Lightning Arrestors [RE-wrenches]

[Chuck Heath, SunPower]

Doug Pratt writes:
> Here's what 15 years in the renewable energy biz has
>taught me about lightning:
Good advice, Doug!

Let me add what broadcast engineers often do at FM/TV transmitter sites:

The coaxial hard-line is grounded to the tower at several points using 
stainless steel clamps as it nears the bottom of the tower. This is 
accomplished by cutting the outer plastic insulation to bond the clamp to 
the solid copper outer shell of the line. If the transmission line is 
semi-flexible, they put in one or two large loops (typically 3-4 ft. in 
diameter for 7/8 in. Heliax) where it turns 90 degrees from the tower to 
the "ice bridge" leading into the transmitter building adjacent to the 
tower.

Since lightning prefers to go in a straight line, this minimizes the 
amount of lightning energy brought into the building. 

Reminds me of the story in HP years ago about the guy who thought putting 
in a kWH meter would solve his lightning problem. He pulled the meter 
when he thought strikes were likely, only to learn the downstrikes easily 
jumped the knife blades in the meter box.

In our RE business, this could mean using shielded output cable from a 
wind genny, then cutting the vinyl insulation about every 2 ft. at a 
dozen or so places in the first 25 ft. up the tower, using stainless 
steel clamps to bond to the shielded cable.

Equally important is using 4 in. wide copper strap extending out from the 
base of the tower several feet (in AM radio, this ground system is needed 
to transmit an effective signal, hence the "best" ground is salt water). 
At the outer end of each ground radial, there is a copper plated ground 
rod. Each of these are interconnected by bare copper wire, forming a 
perimeter ring around the tower. The same concept is often done to the 
transmitter building itself, with bare ground going over the roof and 
down to a perimeter ground surrounding the building, with ground rods 
connected at all corners.

Another popular product with telecom/broadcasters is the Cartana 
Stati-Cat. This piece of hardware sits at the very top of the tower, 
typically on a galvanized pipe rising 8-10 ft. above the top of the 
tower. It's all stainless steel and sort of looks like a porcupine.
     http://mwpersons.com/Cortana.html  (thanks, Sparky, for this url 
last March)

And yes, Polyphaser <www.polyphaser.com> is the first choice of lightning 
protection among broadcasters, but as others have noted, not inexpensive.

My own experience living 12 years on a Sierra mountaintop next to two 140 
ft. commercial broadcast towers with this protection was they never took 
a hit, but a 90 ft. dead cedar tree less than 100 ft. away was struck 3 
times.

Chuck Heath _now serving California's North Coast_

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