Air Breakdown

The charge redistribution associated with an ESD event usually, although not always, includes an air discharge because the event itself lasts only nanoseconds, but most physical movement of electronic products occurs on much longer time scales. It is, therefore, useful to discuss air breakdown.

If an increasing voltage is developed between two fixed spheres in air, a point will be reached beyond which the air ionizes between the two spheres, and a spark travels between them. The electric field strength at which this initial breakdown occurs is dependent upon several factors, including the humidity and motion of the air and the surface roughness of the spheres, but may be conservatively estimated at approximately 1 million volts per meter. (In most cases, it is somewhat less, but it varies greatly and this value is sufficiently accurate for the purposes at hand.) If the distance between the two spheres is one meter, it requires 1 million volts to jump the gap. Viewed another way, if there is a difference of 15 kV between the spheres, they must be separated by at least 15 mm to prevent a discharge.

Note that once ionization occurs, the path between the spheres requires much less voltage differential to conduct because the gases in the path have already been ionized. If a current source exists to continue the spark (discharge), an arc results. This is familiar to horror movie fans as the Jacob's ladder — the vertical, diverging wires with the arc rising across them. The voltage is set just high enough so that the spark can initially form at the bottom (because that is where the wires are closest). Because it is connected to a high voltage tranformer, it has a continuous supply of current, so a (continuous) arc forms. The arc rises because it heats the ionized channel of air through which it is passing. Eventually, the distance between the wires becomes too much to sustain the arc, and it stops; the process then repeats.^[5]

It is important to distinguish the concept of the spark, which is an electrostatic effect (i.e., it does not conduct DC current), from that of the arc, which is an electrodynamic effect (and does conduct a DC current).^[6]