Transcript Lightning protection

Antenna/Equipment
Protection against
catastrophic over-voltage
Lightning
Solar storm
Nuclear EMP
Laird Taylor N7RSR – 25Jun2012
Executive Summary
• Current (through your body) is what kills you
– But voltage is what pushes current through you
– Find a way to keep you & equipment @ same voltage
• Sharpness (small radius)
– Lightning easily enters/departs @edges & points
– Lightning reluctantly goes around sharp corners
• Ground conductors
– 1st instant, lightning travels only on surface
– Foil & straps have more surface per cross-section area
than do thick round wires, so ounce-for-ounce, they work
better as low-impedance conductors
Historical Analogy:
the Wheatstone Bridge
antenna
shack
3 cases of balanced bridge
• Canonical
1000
1
1
laboratory
2000
safety-oriented
0.1 outside 0.001
(you)
1
1
3000
6000
100 inside 1
Links/URLs to design effort
• The ARRL has much of this in public domain
– Lightning protection for the amateur radio station (1..3)
• http://www.arrl.org/files/file/Technology/tis/info/pdf/0206056.pdf
• http://www.arrl.org/files/file/Technology/tis/info/pdf/0207048.pdf
• http://www.arrl.org/files/file/Technology/tis/info/pdf/0208053.pdf
– Lab notes – lightning protection (1..2)
• http://www.arrl.org/files/file/Technology/tis/info/pdf/109481.pdf
• http://www.arrl.org/files/file/Technology/tis/info/pdf/129445.pdf
– Electromagnetic pulse and the radio amateur (1..4)
•
•
•
•
http://www.arrl.org/files/file/Technology/tis/info/pdf/88615.pdf
http://www.arrl.org/files/file/Technology/tis/info/pdf/98622.pdf
http://www.arrl.org/files/file/Technology/tis/info/pdf/108638.pdf
http://www.arrl.org/files/file/Technology/tis/info/pdf/118630.pdf
Know the enemy: Lightning
context of a C-G stroke
• Earth has a near-uniform vertical electric field
– ~350 V/meter
– Earth is negative – sky is positive
• A Thunderstorm is a giant static-electric machine
– The “shadow” of a T-storm may have ~10kV/meter field
– This isn’t enough (by ~10:1) to electrically break down air
– Russian scientists theorize: ion tracks from solar particles
“help” build a localized conductive bridge for the stroke
Know the enemy: Lightning
Parameters of a stroke
• Peak current
– Average: 18kA - 98% of strokes are in range 10..50
• Duration
– Average: 5 msec - 98% of strokes are in range 2..10
• Number of flashes/stroke
– Average: 4 – 98% of strokes are in range 1..11
• Time interval between flashes
– Average: 20 msec – 98% of strokes are in range [ ]
Know the Enemy: Lightning
Daily probability you’ll hear thunder during day
• Varies greatly across America
– There’s a national map from the US Government
– Ft Worth/Denton are at the ~50 days/yr contour,
averaged over decades.
• This year is wet, last year was a 100-year drought
January 2009 Utah County
ARES Training Item
• A "Window" in the world of radio is a way to bring our antenna cables into
our building such that the best lightning protection is offered.
•
• Lightning destroys our radios, power supplies and computers when it finds
its way through our equipment. The high current from a direct strike will
cause damage, even melting and fusing metal in our towers and cables.
– We can't do much to protect our exposed gear against a direct strike, but
– we can mitigate the path of those hyperactive electrons when they come
near. The idea is to direct them to the earth, or "ground", as soon as possible
and keep them from flowing into our equipment, where even a little current
or spike of voltage can destroy sensitive transistors and integrated circuits.
• A Window is the location where the cables enter into our structure. It is
at this Window, or point of entry, where we want to put our lightning
arrestors and protection devices. A key feature for a good lightning
protection scheme is to have a single electrical "Window" into the
building. The idea is to keep all of these electrical services at the same
electrical potential. If there is a differing potential, or voltage, then there
are currents flowing through conductors. Those damaging currents can
take an unexpected path.
1st example – before the strike
a copper pipe J-pole antenna on the peak of my house
• I run the cable through the vent in the gable, through the attic and then
down through spaces in the wall into my radio room in the
basement. Here I hook the antenna to the radio.
• The radio is also directly connected to a power supply that connects to the
mains power of my home. I ensure that all of my equipment is grounded
to the three prong electrical outlet. I also have my rig hooked up to my
computer for digital modes. The computer has connections to the
telephone and a home network.
• Now lightning strikes; it doesn't have to be a direct strike to the J-pole, it
may be a block away. The electrical field will be picked up by the antenna
(that's what antennas do, right?) and send current down through the coax,
through the radio and to ground through the power supply and computer.
• Much of this current stays on the ground wires, but since there is
resistance (and inductance) in every wire, there is voltage directed into
our electronic circuits; it may be direct or it may be induced. The likely
outcome is a dead
–
–
–
–
Radio
power supply
Computer
internet gateway and more. OUCH.
2nd example – after the strike
I have just bought a new rig and computer (ergo, I survived)
• Now I decide to make some changes.
– This time I route the coax from my antenna down the back of my house to the
location where the power meter is. This is the same place where the
telephone company brings in their lines.
– Here [there] is a ground rod that protects both the electrical service and
telephone entry. This is where I place my new lightning protection device and
tie it to ground with a flat braid or heavy solid copper wire (4" wide copper
works best). The coax now enters through the same "Window" as the mains
power and the telephone.
– I minimize the length from the grounds to each of these services to the ground
rod so that they have a "single point ground" that maintains the same
potential.
– If done right, when that next inevitable lightning strike happens, the path to
ground is NOT into my home and through my equipment. There will be a
potential on the cables entering through this common Window relative to the
earth but there will not be a potential, or it will be small, between the
electrical system and my radio gear. Thus the currents will be at a minimum
and the potential for damage is minimized. The voltage on all paths to the
equipment will rise and fall together.
• Hint: I may also consider taking the coax underground and bringing it up at
the Window I created. It is also good to enhance the ground and add
grounds as described in the articles below.