Transcript Ch 6 Lightning Slides 042509

Chapter 6
Lightning Protection
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Overview
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Characteristics of Lightning
Principles of Protection
Precautions for Personnel
Precautions for Electronic Equipment
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Characteristics of Lightning
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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
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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 begins
the 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 induced
current in all metal items within its
influence
 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
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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
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Air Terminal
Discharge Conductor
Water Terminal
Bonding
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Air Terminal
• Also known as a Lightning Rod
• Traditionally ¼ 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 ½”
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Bonding (Chapter 2)
• Bonding is also for Lightning Protection
• Purpose – keep 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
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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
• Put a loop in cables
 Signal
 Power
• Disconnect ALL unnecessary equipment
before, NOT DURING a lightning storm
 All entertainment equipment
 Redundant communications and navigation
equipment
 Disconnect power cables, if accessible
 Disconnect and ground antennas
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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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After Direct Lightning Strike
• First, check crew – CPR Required?
• Next, check hull – repair any new leaks
• Then check navigation & safety equipment
 Radios with test call, or with handheld
 Magnetic compass
• Verify with GPS
• Deviation table may no longer be accurate
• If fixed VHF Radio or GPS inoperative
 Go to backup handheld unit
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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 equipment
 After lightning strike
• First check people
• Then hull
• May have to go to backup electronics
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