Crowbar devices include gas tubes (also known as spark-gaps or gas-gaps) and semiconductor-based active crowbar protection circuits. Although these devices and circuits can shunt a substantial amount of transient energy, they are subject to power-follow problems. Once a gas tube or active crowbar protection circuit has fired, the normal line voltage and the transient voltage are shunted to ground. This power-follow current may open protective fuses or circuit breakers if a method of extinguishing the crowbar clamp is not provided. For example, if a short-duration impulse of a fraction of a millisecond triggers a crowbar device, the shunting action essentially would short-circuit the ac line to which it is connected for at least a half-cycle, and possibly longer. The transient created by short-circuiting the power conductor may be greater than the event that triggered it. The power-follow effect of a crowbar device is illustrated in Figure 15.4. The arc in most gas tube crowbar devices will extinguish at 20 to 30 V. It is fair to point out, how- ever, that crowbar devices are well-suited to low source current applications, such as telco and data lines.
The most common crowbar components include pellet-type and gas-discharge-type surge arrestors. These devices usually are capable of handling high currents because the voltage drop through them becomes very low when the units are conducting.
The selection of a crowbar device, the location(s) where it can be most effective, and applicable safety precautions require a solid understanding of surge voltages and the nature of traveling pulse waves. In general, crowbar devices are most appropriate where current in an inductive circuit element can be interrupted. They also may be warranted where long cables interconnect widely separated ac systems.
Characteristics of Arcs
Defined very broadly, an arc can be described as a discharge of positive ions and electrons between electrodes in air, vapor, or both, which has a potential drop at the cathode on the order of magnitude of the minimum ionizing potential of the air, vapor, or both. A unique characteristic of an arc is the negative resistance effect; increasing current reduces the arc resistance. Consequently, in a given circuit, the arc voltage drop remains fairly constant, regardless of the current magnitude. Larger currents reduce the impedance, but the voltage drop remains the same. Arcs extinguish themselves at the ac zero crossing, requiring a voltage much greater than the arc voltage drop to reignite.