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Lightning rods function...

Externet

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Hello all.
Are lightning rods on a dwelling roof meant to repel lightning strikes; or to attract lightning strikes conducting them o ground; or both ?

How important is the conductivity at those ultra-high voltages? A copper or iron or aluminium connection from the rod on a roof to ground; does it make much of a difference ? Seems there is a lot of speculative and empirical opinions on the subject. What about the intimacy and firmness, cleanliness or weathered corrosion of the connection from the rod to the conductor routed to the soil for grounding ?

The rod tips; always thought to be best if pointy; now a round ball opinions show up.
Light, please, with no strike ?
 

Easy peasy

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They are intended to carry the very high currents to Earth - so that they don't go thru the building and its associated wiring

Hence very capable Cu or AL conductors are required - ( size of your thumb at least ) - steel is not advised

usually down the outside of the building to a very secure Earth stake.

The top part is often a large onion shape with a pointy bit at the top - tests have shown this most efficacious for attracting the feelers that are sent out before a strike.

Don't install it when a storm is brewing ...
 

BradtheRad

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The hope is that a lightning bolt is more attracted to the grounded metal rod, in preference to a nearby chimney or tree or windmill, etc. The lightning rod ought to be higher than nearby objects.

It's no guarantee of protection since 'leaders' of ionized air can extend from any ground object toward a charged cloud overhead. Wherever leaders join in midair, is a channel for a lightning bolt to travel.
 

c_mitra

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The tip must of pointed. It helps even if lighting does not actually strike you (I mean the rod).

There will be induced and opposite voltage and the electric field at the pointed tip will be very high and coronal discharge will take place (whenever a charged could passes over). These coronal charges released from the tip move up and neutralize the charges on the cloud. In effect, a current passes even if no strike happens. This current is of the order of 10-100A but if a strike happens the current may be 10 times more (100-1000A). Some metal from the tip can get lost but usually not replaced unless there is an actual strike.
 
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Externet

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Thanks, gentlemen.
Obviously, being a conductive path from a higher point to ground, it conducts the lightning strike safely avoiding the building to suffer the effects of an immense current. But that is when the lightning strikes.

My interest is for all other non-striking times, the conducting rod presents a path to discharge to ground the buildup of any coronal charges of any polarity ionization in the air; by repelling or attracting or nulling or both or other effects to buildup of charges in the air vicinity ?

The alleged 45 degrees 'cone of protection', who/how was that calculated ?

Is there something that you find wrong in the following explanation, were the protection effect/function is more by repulsion than other ?
Am I interpreting it wrong; or should not apply to real life lightning phenomena ?

---->

(there is other video parts for the same dissertation)

Other words... Installing a lightning rod , in your opinion :
- increases the chance of lightning to strike the rod safely conducting the danger away from a building, or
- decreases the chance of lightning to strike the rod by repelling buildup of charges ?

The NASA launch pad surrounded with 3 lightning rods :

Lightning-protection-system.png


Does not seem effective all the time :

07_lightning_strikes_challenger.jpg
 

c_mitra

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Other words... Installing a lightning rod , in your opinion :
- increases the chance of lightning to strike the rod safely conducting the danger away from a building, or
- decreases the chance of lightning to strike the rod by repelling buildup of charges ?
The lightning rod helps neutralize the charges in the vicinity and reduces the chance of a lightning.

Even though the lightning did not strike, the rod carried 100s of As of current that reduced the electric field stress near the rod.
Simple comparison:
Consider a parallel plate capacitor with air as the dielectric.

If we introduce a sharp point, corona discharge will take place and the charges on the two plates will dissipate.

If the plates are very smooth, you will see arcing (there will be no corona discharge).

High voltage transmission lines have rounded edges of the conductors (where they are joined) to prevent corona.

That video is not entirely satisfactory because it leaves out several aspects.
 

WA9VEZ

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The Journal of the Franklin Institute seems to have the most comprehensive library of lightning research. For example, measurements done at the base of a tower in northern Italy, a researcher, inside a faraday cage, using a Tek oscilloscope and a precision resistor, measured the discharge current until a large bolt destroyed the resistor.

They have years of data including dart leader, stepped leaders, current channels, plasmas, rate of typical current rise, super bolts, enough to get lost for years.

 

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