"Wrightsaerials" <wrightsaerials@aol.comnojunk> wrote in message

[slightly reformatted]
> >>I've told this story before on here, but anyway -
> >>Years ago we had a customer who reached behind the telly to unplug the
> >> aerial just at the moment lightning hit the tree just down the road.
> >> She was
> >>thrown from the front of the house to the back, and was found
> >>unconscious under the kitchen sink with a broken arm. There was a track
> >> across the wall betwen the aerial downlead and the nearby mains outlet.

> >Interesting. What do you think the path of the strike was?
>
> Maybe the shock she received was the result of induced voltage in the
> aerial downlead (EMP as they call it when it comes from a nuclear bomb).

Yes a potential difference but unlikely to be EMP, the dynamics
are much slower in the period building up to a strike when I think
the above conditions occured.

This requires length explanation:

The conditions for a lightning occurs when friction between moving
layers of atmosphere, strip electrons from atoms in the top layer.
(ie a cold layer of air pushes under a warm moist layer forcing it
upwards. The warm air cools rapidly and dumps its moisture as rain)
Static electricity. The electrons are grounded and concentrate in the
ground immediately below the mass of electron deficient atoms above
as they seek to return*. Electron deficient atoms are positively charged
whilst electrons are negatively charged. As the friction continues the
potential difference across the intervening layer of air increases to a
point when breakdown of its insulation occurs. (sheet lightning occurs
when the electrons are not grounded but in a separate area of sky).

What happens when the actual strike occurs is that the bits of air
underneath the positive area is ionised by the stress. 'Feelers'
seek downwards through weaker areas like cracks spread in
a weak dam (more polluted, wettter, more particulates, whatever?)
Once a complete path (crack?) downwards has been established the
electrons will rush upwards through it superheating the air, causing an
electromagnetic pulse and a bang. Note: electricity flows from negative
to positive (it is only considered to flow in the opposite direction for
historic legacy reason) Thus, though the feelers work downward (the
glowing streaks you see before the actual strike), lightning itself
strikes upwards.

Of course an awful lot of things are happening in these
circumstance making precise explanations of consequences hard
to analyse and somewhat speculative.

An electrical lightning conductor, if placed high enough (on a
steeple say) acts more to prevent a strike in its vicinity than attract
one. Electrons flow upwards through the conductor (as there is
a potential difference or electrical field between the ground and the
steeple top) and leech or leak into the atmosphere thus inhibiting the
potential difference in the atmosphere from rising high enough to
cause a breakdown, though this would depend on how fast the
potential is rising. Under these circumstance there will be a heavy
flow up through the conductor, which is why heavy straps are used
to prevent meltdown.

A roof aerial will perform a similar function to a lightning
conductor but in a more limited sense as it is much lower
and the cable is much thinner. But certainly an aerial is
still much higher than the ground and there will be a potential
difference (possibly millions of  V's if directly under the area
of greatest stress) between the top and the  bottom causing a
small (or large) current to flow, especially if the cable has a
reasonably good ground path through a TV. If the  base of the
cable has no such path, unplugged and dangling say,  then no
current can flow through it and the (+ve) potential at its base
may be the same as that at the top. Clearly there will be a much
shorter path between the base of the cable and a electron rich
ground point (-ve) than there is between the roof and the ground,
short enough perhaps for a breakdown to occur across the gap.
Thus there may be a flash from the dangling lead to a power socket
even though there is no general lightning strike at the time.
Touching such a cable (ie unplugging it) during a period building
up to a strike, could be dangerous as the person concerned
would make a good enough ground path across the high voltage.

The moral: Keep away from aerial leads during thunderstorms
especially if not reasonably grounded. Rather a burnt out earthed
cable than a dangerous one with a high voltage on it. Less
likelihood of injury and more work for aerial riggers.

*Cows please note: there will be a potential difference
in the ground radiating outwards from the concentration
of electrons, you are advised to stand on one leg during
storms.

-- 
Les Hellawell

Greetings from :
YORKSHIRE - The White Rose County