Lightning protection

[Tom MacIntyre]

Hi folks...this was in a baseball newsgroup that I participate in
(rec.sport.baseball). Faraday cages have been mentioned, implying that
it could be safe. I don't know that I'd trust this against lightning
(maybe a Van de Graff generator). What do you think?

"Just wondering if sitting on a wood bench inside a metal cage (i.e.
metal on top and 4 sides) is a safe place to be during a lightning
storm. This is similar to the situation of being in a car (except for
the rubber tires!). I have seen younger kids wait out a passing storm
this way and am wondering if it IS in fact safe? What would the effect
of metal cleats be? I'd appreciate any thoughts from anybody but
especially from physics teachers/physicists. Thanks!"

Tom

 

[TimPerry]

Hi folks...this was in a baseball newsgroup that I participate in
(rec.sport.baseball). Faraday cages have been mentioned, implying that
it could be safe. I don't know that I'd trust this against lightning
(maybe a Van de Graff generator). What do you think?

"Just wondering if sitting on a wood bench inside a metal cage (i.e.
metal on top and 4 sides) is a safe place to be during a lightning
storm. This is similar to the situation of being in a car (except for
the rubber tires!). I have seen younger kids wait out a passing storm
this way and am wondering if it IS in fact safe? What would the effect
of metal cleats be? I'd appreciate any thoughts from anybody but
especially from physics teachers/physicists. Thanks!"

Tom

there is no guarantee that a baseball cage is adequately grounded. the
support poles are usually set in concrete.

faraday cages are meant to block RF. lightning is DC (mostly).

refer to http://www.lightningsafety.com/nlsi_pls/lst.html


http://www.securityworld.com/community/quicktips/lightningsafety.html

http://www.nhoem.state.nh.us/mitigation/fig 3-16.htm

http://www.metlife.com/Applications/Corporate/WPS/CDA/PageGenerator/0,1674,P4091,00.html

http://www.scouter.com/compass/Leaders_Resource/Troop_Safety/Lightning_Safety/


i routinely repair lightning damaged equipment. when you see first hand
large molten globs of metal, ceramic, and other material that used to be
perfectly good electronics you tend to get real cautious about where you go
and what you do when a lightning storm passes through.

 

[w_tom]

Effectiveness of protection from lightning is determined by how that
electricity gets to earth ground. Not earth ground beneath you. If
seeks charges via earth that is maybe miles away. In the process, the
struck batting cage then carries electricity underneath your feet to to
take a mile plus path to those distant charges.

For example, if standing on earth with feet apart, and if the nearby
tree is struck, then electricity might pass through earth, up one leg,
down the other, and then continue through earth. Therefore the human
is electrocuted.

This is also why four legged animals tend to be at risk when a nearby
tree is struck. Four legged animals do not make a single point ground
connection to earth; therefore can be harmed by electricity flowing
through the earth.

The baseball cage may act as a good lightning rod. But then
electricity passes beneath a human to cause harm. IOW the faraday cage
is not complete because the earth beneath the human was not
sufficiently conductive.

BTW, lightning is not DC. Lightning is AC current - mostly in radio
frequency ranges. This AC nature is why longer conductors, sufficient
for 60 Hz AC electric power, can be poor conductors to lightning; its
energy in higher frequencies. This AC nature of lightning is why so
many humans think lightning is capricious.

 

[w_tom]

Effectiveness of protection from lightning is determined by how that
electricity gets to earth ground. Not earth ground beneath you. If
seeks charges via earth that is maybe miles away. In the process, the
struck batting cage then carries electricity underneath your feet to to
take a mile plus path to those distant charges.

For example, if standing on earth with feet apart, and if the nearby
tree is struck, then electricity might pass through earth, up one leg,
down the other, and then continue through earth. Therefore the human
is electrocuted.

This is also why four legged animals tend to be at risk when a nearby
tree is struck. Four legged animals do not make a single point ground
connection to earth; therefore can be harmed by electricity flowing
through the earth.

The baseball cage may act as a good lightning rod. But then
electricity passes beneath a human to cause harm. IOW the faraday cage
is not complete because the earth beneath the human was not
sufficiently conductive.

BTW, lightning is not DC. Lightning is AC current - mostly in radio
frequency ranges. This AC nature is why longer conductors, sufficient
for 60 Hz AC electric power, can be poor conductors to lightning; its
energy in higher frequencies. This AC nature of lightning is why so
many humans think lightning is capricious.

