[TowerTalk] Ground Rods

[Tom Rauch]

> Does it really matter to a multi-million volt lightning
strike whether
> the ground is 300 ohms or five?  The difference would seem
to be
> inconsequential.

If the current is 1mA, then the difference means nothing.

If the current is 100 amperes, then the difference in
voltage between your equiment and alternate paths like the
power line system would be 100*300 = 30 kV and 100*5=
500volts

I'll take the 500 volts any day, or better yet the couple
volts my ground systems provide at that current.

> This reminds me of the school of thought that says the
connection to
> your ground rods must be tight.  As if lightning, having
traveled
> thousands of feet through the air, is going to be stopped
by a gap of a
> few thousands of an inch.

What you will find is the lightning melts the wire at the
gap or poor connection, and often blows it apart. A good
example of how this works is a welding rod. The rod rapidly
melts at the arc-point, and even penetrates deep into the
target metal. At solid connections points, or if you stick
the rod to the metal, heating is minor even when
steady-state current is dozens of times higher than arc
current.

One of W4AN's yagis had a large hole through a heavy wall 3"
boom from a lightning hit on an element. The hole was there
because the element was insulated from the boom and there
was a short arc path between the element and the boom.

The #16 wires in my radial system take lightning hits all
the time without damage. At the same time a large 7/8th inch
coaxial cable had the shield totally blown away where it
crossed a copper flashing for the radial buss. Had that been
a solid bonded connection, there is little doubt the shield
would be intact.

You need a good ground and good connections. Make no mistake
about it. It doesn't stop the hit, it stops the voltage rise
and the power dissipated by reducing E*I . If E across a
junction is zero, it won't heat. If the ground resistance is
low, it stops much of the crap from getting into the
equipment in the house from tower hits. You do, however,
have to bond the power line ground directly to the station
ground with a very low impedance connection to prevent high
current in paths to and from the powerline from going
through all the devices in the house.

73 Tom