Grounding Review of Grounding Considerations and Options Rick Fletcher, W7YP FVARC November 22, 2017
Three Ground Types: Safety (electrical) ground Protects against shocks, burns and death Lightning ground Protects against damage and injury from cloud-to-ground lightning RF ground Required only by certain kinds of antennas where the RF ground replaces a missing part of the antenna: Most vertical antennas Ground rod with radials (half-wave vertical often only needs the ground rod)
Safety (Electrical) Ground As little as 5ma of current can kill you Make sure electrical system is up to code Use GFCI/AFCI protected-circuits Use 3-wire outlets and power cords Make sure all radio gear is connected to the electrical system s ground
Lightning What Are Its Characteristics? Complex AC surge with a typical frequency of between 20-500 KHz IEEE Standard Strike has an 8us rise time and a 20us fall time That s 50 to 125 Khz Average current is 18,000 amps for the first stroke and half that for the second and third strokes 10% will exceed 65,000 amps 1% will exceed 140,000 amps Average of 3-4 strokes It DOES strike the same place twice! NOTHING can be done to prevent a strike
Does Lightning Strike Down or Upwards? BOTH! The first stroke comes from the clouds down to the ground It begins with a bidirectional channel of ionized air called a leader This builds between the negatively charged clouds and the positively charged ground Leaders sometimes spit to form branches The return stroke (the one your eyes see) Once a negatively charged leader from the clouds attaches to a positively charged leader from the ground, a huge current will flow from the clouds to the ground This current will move through the ground very quickly, taking lower impedance paths over higher impedance routes These currents have gone up into a person s leg and out the other leg
Equipment Damage Power line surges are by far the greatest source of damage to equipment Surge sources Lightning this is the greatest threat Utilities (power company, phone and cable) Neighbors Your own equipment All radio gear should be behind good surge protectors Most inexpensive surge protectors use MOVs (Metal Oxide Varistors) Degrade over time under exposure to common smaller transients Tranzorb (reverse-paired Zener diodes) Gas Discharge Tubes (GDT)
Equipment Damage By Lightning Destruction from Direct lightning strike Exposure to the stroke channel Massive current flows Heating caused by the currents Electromagnetic pulse Electrostatic pulse Earth currents
Defensive Measures Give lightning a LOW IMPEDANCE path to ground Short, fat, straight Give it a HIGH IMPEDANCE path to your equipment! Long, thin, coiled YOU MUST HAVE A GOOD GROUND SYSTEM
Good Lightning Ground System Multiple 8 ground rods as near the station as possible Interconnect them with heavy tinned straps Connect all radio gear to the grounding system as the SAME point Do not daisy-chain Do not substitute cold water pipes or gas lines (Yikes!) At least one 8 ground rod per tower leg Keep paths as short as possible between tower legs and radio gear Use 6 gauge or larger wire or grounding straps If your soil has poor conductivity, use a larger number of ground rods Wet soil generally conducts better Unplug your equipment and disconnect coax lines going to them when lightning is expected
Protecting the Shack Have a single ground plate where the cables enter the house Use Lightning arrestors at the ground plate Alpha Delta Polyphaser Put lightning arrestors on control lines as well
Radio Grounded Antenna Switch Coax Surge Protector To antenna, coil coax once inside the house, ~90 deg corners, go DOWN just before the coax goes outside Power Supply Ground Buss Bar To ground, heavy gauge, no sharp turns, go DOWN, no coils of wire AC Surge Protector PC Ethernet Surge Protector To AC Supply
Out At The Tower Use lightning arrestors on coax feeding antennas on the tower Coil coax into loops after the arrestors MYTH: the cone of lightning protection
Ask AA4XX About Lightning Into The Shack
Ask AA4XX About Lightning Into The Shack
Forget to Disconnect One Radio?
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