 

[NSM]

i routinely repair lightning damaged equipment. when you see first hand
large molten globs of metal, ceramic, and other material that used to be
perfectly good electronics you tend to get real cautious about where you go
and what you do when a lightning storm passes through.

Yep, you never know. I hide under the bed myself.

N

 

[TimPerry]

BTW, lightning is not DC. Lightning is AC current - mostly in radio

frequency ranges. This AC nature is why longer conductors, sufficient
for 60 Hz AC electric power, can be poor conductors to lightning; its
energy in higher frequencies. This AC nature of lightning is why so
many humans think lightning is capricious.

utter and complete crap.

lightning is an electrostatic discharge.

 

[N Cook]

Effectiveness of protection from lightning is determined by how that
electricity gets to earth ground. Not earth ground beneath you. If
seeks charges via earth that is maybe miles away. In the process, the
struck batting cage then carries electricity underneath your feet to to
take a mile plus path to those distant charges.

For example, if standing on earth with feet apart, and if the nearby
tree is struck, then electricity might pass through earth, up one leg,
down the other, and then continue through earth. Therefore the human
is electrocuted.

This is also why four legged animals tend to be at risk when a nearby
tree is struck. Four legged animals do not make a single point ground
connection to earth; therefore can be harmed by electricity flowing
through the earth.

The baseball cage may act as a good lightning rod. But then
electricity passes beneath a human to cause harm. IOW the faraday cage
is not complete because the earth beneath the human was not
sufficiently conductive.

BTW, lightning is not DC. Lightning is AC current - mostly in radio
frequency ranges. This AC nature is why longer conductors, sufficient
for 60 Hz AC electric power, can be poor conductors to lightning; its
energy in higher frequencies. This AC nature of lightning is why so
many humans think lightning is capricious.

Lightning is AC ?!!!. I've never seen any rotating clouds,
in the UK we get plenty of lighning but no rotating tornados with lightning.

 

[Choreboy]

The baseball cage may act as a good lightning rod. But then
electricity passes beneath a human to cause harm. IOW the faraday cage
is not complete because the earth beneath the human was not
sufficiently conductive.

With no mesh underfoot, it might depend on the shape of the cage. If
you were surrounded on three sides, there might not be much current
underfoot because the metal fence would be a better conductor. At the
nearest playground, the "batter's cage" is just a wide V backstop, so I
wouldn't stand there in a storm.

Pointy rods on the top of the cage might prevent a lot of strikes by
bleeding off charges.

BTW, lightning is not DC. Lightning is AC current - mostly in radio
frequency ranges. This AC nature is why longer conductors, sufficient
for 60 Hz AC electric power, can be poor conductors to lightning; its
energy in higher frequencies. This AC nature of lightning is why so
many humans think lightning is capricious.

I think I've read that there can be several phases in a lightning
strike, and maybe electricity flows back and forth. I can't remember.
Anyway, lightning doesn't have to be AC for a power line to be a poor
conductor. Inductance reacts against a sudden change in current. With
lightning, the change is so sudden that a little inductance means a lot
of impedance.

Leading from a 220V transformer to a house, the three conductors are
usually twisted together nowadays. The reduces inductance. I'll bet
we'd have less damage from surges with fast rise times, whether or not
caused by lightning, if power companies spaced the conductors a foot
apart as they once did.

 

[CJT]

w_tom wrote:

BTW, lightning is not DC. Lightning is AC current - mostly in radio
frequency ranges. This AC nature is why longer conductors, sufficient
for 60 Hz AC electric power, can be poor conductors to lightning; its
energy in higher frequencies. This AC nature of lightning is why so
many humans think lightning is capricious.

Huh? Cite?
<snip>
-
The e-mail address in our reply-to line is reversed in an attempt to
minimize spam. Our true address is of the form [email protected].

 

[w_tom]

To make it easier for some to understand, I should have first noted
that lightning is an impulse. The classic current transient is an 8/20
microsecond impulse. That impulse is a summation of frequency
components; AC waveforms at various radio frequencies. This is basic
knowledge to those with electrical engineering training AND those with
math backgrounds that include Fourier Series.

