John Woodgate wrote:
In message <[email protected]>
<mailto:[email protected]> ,
dated Fri, 6 Nov 2009, Mick Maytum on Gmail <[email protected]>
<mailto:[email protected]> writes:
Also please be careful when talking about lightning energy -
this can lead
to dreadful statements like "The lightning protection system diverts the
lightning energy to earth". It doesn't, current gets diverted to ground and
the amount of energy in the earth depends on the earth potential rise -
something the lightning doesn't have control over.
Where does the energy go, if not into the mass of the planet?
Nothing to do with energy being moved but charge moving.
The initial potential difference between the cloud charge region and ground is
estimated to be between 50 MV and 500 MV (Rakov & Uman). Any voltage developed
by a lightning struck object is going to be insignificant compared the cloud
ground voltage. Thus the cloud to ground discharge can be thought of as a
current source. A lot of the discharge energy isn't developed in the struck
object (Rakov & Uman reckon values of some 50 GJ occur)
There is a long-duration low-level lightning current that is effectively a
rectangular current pulse - this makes the maths simple. The typical
parameters for this (IEC 61312-1) are Q = 150 C, Td = 0.5 s and I = 300 A.
Assume the LPS managed to divert all this current to earth. If the earthing
resistance was 1 ohm a voltage of 300 V would be developed, making a power of
90 kW for 0.5 s and a ground energy of 45 kJ. Now a 1 ohm ground resistance is
exception. If I had used 10 ohms everything would have been 10 times bigger -
then I would have to have brought in HV soil ionisation effects which lowers
the effective earth resistance.
Thus think current and the voltage the object develops — not energy. (The
closest the lightning people get to energy is something called the current
action integral, which you and I know as i-squared-t. This factor is useful
for wire fusing)
Once the lightning current enters a network of multiple paths then the surge
source impedance becomes that of the network, but that's another story!
Mick Maytum
UK
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