Open-Flame Arc

an electric arc that burns freely in air and is not subjected to any special action that speeds up the process of its extinction. The energy balance of an open-flame arc depends primarily on the natural heat exchange between the arc path and the environment. The fundamental characteristics of both DC and AC open-flame arcs are the static current-voltage characteristic (for AC, the amplitude current-voltage characteristic), the length of the arc, and the burning time. An open-flame arc is extinguished spontaneously when its length increases—and the current magnitude decreases—to critical values, such that the energy balance in the path of the arc becomes negative.

An open-flame arc occurs when low currents are cut off by switching apparatus that has no arc arresters (for example, by circuit breakers in high-voltage installations), and also in cases of short circuits on power switchboards and aerial power lines. When low currents are cut off in low-voltage and high-voltage circuits, the open-flame arc is extinguished spontaneously. In case of a short circuit, an open-flame arc is usually extinguished spontaneously only when one phase of a power line in a system having an insulated neutral is short-circuited to ground. In other cases, the open-flame arc is extinguished by disconnecting the site of the short circuit from the source of power, often in combination with an automatic reclosing.