Lightning obviously is not DC. If it was, then only wire resistance
would be relevant. An impulse due to lightning contains numeorus AC
sine waves. Much of that energy is in the 100 kilohertz and Megahertz
range. These obviously are not DC. Wire resistance has very little
relevance. Wire impedance is a dominant parameter due to sine waves in
an 8/20 usec impulse called CG lightning. Wire impedance would be
completely irrelevant if lightning were DC.

Nothing new about this. First year electrical engineers even study
impulse response. Those who have a problem with this concept will have
difficulty understanding the electrical nature of lightning. Clearly
lightning is not DC because lightning current is an impulse in the
microsecond range.

 

[Aidan Grey]

To all:

This statement is correct. A lightning bolt often consists of as many as
40 to 50 individual discharges. This is not apparent unless the lightning
is captured by high speed photography.

So, since the lightning discharge turns on and off 40 to 50 times in a
fraction
of a second, it is not pure DC.

The current flows in only one direction. However, the energy is split
between a DC component, and a considerable part that is effectively RF
energy. Note this is RF energy, not low frequency AC.

I don't have a Web source to cite for this, but I have read about this
before. Specifically, that a ground for a lightning rod must not go through
a coil of wire.


Aidan Grey

 

[N Cook]

To all:

This statement is correct. A lightning bolt often consists of as many as
40 to 50 individual discharges. This is not apparent unless the lightning
is captured by high speed photography.

So, since the lightning discharge turns on and off 40 to 50 times in a
fraction
of a second, it is not pure DC.

The current flows in only one direction. However, the energy is split
between a DC component, and a considerable part that is effectively RF
energy. Note this is RF energy, not low frequency AC.

I don't have a Web source to cite for this, but I have read about this
before. Specifically, that a ground for a lightning rod must not go through
a coil of wire.


Aidan Grey

chopped DC makes sense - anyone know what initiates the chopping and the
rate of chop?
Is chopped DC the discharge mechanism from van de graff's etc?

 

[CJT]

To all:

This statement is correct. A lightning bolt often consists of as many as
40 to 50 individual discharges. This is not apparent unless the lightning
is captured by high speed photography.

So, since the lightning discharge turns on and off 40 to 50 times in a
fraction
of a second, it is not pure DC.

The current flows in only one direction. However, the energy is split
between a DC component, and a considerable part that is effectively RF
energy. Note this is RF energy, not low frequency AC.

A small multiple of 40-50 Hz is not exactly RF. So what's the risetime
of these pulses? Duration?

I don't have a Web source to cite for this, but I have read about this
before. Specifically, that a ground for a lightning rod must not go through
a coil of wire.

Why would you want one to run through a coil, anyway?

I suspect the warning is to avoid somebody being electrocuted by induced
current from the pulse(s).

 

[w_tom]

Risetime of these current pulses was provided in a previous
post: exponentially rising and falling edges of 8/20
microseconds.

Obviously the ground wire would not be run through a coil.
But due to the high frequency nature of transients, that
ground wire must not be spliced, no sharp bends, and not
routed inside metallic materials. A violation would only
increase wire impedance unacceptably. Why? Because even
sharp bends cause significant impedance increases when
discussing the higher frequency components of lightning.

We could play a game of 50,000 questions since even the most
basic nature of lightning - its RF components - is new
information. These and other questions have been answered
previously in a list of citations at:
http://makeashorterlink.com/?X61C23DCA

Figure about one days worth of reading.

 

[TimPerry]

Risetime of these current pulses was provided in a previous
post: exponentially rising and falling edges of 8/20
microseconds.

Obviously the ground wire would not be run through a coil.
But due to the high frequency nature of transients, that
ground wire must not be spliced, no sharp bends, and not
routed inside metallic materials. A violation would only
increase wire impedance unacceptably. Why? Because even
sharp bends cause significant impedance increases when
discussing the higher frequency components of lightning.

We could play a game of 50,000 questions since even the most
basic nature of lightning - its RF components - is new
information. These and other questions have been answered
previously in a list of citations at:
http://makeashorterlink.com/?X61C23DCA

citing a newsgroup discussion is not a valid reference.

elaborate (and probably inaccurate) discussion as to the exact nature of
lightning will not be of benefit to the original poster.

a DC pulse is still a DC pulse no amount of wishful thinking will turn it in
to AC (granted that there are some aspects that generate RF and perhaps some
other characteristics that may be able to be handled by AC analysis)

looking at a few of toms 16,900 archived posts i see he also claims that a
ground rod in rock will make a perfectly good ground. from that it follows
that a batting cage set in concrete is adiquitly grounded. i dont think so.

