lightning slides
TRANSCRIPT
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Chapter 6
Lightning Protection
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Overview
Characteristics of Lightning Principles of Protection
Precautions for Personnel
Precautions for Electronic Equipment
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Characteristics of Lightning
Static Electricity Ultra-High Voltage Generation
The Discharge
Surface Dispersion Basic Laws of Electricity
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Lightning
Static Electricity Separation and storage of electrical charge
A spark is an extremely small lightning discharge
Ultra-High Voltage Generation
Surface of earth is normally negatively charged
Top of storm clouds are positively charged
Forces nearby earth to become positively charged
Develops a multimillion-volt potential
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The Discharge
Clouds send negative charged leaders down
Earth sends positive leaders up
From grounded sharp metal objects
Conducting path when leaders meet
Unidirectional (DC) current flow
Voltage potential 100 to 1,000 million volts Current range 10,000 to 200,000 amperes
Duration from 1 microsecond to 1 second
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Anatomy of a Lightning Stroke
Electrons begin
zigzagging
downward in a
forked pattern.
This is the
stepped leader.As the stepped leader
nears the ground, it
draws a streamer of
positive charge upward.
As the leader and
streamer come together,
a powerful electrical
current begins flowing.
Current beginsthe return stroke,
an intense wave
of positive
charge traveling
upward about
60,000 miles per
second.
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Surface Dispersion
Surface dispersion
Main portion of lightning bolt penetrates earth
Spurs find far-reaching paths along surface
Surface dispersion is deadly
Stay out of the water
Keep away from trees
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Basic Laws or Electricity
Lightning creates magnetically inducedcurrent in all metal items within itsinfluence
The longer the wire, greater the current
The closer the strike, greater the current
Any impedance to current flow results in
Build up of high voltage at that location Resulting in arc-over to reach ground
Ignites flammable material
Vaporizes metal of insufficient cross-section
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Principles of Protection
Cone of Protection
Lightning Protection System
Power Boat Applications Sail Boat Applications
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Cone of Protection
Lightning rod protects areas within its cone
60 degree cone is 99% effective
45 degree cone is 99.9% effective
Less current flow from sharp pointed tip
More current flow from blunt or ball tip
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Lightning Protection System
Air Terminal
Discharge Conductor
Water Terminal Bonding
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Air Terminal
Also known as a Lightning Rod Traditionally inch copper rod
With sharpened point
Six inches above object to be protected
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Discharge Conductor
Not less than #4 AWG Uninsulated stranded copper wire
Straight from Air Terminal to Water Terminal No sharp bends (bend radius of 1 foot)
Should be run outside of hull
Electrical wiring should be at right angles
#4
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Water Terminal
Also known as a Ground Plate
External to hull Sailboats use metal keel
Area of one square foot
Not painted No water film between plate and hull
Use bedding compound
Size: 18 x 6 x
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Bonding (Chapter 2)
Bonding is also for Lightning Protection
Purposekeep all metal surfaces at zero potential To prevent electrical shock
To prevent stray current corrosion
To prevent induced potential from lightning strike
Bonding conductor Cross section of #6 AWG
Strap not less than #20 gauge (0.032 inch thick)
Normally #6 bare copper wire
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Powerboat Application
Fiberglass antennas provide NO protection
Add lightning rod on other side for protection e.g. grounded metal whip antenna
Ground the signal mast or Tuna Tower
Need a ground plate
Stay within the cone of protection
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Powerboat Application
Grounded metal whip provides protection
Grounded HF whip antenna with loading coil No protection above loading coil
Unless loading coil bypassed with large conductor
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Sailboat Application
Metal masts are grounded (bonded)Add discharge conductor to wood mast
Shrouds and stays grounded
Keel is ground plate
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Precautions for Personnel
Remain inside the boat Trust lightning protection system
Stay Out of the Water
Surface dispersion
Avoid contact with metal surfaces
Induced voltage
Handle only one metal control at a time
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Precautions for Electronics
Before a lightning storm
After a lightning strike
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Before Lightning Storm 2
If underway, keep operational One VHF radio
One GPS / chart plotter
Radar, if so equipped
Handhelds stored below
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Summary
Tremendous energy in lightning strike
Lightning protection Based on cone of protection Components
Air terminal Discharge conductor (#4 AWG stranded) Water terminal (1 sq ft)
Bond all metal above deck Before lightning storm
Disconnect all unnecessary equipmentAfter lightning strike
First check people Then hull
May have to go to backup electronics