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Lightning and aerials - LONG POST



 
 
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  #1  
Old July 24th 03, 01:15 PM posted to uk.tech.digital-tv
Duncan Ross
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Posts: 14
Default Lightning and aerials - LONG POST

Interesting post - if I can add a few points;

Lightning ground strikes occur when the potential difference between the cloud
and earth is allowed to build up to such a voltage that the resistance of the
air breaks down and the air is ionised. To do this you need lots of volts just
to get there and enough power (volts x amps) to actually get the work done.
When this happens the resistance of air collapses and the floodgates are opened
for huge currents to flow. Before this happens the current is still flowing but
to a much lesser extent. However wet air has a much lower resistance than dry
air and this causes the centre of the cloud (where it is actually raining) to
discharge quicker to earth and lose a lot of its potential difference. Because
of this, strikes through rainfall are almost unheard of - cloud to ground
lightning almost always occurs around the edges of the cloud where air
resistance is greatest. Try it yourself, pull the spark plug connectors off the
spark plugs in your car, pour water over the plugs and plug in again and then
try to start the car. The water dissipates the spark to earth before it can
reach a high-enough level to break down the air resistance in the cylinder.

Back to your post!

When the aerial is in an area of high potential difference compared to earth,
the unplugged connector acts like a mini ioniser, producing a stream of
negative ions into the room. However despite the voltage difference between it
and (say) the earth on a nearby mains socket it cannot deliver a flash because
the current isn't available - this would require the resistance in the air
above the aerial to break down too. So although your sums are dead on, you
aren't going to get a flash unless a large amount of current becomes available
in the vicinity of the aerial. Bizarely this can happen through a nearby
strike, and suddenly all the aerials in a block that have happily been acting
as ionisers for ten minutes suddenly get enough current from the cloud (which
has finally managed to reduce the resistance of the surrounding air to almost
zero) to zap all the televisions in one go!

Sounds like a job for Wrights Aerials!
Ads
  #2  
Old July 24th 03, 06:16 PM posted to uk.tech.digital-tv
Wrightsaerials
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Posts: 93
Default Lightning and aerials - LONG POST

Sounds like a job for Wrights Aerials!


Super!

Some years ago I was on the roof of a retail store very close to Bradford
railway station (Forster Sq I thing it's called). It was a sunny day, very
bright but also very windy. A black clound came across very quickly. I thouight
there might be lightning and told the two blokes with me that we must get off
the roof very quickly. They thought I was crazy because the sun was still
shining and on the face of it it was still a nice day, but I insisted and we
set off across the very big roof to the ladder. At the monent when the first
man was on the ground, the second half way up the ladder and I was still on the
roof lightning struck a railway engine, only a few yards away from us. I
happened to be looking in that direction and was momentarily blinded. The man
half way up the ladder got a fright but didn't fall. The man on the ground was
knocked to the floor. This took place when the IRA was active and the people in
the shop thought they'd been bombed. The staff came bursting out of the loading
bay really scared. One bloke just broke down and curled up into a ball and
sobbed. Meanwhile the engine that had been struck burst into flames. Then a
torrential downpour started, and although the staff had their indoor clothing
on they wouldn't go back in the shop because they still thought there had been
a bomb and they thought there might be another. They all got absolutely soaked
to the skin, but they wouldn't go in. It was a weird experience. We finished
the job in somber mood.
Bill
  #3  
Old July 24th 03, 07:57 PM posted to uk.tech.digital-tv
Les Hellawell
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Posts: 20
Default Lightning and aerials - LONG POST


"Duncan Ross" wrote in message

Interesting post - if I can add a few points;


Lightning ground strikes occur when the potential difference between the
cloud and earth is allowed to build up to such a voltage that the
resistance of the air breaks down and the air is ionised. To do this you
need lots of volts just to get there and enough power (volts x amps) to
actually get the work done.


Right. It is the quantity of electricity that determines the potential
difference between points (measured in Coulombs?) Big capacitors
carry higher volts than small ones. Remember this is static or stored
electricity not dynamic electricity as produced by a battery or
generator.

When this happens the resistance of air collapses and the floodgates are
opened for huge currents to flow. Before this happens the current is still
flowing but to a much lesser extent. However wet air has a much lower
resistance than dry air and this causes the centre of the cloud (where it
is actually raining) to discharge quicker to earth and lose a lot of its
potential difference.


