the standpoint of protection the most important value is lightning current
because it is passing through the hit object. That current is not constant.
It increases rapidly till the largest (peak) value. The time between the
beginning of this appearance and the peak current value is called forehead
The negative lightning hit produces current waves that can be relatively
different. During the first negative discharge the length of forehead is
equal 10-15 Ms. With the next discharge (if exists) the duration of forehead
becomes shorter. The peak value of current is smaller than in first discharge.
The positive lightning hits are usually made of one discharge which has
duration 0,1-0,2 s. The duration of forehead is relatively long and it's
value is somewhere between 20 and 50 ms and peak value of positive current
can grow even larger than 1000 kA.
PEAK VALUE OF LIGHTNING CURRENT
Peak value of lightning current is the most important value, because with
it we can calculate the drop of voltage that it makes passing through
some object on the earth: U=I*R. Because of that it is important for calculating
the protection from lightning strike. Of course we don't know in advance
what value of current we can expect so we can't construct the protection.
However we know the possibility of appearance of certain current of lightning
and that information we can use in calculating the protection. More about
that in chapter Probability of lightning strike.
STEP OF THE LIGHTNING CURRENT
Steep of the lightning current is actually the speed of achieving peak
value of lightning current. We calculate it by dividing the peak value
of current with duration of wave forehead. It is important to us because
these sudden changes of current in their nearness create changeable magnetic
field. The value of voltage that is induced on objects in that field directly
depends on the speed of variation of magnetic field.
Briefly: the greater the steeps of lightning current the grater are voltages
and current values on objects close to discharge, and they even don't
have to be directly hit by lightning strike. If voltage and electric current
do not damage those objects, electromagnetic forces that are inducted
by passing current still can damage them (If electric current passes through
two conductors, a force is created among them. Direction of force depends
on direction of electric current and its value depends on value of electric
current and the distance between conductors.)
During the calculation of lightning rod protection we have to think about
forces effecting the lightning rod. If they are too strong they could
damage lightning rod itself.
CHARGE OF LIGHTNING CURRENT
Charge of lightning current is the charge that is neutralized during one
strike. By itself that isn't so interesting information, but whole energy
freed during the strike depends on it. On that energy depends melting
of lightning rod or part of aluminum plate of plane.
That energy can be calculated as multiplication of charge and cathode
voltage drop: W=Q*U
SQUARE IMPULSE OF LIGHTNING CURRENT
That is a value that is magisterial for calculating the heating of lightning
rod installation, which conducts the current of lightning. By definition
it is calculated by formula:
where "R" is the resistance of conductor, "i"
is the value of lightning current and "t" is the time.