radio grounding for lightning protection? (the “electrical safety ground” and an “rf ground”...
Post on 12-Jan-2016
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Radio Grounding for Lightning Protection?(The electrical safety ground and an RF ground are not effective as a lightning ground.)
What are we dealing with?50% -18,000+ amps
10% - 65,000+ amps
1% - 140,000+ amps
Voltages easily exceeding 100 KV+
Avg near 50 thunder-storms/yr MN-WIOften 2 or 3 strokes per strike
How does lightning strike?Charge in cloud ionizes area beneath Leader extends as air breaks down pathMultiple leaders inter-connect approaching opposite polarity of earth
As charge increases, upward seeking leader extends toward the approaching stepWhen leaders connect, huge currents flowPolarity usually positive
A lightning strike can really mess up your day . K0KFC
A lighting strike has predominant AC components, hence inductance (L) and impedance (Z) are primary considerations.Main components are 1- 2 Mhz and higher.
Start with Single Point Ground BarAll metallic chassis direct connected to SPGBIncludes transceivers, rotor controls, TV, telephone, racks, computer, everythingMust have low inductance (L) pathsAccomplishes common bondingPower cord 3rd wire ground of little value
Coax ProtectorsEach coax must have coaxial feed thru arrestor. DC continuity vs DC blockedGas tubes? (single vs multi-electrode)Protector placed as close to transceivers as possibleWell grounded at SPGB Wave Stub must be low Q to minimize ringing (Usually very little effectiveness)
Rotor Cables, Tel lines, Relay control boxesMOV (Metal Oxide Varistors)Well groundedClamp rotor lead during voltage riseMetal enclosureMOV must have short lead length
Establish Perimeter Ground FieldMinimum 8 foot rod length (1/2 or 5/8)Interconnect w/ #4 AWG minimum2 copper strap desirable (min L) Rod spacing X2 its length (16 min) Exothermic bonds most desirablePlan perimeter field w/duck foot radials
Ground Field (cont)Interconnected ground rods become energy dissipation systemHow many ground rods?Common bonds to electrical safety ground and metallic waterProvide low L path to direct energy away from the shackHow many ohms? XL? L? R?
TowersGuyed vs Self-Supporting?
Properly grounded guy wires place added low L paths in parallel with tower L resulting in overall lower impedanceLocate tower 20 minimum from shack to minimize EM flashoverTower leg joints not a concernUfer ground is useful as a part of the overall system (less effective when used alone)
Towers (cont)Be aware of corrosion issuesAll dis-similar connections isolated with stainless steel hardwareUse joint compounds (protective coatings)Ground ALL coax sheath at both top and bottom of towerBring coax off at very bottom of tower to minimize voltage divider effect
Towers, (cont)Orient copper strap broadside to tower to minimize mutual couplingNo sharp bends anywhere (>2)Avoid scratching off galvanized coatings
AC Power ConsiderationsSurge Protection (TVSS) at main entrance service panelTVSS should be physically closest to main breakers in panelInterconnect electrical ground with tower and perimeter ground fieldsRun power in grounded metallic conduit to shack to minimize inductive flashover
AC Power, (cont)Surge protectors on all devices at the operating deskMinimize AC cord lengths (Cut to length)NO cheapie plastic (K-mart specials) power strips (fire hazzard)Power strip must have robust metal enclosure to minimize fire hazzards
Two more observations..The arc/flashover resulting from an un-controlled medium sized lightning strike is hotter than the surface of the sun. Never operate when you can hear thunder or see lightning flashes. (Grounding is intended to protect equipment, not operator.)
References/Questions?www.polyphaser.comLightning Protection code NFPA 780www.harger.comwww. arrl.org/tis/info/lightning.html
A lightning strike can really mess up your dayA strike produces temperatures hotter than the surface of the sunTime limitations (20 min) so we will take quick look at important items.Happy to talk off line (in the hall) abut any .will try to save a couple of minutesfor questions a the endBetter if entire unit is mounted on copper sheet panelmin of .060 thicknessMov part numberwell grounded means #4 or 2 copper strapNote metal box (air conditioner disconnect box)Ordinary cinch-Jones barrier strip mounted on copper surfaceElectrical style ground bus on each sideMOVs are XXXXXXXXXXXXXXXXXXXXX#4 AWG ground wire (better if strap to MGB)Better if another foot deeper?Use exothermic??? $$$$Place removeable cover for future inspection if mechanical (yearly)
Bury ground wires in slitted earth minimum 6-10 inches or betterHomebrewed coaxial sheath pickoff straps.Bent from about .060 - .080 copper or brass stockUse brass or stainless hardwareProtect from moisture and resulting corrosion effects
Important for both aerial and buried power entrancesWill help control surges resulting from events distance away from home
Caution: cut the ground lead from the TVSS device to actual length needed. Electricians like coil it up and shove into the back.NO No!Rearrange existing breakers to allow for first position of TVSS