 

[CJT]

To make it easier for some to understand, I should have first noted
that lightning is an impulse. The classic current transient is an 8/20
microsecond impulse. That impulse is a summation of frequency
components; AC waveforms at various radio frequencies. This is basic
knowledge to those with electrical engineering training AND those with
math backgrounds that include Fourier Series.

Lightning obviously is not DC. If it was, then only wire resistance
would be relevant. An impulse due to lightning contains numeorus AC
sine waves.

That a pulse is susceptible to Fourier analysis does not make it
"contain numerous AC sine waves."

Much of that energy is in the 100 kilohertz and Megahertz

 

[CJT]

Risetime of these current pulses was provided in a previous
post: exponentially rising and falling edges of 8/20
microseconds.

I took what you wrote previously to be the pulse duration, not its
risetime.

Obviously the ground wire would not be run through a coil.
But due to the high frequency nature of transients, that
ground wire must not be spliced, no sharp bends, and not
routed inside metallic materials. A violation would only
increase wire impedance unacceptably. Why? Because even
sharp bends cause significant impedance increases when
discussing the higher frequency components of lightning.

An 8 microsecond risetime doesn't imply all that high a frequency.
Sharp bends in thick wire at ultrasonic frequencies aren't a big deal.

 

[w_tom]

You are joking. Right? The Fourier analysis of lightning
reveals energy at numerous frequencies. Lightning is RF
electricity. That RF energy is why lightning even causes
noise on radios. So now you say all those RF sine waves,
demonstrated by Fourier analysis, really do not exist? Why
not just say the world is still flat? If lightning did not
have so much energy in RF, then radios would not receive that
RF noise. An example that demonstrates the RF nature of
pulses and of lightning. Pulses are not DC.

If lightning were DC, then wire impedance would not be
relevant. But lets put numbers to it. A 50 foot 20 amp wire
may be 0.2 ohms resistance. But wire reactance means same
wire has something like 120 ohms impedance to a lightning
transient. Why is impedance so much larger than the
resistance? Because the RF components of lightning make
impedance relevant and significant. If lighting were DC, then
wire resistance and wire impedance would be same. If
lightning were DC, then lightning would not have such
destructive consequences.

Fourier analysis demonstrates lightning is AC electricity at
many frequencies. Putting a number to the impulse - 8/20
microseconds - explains why so much energy is in the Megahertz
range. Using DC analysis to explain lightning means that
person never first learned basic transient analysis taught to
first year EE students. Lightning has massive energy in radio
frequencies which makes lightning so uniquely destructive.
Those radio frequencies even mean that an earthing wire
bundled with other wires will induce transients on those other
wires. DC electricity would not do that inducing.

Why are induced transients a problem with lightning?
Because lighting is AC electricity - as demonstrated by
Fourier analysis, as demonstrated by discussion about
impedance, as demonstrated by what first year EE students are
taught, as demonstrated by noise on the radio, and as
demonstrated by so many industry professional cited in:
http://makeashorterlink.com/?X61C23DCA

Show me. Show me how lightning is only DC. Provide
supporting facts. Provide industry citations as I have done
repeatedly. Declaring something without supporting facts is
classic junk science reasoning. Where are those supporting
facts, the numbers, and industry professional citations that
declare lightning as DC electricity? Without supporting
facts, et al, then your claim is really nothing more than
personal speculation or urban myth. Show me. Provide facts
and numbers. Provide the underlying math. Show me that
capacitors and inductors are irrelevant to conducting
lightning's DC electricity. Show me rather than make
unsubstantiated claims. Show me how you know that lightning
is DC electricity. Show me that you also know the difference
between ultrasonics and electricity.

 

[w_tom]

Point one. Deny citations from industry professionals by
refusing to read them. An ostrich does that; not an informed
human. Cited are numerous technical papers and other
technical facts on lightning - literally a full days worth of
reading. Instead TimPerry pretends those citations did not
exist in:
http://makeashorterlink.com/?X61C23DCA

A DC pulse is an oxymoron. As confusing as another
ridiculous term: DC triangle wave. Either it is a pulse
affected by reactance (inductors and capacitors), or it is
only DC that completely ignores reactance. One cannot have
it both ways. Either it is DC that ignores reactance, or it
is a pulse that makes reactance relevant. Which is it? If
lightning is DC, then inductance, capacitance, and impedance
is not discussed. Why do industry professionals discuss these
repeatedly? Because lightning is not DC.

