TR 61400-24 IEC:2002(E) – 13 –
these basic types are referred to as “cloud-to-ground flash” or “downward flash” and “ground-
to-cloud flash” or “upward (initiated) flash”, respectively.
Both types of lightning are further sub-divided according to the polarity of the charge removed
from the thundercloud. A negative discharge lowers negative charge from the thundercloud to
the earth. A positive discharge results in positive charge being transferred from the
thundercloud to the earth. The majority of lightning discharges are negative, making up about
90 % of all cloud-to-ground flashes. Positive discharges make up the remaining 10 % of all
cloud-to-ground flashes. Normally, the latter exhibits higher electrical parameters.
Each lightning stroke is different due to the natural variations in the thundercloud that
produced it. For example, it is not possible to predict that the next lightning stroke to a
particular structure will have a peak current of a given value. What can be said is that the
structure has a given probability of being struck by a lightning stroke exceeding a certain
value.
Probability distributions of the electrical parameters that are used to describe a lightning
stroke have been produced using direct measurements of actual strokes to tall towers
[33] [34]. Further information is now becoming available worldwide from regional and national
lightning location systems. These can record the location of a lightning stroke and estimate
the peak current.
The probability distributions that describe the electrical parameters of a lightning stroke are
different for each type of lightning (upward/downward and positive/negative). The appropriate
probability distributions are described below along with the typical wave shape of each type of
discharge. The probability level given indicates the probability of the specified electrical
parameter exceeding the tabulated value during a lightning stroke. Empirical methods to
estimate the probability of the electrical parameters exceeding a specific value exist [4].
3.3 Cloud-to-ground flashes
A cloud-to-ground flash (downward initiated discharge) is initially formed by a preliminary
breakdown within the cloud. The physics of this process are not fully understood at this time.
The parts of the discharge process taking place below cloud level are much better known.
3.3.1 Negative cloud-to-ground flashes
In the case of a negative flash, a stepped leader descends from the cloud towards the
ground in steps of several tens of meters with a pause time between the individual steps of
approximately 50 µs. The steps have short duration (typical 1 µs) impulse currents of more
than 1 kA. The leader channel contains, when fully developed, a total charge of about 10 C, or
more. The channel diameter is in the range of up to a few tens of metres. The total duration of
the stepped leader process is a few tens of milliseconds. The faint leader channel is not
visible to the naked eye.
The end of the leader, the leader tip, is at a potential in excess of 10 MV with respect to the
earth. As the leader tip approaches the earth this high potential raises the electric field
strength at the surface of the earth. When the electric field at ground level exceeds the
breakdown value of air “answering” (upward moving) leaders are emitted from the earth or
from structures on the ground. These upward moving leaders are commonly termed
connecting leaders. Connecting leaders play an important role in determining the attachment
point of a lightning flash to an object.