not contain discontinuities in contrast to the crowbar device, which exhibits a turn-on action.