Chapter 6 Lightning Protection MElec-Ch6 - 1 Overview • • • • Characteristics of Lightning Principles of Protection Precautions for Personnel Precautions for Electronic Equipment MElec-Ch6 - 2 Characteristics of Lightning • • • • • Static Electricity Ultra-High Voltage Generation The Discharge Surface Dispersion Basic Laws of Electricity MElec-Ch6 - 3 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 MElec-Ch6 - 4 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 MElec-Ch6 - 5 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. MElec-Ch6 - 6 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 MElec-Ch6 - 7 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 MElec-Ch6 - 8 Principles of Protection • • • • Cone of Protection Lightning Protection System Power Boat Applications Sail Boat Applications MElec-Ch6 - 9 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 MElec-Ch6 - 10 Lightning Protection System • • • • Air Terminal Discharge Conductor Water Terminal Bonding MElec-Ch6 - 11 Air Terminal • Also known as a Lightning Rod • Traditionally ¼ inch copper rod With sharpened point Six inches above object to be protected MElec-Ch6 - 12 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 MElec-Ch6 - 13 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” MElec-Ch6 - 14 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 MElec-Ch6 - 15 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 MElec-Ch6 - 16 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 MElec-Ch6 - 17 Sailboat Application • • • • Metal masts are grounded (bonded) Add discharge conductor to wood mast Shrouds and stays grounded Keel is ground plate MElec-Ch6 - 18 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 MElec-Ch6 - 19 Precautions for Electronics • Before a lightning storm • After a lightning strike MElec-Ch6 - 20 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 MElec-Ch6 - 21 Before Lightning Storm 2 • If underway, keep operational One VHF radio One GPS / chart plotter Radar, if so equipped • Handhelds stored below MElec-Ch6 - 22 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 MElec-Ch6 - 23 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 MElec-Ch6 - 24