Right. But this would only be true as long as the rate of ionisation
matches the rate at which the quantity of electricity (electrons)
is being generated. Some thunderstorms generate electric fields
fast enough to cause frequent flashes whilst others are quite slow.
(The recent one was quite slow where I lived)

Because
of this, strikes through rainfall are almost unheard of - cloud to ground
lightning almost always occurs around the edges of the cloud where air
resistance is greatest.


You seem to be arguing that the discharge will take a longer and harder
path ignoring a shorter path of lower resistance?

When the aerial is in an area of high potential difference compared to
earth, the unplugged connector acts like a mini ioniser, producing a
stream of negative ions into the room. However despite the voltage
difference between it and (say) the earth on a nearby mains socket it
cannot deliver a flash because the current isn't available - this would
require the resistance in the air above the aerial to break down too.
So although your sums are dead on, you aren't going to get a flash
unless a large amount of current becomes available
in the vicinity of the aerial.


Well certainly ionisation into the air would serve to inhibit the voltage
reaching high enough for breakdown. It is rather like trying to fill a
bucket with a hole, but this will only occur for as long as the
ionisation flow can match the current flow which will increase as
the stress between ground and cloud increases (building up to
a strike). Pour more water into a bucket than flows out through the hole
and the water level will rise until it runs over the top.

If the voltage does reach breakdown point (on an unplugged lead),
and we have heard cases in this thread when it has, it is because the
quantity of electricity gathered is sufficient to generate the potential
difference needed and sufficient to cause breakdown even if not
sufficient to sustain it. The same is true of a full lightning strike except
on a larger scale, the lightning path only last long enough for the
charge to discharge, it does not sustain longer than that. (we are
talking tiny amounts of time of course).

Bizarely this can happen through a nearby
strike, and suddenly all the aerials in a block that have happily
been acting as ionisers for ten minutes


They will only ionise current into the air if unplugged of course,
otherwise the current flows into and through the TV (a much
larger hole in the bucket). Damage will occur once this current gets too
high (due to the rising electrical field) or, if the aerial is unplugged,
the ionisation cannot match the current flow and the voltage rises
to the flash point. There are two distinct circumstances here.

suddenly get enough current from the cloud (which
has finally managed to reduce the resistance of the surrounding air to
almost zero) to zap all the televisions in one go!


The short circuit will only be in the narrow corridor where the
lightning discharge occurs, the surrounding air will still have the
same resistance surely? Also, if the resistance of surrounding air
does drop to zero as you suggest, why would the current
then flow down a cable (with resistance) and through the TV
(also with resistance) when it can take a more direct path through
this zero resistance air, or is the air only zero resistant down
to the roof top? Sorry but that is not logical.

Surely once the nearby strike occurs the potential difference between
cloud and ground would instantly drop to zero due to the short circuit,
collapsing the electrical field in the air with it and the potential
difference between aerial top and ground? Thus any current flow
down the aerial cable (if not already burnt out) would instantly drop
not rise as you suggest? Ohms law still applies. Thus the flash from
the unconnected aerial or TV/cable damage due to excess current
must surely have occurred a finite time before the strike, driven by
the high electrical field between the ground and the aerial?

--
Les Hellawell

Greetings from :
YORKSHIRE - The White Rose County









  #4  
Old July 24th 03, 08:04 PM posted to uk.tech.digital-tv
Les Hellawell
external usenet poster
 
Posts: 20
Default Lightning and aerials - LONG POST


"Wrightsaerials" wrote in message
...
Sounds like a job for Wrights Aerials!


Super!

Some years ago I was on the roof of a retail store very close to Bradford
railway station (Forster Sq I thing it's called). It was a sunny day, very
bright but also very windy. A black clound came across very quickly. I

thouight
there might be lightning and told the two blokes with me that we must get

off
the roof very quickly. They thought I was crazy because the sun was still
shining and on the face of it it was still a nice day, but I insisted and

we
set off across the very big roof to the ladder. At the monent when the

first
man was on the ground, the second half way up the ladder and I was still

on the
roof lightning struck a railway engine, only a few yards away from us. I
happened to be looking in that direction and was momentarily blinded.