Pulses are transient responses - taught in 1st year
engineering - an introduction to AC characteristics. AC
analysis makes wire inductance and capacitance significant.
DC analysis ignores inductance and capacitance. To understand
how lightning works - as cited in
http://makeashorterlink.com/?X61C23DCA - one cannot pretend
lightning is DC. A lightning impulse is an AC or transient
event. Lightning is a composition of many frequencies. Show
me a DC wave that has frequency components? You cannot. That
is the oxymoron of DC pulse. A 'pulse' has AC components.
'DC' has no AC components. Oxymoron.

Equally confusing is to say "DC triangle wave". DC pulse or
DC triangle wave - both are oxymorons. Both contain numerous
frequency components. Therefore a 'DC pulse' cannot be
analyzed using DC analysis. Lightning requires AC analysis.
Lightning is not a DC event.

Point two. TimPerry believes a batting cage set in concrete
is not conductive. TimPerry should first read those
discussions he refused to learn from in:
http://makeashorterlink.com/?X61C23DCA
A most superior earth ground is an Ufer ground. Why? The
earthing is through concrete. Had TimPerry first learned what
industry professionals teach, then he would have known the
batting cage mounted in concrete makes a best earthing ground
- exactly like Ufer grounds. But then TimPerry also promotes
an oxymoron called 'DC pulse'. Somehow he knows without first
learning the facts. If only he had first read those citations
and first learned about Ufer grounding. Then he would not
have posted, "i dont think so." One should first learn before
knowing something.

The energy of lightning is AC electricity.

 

[Brian Clark]

You are joking. Right? The Fourier analysis of lightning
reveals energy at numerous frequencies. Lightning is RF
electricity. That RF energy is why lightning even causes
noise on radios. So now you say all those RF sine waves,
demonstrated by Fourier analysis, really do not exist? Why
not just say the world is still flat? If lightning did not
have so much energy in RF, then radios would not receive that
RF noise. An example that demonstrates the RF nature of
pulses and of lightning. Pulses are not DC.

If lightning were DC, then wire impedance would not be
relevant. But lets put numbers to it. A 50 foot 20 amp wire
may be 0.2 ohms resistance. But wire reactance means same
wire has something like 120 ohms impedance to a lightning
transient. Why is impedance so much larger than the
resistance? Because the RF components of lightning make
impedance relevant and significant. If lighting were DC, then
wire resistance and wire impedance would be same. If
lightning were DC, then lightning would not have such
destructive consequences.

Fourier analysis demonstrates lightning is AC electricity at
many frequencies. Putting a number to the impulse - 8/20
microseconds - explains why so much energy is in the Megahertz
range. Using DC analysis to explain lightning means that
person never first learned basic transient analysis taught to
first year EE students. Lightning has massive energy in radio
frequencies which makes lightning so uniquely destructive.
Those radio frequencies even mean that an earthing wire
bundled with other wires will induce transients on those other
wires. DC electricity would not do that inducing.

Why are induced transients a problem with lightning?
Because lighting is AC electricity - as demonstrated by
Fourier analysis, as demonstrated by discussion about
impedance, as demonstrated by what first year EE students are
taught, as demonstrated by noise on the radio, and as
demonstrated by so many industry professional cited in:
http://makeashorterlink.com/?X61C23DCA

Show me. Show me how lightning is only DC. Provide
supporting facts. Provide industry citations as I have done
repeatedly. Declaring something without supporting facts is
classic junk science reasoning. Where are those supporting
facts, the numbers, and industry professional citations that
declare lightning as DC electricity? Without supporting
facts, et al, then your claim is really nothing more than
personal speculation or urban myth. Show me. Provide facts
and numbers. Provide the underlying math. Show me that
capacitors and inductors are irrelevant to conducting
lightning's DC electricity. Show me rather than make
unsubstantiated claims. Show me how you know that lightning
is DC electricity. Show me that you also know the difference
between ultrasonics and electricity.

well I'd never even thought about whether it was AC DC or whatever. But
its an interesting discussion... Its certainly disruptive across a wide
band of the rf spectrum. I wouldn't make a phone call if it was overhead
anyway! hmmm