lightning rod question

[izzi4]

Hello, I was wondering why there is a ball on lighting rods? example
http://www.period1.com/lightni.jpg I was just wondering what the purpose of
the ball is, I know that the point creates a concentrated electric field at
the tip but i can't imagine the balls purpose. Thanks in advance

 

[Bert Hickman]

Hello, I was wondering why there is a ball on lighting rods? example
http://www.period1.com/lightni.jpg I was just wondering what the purpose of
the ball is, I know that the point creates a concentrated electric field at
the tip but i can't imagine the balls purpose. Thanks in advance

Decoration. Most of these balls are colored glass.

-- Bert --

 

[Uncle Al]

Hello, I was wondering why there is a ball on lighting rods? example
http://www.period1.com/lightni.jpg I was just wondering what the purpose of
the ball is, I know that the point creates a concentrated electric field at
the tip but i can't imagine the balls purpose. Thanks in advance

A metal rod sticking up into the air is both a silly thing and nearly
indestructible. By adding apical decoration it is transformed into an
asethetic statement, the price goes up, and there is something to be
damaged and repaired/replaced.

 

[Sam Wormley]

Hello, I was wondering why there is a ball on lighting rods? example
http://www.period1.com/lightni.jpg I was just wondering what the purpose of
the ball is, I know that the point creates a concentrated electric field at
the tip but i can't imagine the balls purpose. Thanks in advance

Oh the irony--you're standing in the front yard when the lightning strikes
the lightning rod... potentially saving your life. The thermal shock
of the strike fractures the glass ball, half hurled striking your head
killing you! Oh the irony.

 

[Don Kelly]

It seems there is a metal one in this picture which has some arcing under
it, Its hard to believe they serve no purpose.
http://www.francetechnologie.com/cie-pouyet/images/img11.jpg

Still decoration. At one time (early 30's or late 20's ) there were rods
made with a ball at the top based on some half baked idea that came into
vogue (to raise the price, probably) that it would provide better shielding.
To sum up- it didn't work. There have been, are, and will be a variety of
devices made and sold to provide lightning shielding. Typically they don't
do as well as simply sticking a bunch of old bedspring on the roof and
grounding them well - but they are shinier and more expensive.

 

[Vermin]

It seems there is a metal one in this picture which has some arcing under
it, Its hard to believe they serve no purpose.
http://www.francetechnologie.com/cie-pouyet/images/img11.jpg

That would appear to be an "early streamer" type lightning rod. i.e.
it is supposed to attract lightning better than a Franklin rod, by
some means of ionizing the air around it.

There is a lot of debate as to the effectiveness of these devices, in
my opinion they are no better than snake oil and the Australian
standard at least is unsupportive of them (despite appreciable
pressure from manufacturers of these devices).

V.

 

[Doug]

Oh the irony--you're standing in the front yard when the lightning
strikes the lightning rod... potentially saving your life. The thermal
shock of the strike fractures the glass ball, half hurled striking your
head killing you! Oh the irony.

If you get killed by this sequence than it was definitely your time to go!

 

[rayjking]

Hi,

The ball is usefull in spreading heat from a direct strike. A #8 wire can
carry any known strike but it will bern at the point of the strike without
more thermal mass.

Ray

 

[Gordon Youd]

What's lightning, life just gets darker with age.

----------------------------------------------------------

 

[izzi4]

wouldn't the head produced be caused by the current through the wire, or is
there another method of heating in lightning strikes i'm not aware of?
Wouldn't this spread the heat through the region of the ball but not farther
along the path of the wire? I hadn't considered the heat involved before,
intresting.

 

[rayjking]

The heat generated is much more in the air above the wire. The power
generated causes the vapor in the air to turn to steam
and the rapid expansion of the steam creates the thunder.

Ray

 

[Bert Hickman]

The heat generated is much more in the air above the wire. The power
generated causes the vapor in the air to turn to steam
and the rapid expansion of the steam creates the thunder.

Ray

Not exactly... thunder does not require the presence of water in any
form. Like all sparks, the rapid expansion of the spark channel
creates a shock wave that ultimately creates thunder. For lightning,
the exact mechanisms are subject to some debate, but thunder simply
does not require the presence of water in either vapor or liquid form.

-- Bert --
--

 

[Tom Biasi]

Hi,

The ball is usefull in spreading heat from a direct strike. A #8 wire can
carry any known strike but it will bern at the point of the strike without
more thermal mass.

Ray

I didn't catch the original post but I thought that this may be a good time
to mention that lightning rods are/were not designed for direct hits. The
purpose is to keep the accumulated charge below a safe level.
Tom

 

[Don Kelly]

I didn't catch the original post but I thought that this may be a good time
to mention that lightning rods are/were not designed for direct hits. The
purpose is to keep the accumulated charge below a safe level.
Tom

---------------
Actually this is extremely questionable. Design that I know of is based on
the probability of the stroke striking the rod rather than the protected
area. Granted, in some situations, particularly with tall structures, such
draining does occur (possibly increasing the chance of side flashes-another
problem) but this is a bonus, not the basis for design.
I am aware of a purveyor of lightning protection that claimed that his
protection works on the basis of charge dissipation. I have seen no evidence
that it does. I have no idea if he is still is in business.

Generally the source of the charge is several miles overhead -the rod
doesn't get seen by it. When a leader gets near a rod then it may be a
preferred target for the next step and if it is, then the main stroke will
be to the rod-if not something else gets hit. Design is based on it being
such a target for higher current strokes (but not necessarily lower current
strokes). This is true for protective systems for transmission lines as
well as structures. Catch the damaging strokes know that some of the little
ones will get by.

