Lightning Arrestors [RE-wrenches]

[Joel Davidson]

Lightning results from a charge separation in cumulo-nimbus clouds, which cover large areas
and can extend to 40,000 feet. The mechanism is not well understood or agreed upon, but it
is known that a region of negative charge forms at the base of the cloud, and a region of
positive charge forms at the top.

One suggestion is that as water droplets freeze into ice crystals, and electric potential
of the reaction charges the small water particles positively. These are lighter than the
ice, which carries a negative charge. Air currents move the light water upwards and leave
the negatively charged region to form near the bottom of the cloud.

A charge difference builds up between earth and cloud until the insulating power of the air
in between breaks down. This occurs at about 5-10Kv/m.

A small corona discharge called a pilot streamer moves downward carrying a negative charge.
This is followed by a surge of current called a stepped leader. This leader moves about 50
meters in 1 microsecond than stops for about 50 microseconds, then takes another step and
stops, etc. Thus the leader moves toward earth in random steps at about 10 to the 5th power
to 10 to the 6th power meters per second.

When this leader is near earth, (within a few 100 meters) an upward streamer rises from the
positive charged earth's surface to meet it. The distance from the earth, or object on the
surface, to produce this upward streamer is called the "striking distance" and it depends
on the strength of the downward stepped leader. The more current carried downwards, the
greater the striking distance.

When the two streamers connect somewhere near above the earth or object, a huge discharge
of positive current flows upward to the cloud from the earth to neutralize the charge
imbalance. This is the return stroke and is the main flash and cause of thunder associated
with lightning. It travels about 10 to 100 times faster than the downward leader.

After about 10 to 30 microseconds, another downward leader leaves the cloud and travels
down the ionized path. When it reaches earth, another return stroke discharges upward. As
many as 30 return strokes can occur in one "flash'.

The current in the return stroke rises very quickly to a very high value and then decays.
Peak currents of 30kA (but as high as 200kA) occur within usually 5 to 10 microseconds,
while average currents over the life of the stroke are usually 10kA. The rate of current
rise ranges from 5 to 3000kA/microsecond. It is these rapid current rises and their high
value that makes lightning so formidable a power to contend with.

The temperature of a bolt can be 30,000 degrees C. causing melting of metal conductors if
they are not large enough. High current peaks may bend or tear metal due to electromagnetic
forces proportional to the square of the current. Therefore, one must securely fasten all
metal components of a lightning protection system. Catastrophic damage and explosions may
occur to non-conductors (wood, brick, concrete, masonry) due to the instantaneous
vaporization of entrapped water.

A direct strike of lightning to a conductor to ground will allow tremendous current to flow
for a short time in the conductor. If a 30 meter run of copper wire (0.35 inches in
diameter) carries a current of 20kA, it will develop an IR drop of only 173 volts, not a
serious threat to insulation. However, the rapid rise time of the current results in an
inductive voltage of over one million volts. This is enough to cause insulation to break
down, resulting in a "flashover" or discharge to nearby conductors or personnel. Routing
normal down conductor around corners for aesthetic purposes greatly increases the chance od
side flashing. For these reasons, a coaxial cable, with outer shield properly connected to
ground, is advised.

Besides flashover, these rapid rise times cause electro-magnetically induced voltage surges
to be created in nearby conductors. These surges may be induced by currents flowing through
a down conductor or structural member, or stroke to earth in the vicinity of buried cables,
or cloud to cloud discharges occurring in parallel to long cable runs above or below
ground.

(Wowl. Don't you wish you'd said this. Me too. It's from the 1986 Arco Solar training
manual. There's much more, but perhaps this is enough to inspire someone to dig out their
old manual and scan it with character recognition software.)

"Todd Cory, Bald Mtn. Solar" wrote:

> List members,
>
> I saw something on TV (TLC?) showing high a speed film of a lightning strike.
> Interestingly enough the bottom part of the strike actually started at the ground and
> went UP to meet the downward arc, so as I understand it the grounding is not so much to
> absorb and dissipate a direct strike (although it does that too) but more to reduce any
> attractive charge level so an arc would not start up to meet the lightning bolt in the
> first place. Kind of as a preventative measure. Anyone have comments on this?
>
> also:
>
> Doug Pratt wrote:
>
> > 3. Multiple ground rods in a single system are fine, but they MUST all be connected
> > together.  If those connections are bare #8 wire in direct contact with the ground
> > (not in conduit), then it's even easier for the lightning energy to dissipate.
> >   So put in a ground rod at your PV array, put in a ground rod at the base of your
> > wind tower, AND at each guy wire anchor, but HOOK THEM ALL TOGETHER with buried
> > wire!
>
> Sometimes this is not practical when running long 150+' runs in from the PV/ wind
> gennys. Are you saying you want 150' of #8 or #4 bare copper in the trench with the RE
> delivery wires?
>
> Todd
>
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