Lightning phenomena (continued)

Stroke formation 

Types of strokes

There are a number of different types of lightning strokes. These include strokes within clouds, strokes between separate clouds, strokes to tall structures, and strokes that terminate on the ground. The positive and negative strokes terminating on the ground are the types of most interest in designing shielding systems and the following discussion will be confined to those types.

Stepped leaders

The actual stroke development occurs in a two-step process.  The first step is ionization of the air surrounding the charge center and the development of stepped leaders, which propagate charge from the cloud into the air. Current magnitudes associated with stepped leaders are small (in the order of 100 A) in comparison with the final stroke current.  The stepped leaders progress in a random direction in discrete steps from 10 to 80 m in length. Their most frequent velocity of propagation is about 0.05% the speed of light, or approximately 150 000 m/s. This produces electric fields near ground with rise times on the order of 100 to 500 microseconds. Electric fields of 250 microseconds from switching surge overvoltages tend to produce the minimum electrical strength of large air gaps compared to 1.2/50 microsecond lightning overvoltages. It is not until the stepped leader is within striking distance of the point to be struck that the leader is positively diverted toward this point. Striking distance is the length of the last step of leader under the influence of attraction toward the point of opposite polarity to be struck.  

Return stroke

The  second  step  in  the  development  of  a  lightning  stroke  is  the  return  stroke.  The return stroke is the extremely bright streamer that propagates upward from the earth to the cloud following the same path as the main channel of the downward stepped leader. This return stroke is the actual flow of stroke current that has a median value of about 24 000 A and is actually the flow of charge from earth (flat ground) to cloud to neutralize the charge center. The velocity of the return stroke propagation is lower than the speed of light and varies with atmospheric conditions; an approximate value can be 10% of the speed of light. The  amount  of  charge  (usually  negative)  descending  to  the  earth  from  the  cloud  is  equal  to  the  charge (usually positive) that flows upward from the earth. Since the propagation velocity of the return stroke is so much greater than the propagation velocity of the stepped leader, the return stroke exhibits a much larger current flow (rate of charge movement). The various stages of a stroke development are shown in Figure 2.

Approximately 55% of all lightning flashes consist of multiple strokes that traverse the same path formed by the initial stroke. The leaders of subsequent strokes have a propagation velocity much greater than that of  the  initial  stroke  (approximately  3%  the  speed  of  light)  and  are  referenced  as  "dart  leaders".

Figure: 02