References:
Moussa & Shrinivasta, "Shielding of Tall Structures Against Direct Lightning
Strokes" Canadian Conference on Electrical and Computer Engineering,
Vancouver, BC, 1988
Energy Systems Journal, Vol. 11, N0.1 1991
EPRI Transmission Reference Book, 345KV and Above.
I believe that IEEE changed its standards to reflect the concepts involved
in these references.
Sorry that I do not have more recent references but I have been retired for
some time.

I agree with Vermin on the lack of usefulness of the ball. There appears to
be no rational basis for its use. Note that its thermal mass is generally
pretty small and by the time it starts to dissipate heat, any damage is
done.

 

[Tom Biasi]

---------------
Actually this is extremely questionable. Design that I know of is based on
the probability of the stroke striking the rod rather than the protected
area. Granted, in some situations, particularly with tall structures, such
draining does occur (possibly increasing the chance of side
flashes-another
problem) but this is a bonus, not the basis for design.
I am aware of a purveyor of lightning protection that claimed that his
protection works on the basis of charge dissipation. I have seen no
evidence
that it does. I have no idea if he is still is in business.

Generally the source of the charge is several miles overhead -the rod
doesn't get seen by it. When a leader gets near a rod then it may be a
preferred target for the next step and if it is, then the main stroke
will
be to the rod-if not something else gets hit. Design is based on it being
such a target for higher current strokes (but not necessarily lower
current
strokes). This is true for protective systems for transmission lines as
well as structures. Catch the damaging strokes know that some of the
little
ones will get by.

References:
Moussa & Shrinivasta, "Shielding of Tall Structures Against Direct
Lightning
Strokes" Canadian Conference on Electrical and Computer Engineering,
Vancouver, BC, 1988
Energy Systems Journal, Vol. 11, N0.1 1991
EPRI Transmission Reference Book, 345KV and Above.
I believe that IEEE changed its standards to reflect the concepts involved
in these references.
Sorry that I do not have more recent references but I have been retired
for
some time.

I agree with Vermin on the lack of usefulness of the ball. There appears
to
be no rational basis for its use. Note that its thermal mass is generally
pretty small and by the time it starts to dissipate heat, any damage is
done.

Thanks for the info, I'll see what else I can find. Probably the foremost
expert on lightning in this group is Mark Kinsler. I believe he did his
doctorate on the subject. I have not seen posts from him in a long while.
Tom

 

[w_tom]

A best source of information in this discussion are both
posts from Don Kelly. Especially where he defines what makes
a lightning rod effective: earth ground. We tend to
rationalize only upon what we see. We see the lightning rod
and assume IT is the protection. We then get all hyped over
blunt rods verses pointed rods verses one with a ball. All
irrelevant once the facts are considered. Since we don't see
earth ground, then we tend to forget the most important
component of a lightning protection system.

Don also cites another well regarded expert on the subject -
Dr Abdul Mousa. Dr Mousa's IEEE reviewed papers contain good,
scientific language. But to summarize into laymen's terms -
Early Streamer Emission (ESE) protectors that are suppose to
discharge the air are scams. Air terminals provide the best
or more conductive path from cloud to earth. Which again
demonstrates what makes lightning rods effective - quality of
that earth ground system.

Don't worry about the ball on the end of a lightning rod.
Worry about what makes that lightning rod effective - earth
ground. Lightning seeks earth ground. Either it obtains
earth ground via the building or it obtains earth ground via
the lightning rod. Which is the better conductor - a then
damaged building or a blemished lightning rod? Lightning
protection is about the electrical connection from cloud to
earth which is why protection is only as effective as the
earth ground system and connection to that earthing system.
The most important component is the one we forget about
because we do not see it - earth ground.

 

[Bob Masta]

Actually this is extremely questionable. Design that I know of is based on
the probability of the stroke striking the rod rather than the protected
area. Granted, in some situations, particularly with tall structures, such
draining does occur (possibly increasing the chance of side flashes-another
problem) but this is a bonus, not the basis for design.

I'm no expert on this, but I do recall my undergrad Electromagnetic
Fields instructor coming down on the draining side of this issue.
His examples were opposite to yours: He pointed out that the
size of the ground wires on the typical barn or home lightning rod
was far too small to withstand a direct hit. He noted that tall
buildings, which do sustain repeated hits, have massive ground
conductors to handle the current.

That was probably the only meaningful thing I recalled from that
class. All the rest was curls, dels, and other funny symbols.
(That was over 35 years ago. A lot of synapses could have
drained to ground since then!)


Bob Masta
dqatechATdaqartaDOTcom

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com

 

[Rich Grise]

Don't worry about the ball on the end of a lightning rod.

Hm. And here, I thought it was for catching ball lightning.

;-)
Rich

 

[Rich Grise]

The heat generated is much more in the air above the wire. The power
generated causes the vapor in the air to turn to steam
and the rapid expansion of the steam creates the thunder.

The water vapor in the air is already steam. That's what steam
is, is water vapor.

The thunder is the sonic boom from the explosive expansion from the
ionization.

Y'know, in a dry climate, how you can shuffle across the carpet and
draw a spark? Snap! Well, thunder is that Snap!, but from a spark
three miles long!

God shuffles across the carpet in Heaven, see, and ....

Cheers!
Rich

 

[Rich Grise]

Still decoration.

That's what I was going to say!

Actually, I was going to guess, "It's a world globe - note the wind
vane"

At one time (early 30's or late 20's ) there were rods

made with a ball at the top based on some half baked idea that came into
vogue (to raise the price, probably) that it would provide better
shielding. To sum up- it didn't work. There have been, are, and will be
a variety of devices made and sold to provide lightning shielding.
Typically they don't do as well as simply sticking a bunch of old
bedspring on the roof and grounding them well - but they are shinier and
more expensive.

Cheers!
Rich