Was there a bang? I once saw lightning strike a nearby pole mounted
YEB sub-station only about 30-40ft away from my rear window. It burnt
out one of the cables which dropped to the ground setting poles and
remaining cables shaking violently all down the field. The only noise was
a slight 'fizzing' sound and no bang. Duncan please note: it was raining
at the time.


--
Les Hellawell

Greetings from :
YORKSHIRE - The White Rose County



  #5  
Old July 24th 03, 09:32 PM posted to uk.tech.digital-tv
M. J. Powell
external usenet poster
 
Posts: 103
Default Lightning and aerials - LONG POST

In message , Les Hellawell
writes

"Duncan Ross" wrote in message

Interesting post - if I can add a few points;


Lightning ground strikes occur when the potential difference between the
cloud and earth is allowed to build up to such a voltage that the
resistance of the air breaks down and the air is ionised. To do this you
need lots of volts just to get there and enough power (volts x amps) to
actually get the work done.


Right. It is the quantity of electricity that determines the potential
difference between points (measured in Coulombs?) Big capacitors
carry higher volts than small ones. Remember this is static or stored
electricity not dynamic electricity as produced by a battery or
generator.


Well, no. For a given quantity of charge (in Coulombs) a small capacitor
will show a higher voltage than a small one.

Mike
--
M.J.Powell
  #6  
Old July 25th 03, 12:19 AM posted to uk.tech.digital-tv
R. Mark Clayton
external usenet poster
 
Posts: 152
Default Lightning and aerials - LONG POST


"Les Hellawell" wrote in message


Was there a bang?


There often is.

I was once having breakfast in the Boar's Head (PH / Hotel) in Bishops
Stortford (formerly the palace) when lightening struck the church spire
about 30m away on the other side of the road and passed down the conductor.
It was summer and the door was open.

The flash rendered me temporarily blind (~10s), and the bang meant I
couldn't hear anything for about a minute.

When at school lightnening struck the building opposite (~50m) - the bang
was much worse than the flash.




  #7  
Old July 25th 03, 08:00 AM posted to uk.tech.digital-tv
Duncan Ross
external usenet poster
 
Posts: 14
Default Lightning and aerials - LONG POST

From: "Les Hellawell"
Date: 24/07/2003 20:57 GMT Daylight Time
Message-id:



Because
of this, strikes through rainfall are almost unheard of - cloud to ground
lightning almost always occurs around the edges of the cloud where air
resistance is greatest.


You seem to be arguing that the discharge will take a longer and harder
path ignoring a shorter path of lower resistance?


No, the ground to cloud potential difference in the area of rainfall is much
lower than the potential difference around it. In addition wet air conducts,
and a spark cannot pass through a conductor, only across an insulator (dry
air).

suddenly get enough current from the cloud (which
has finally managed to reduce the resistance of the surrounding air to
almost zero) to zap all the televisions in one go!


The short circuit will only be in the narrow corridor where the
lightning discharge occurs, the surrounding air will still have the
same resistance surely? Also, if the resistance of surrounding air
does drop to zero as you suggest, why would the current
then flow down a cable (with resistance) and through the TV
(also with resistance) when it can take a more direct path through
this zero resistance air, or is the air only zero resistant down
to the roof top?


This is from inductance, a large current flowing down a nearby conductor (the
lightning channel) will induce a smaller current flow in the aerial conductor -
this is the current flow that it was waiting for to kick things off.
  #8  
Old July 25th 03, 11:33 AM posted to uk.tech.digital-tv
M. J. Powell
external usenet poster
 
Posts: 103
Default Lightning and aerials - LONG POST

In message , Les Hellawell
writes

"Wrightsaerials" wrote in message
...
Sounds like a job for Wrights Aerials!


Super!

Some years ago I was on the roof of a retail store very close to Bradford
railway station (Forster Sq I thing it's called). It was a sunny day, very
bright but also very windy. A black clound came across very quickly. I

thouight
there might be lightning and told the two blokes with me that we must get

off
the roof very quickly. They thought I was crazy because the sun was still
shining and on the face of it it was still a nice day, but I insisted and

we
set off across the very big roof to the ladder. At the monent when the

first
man was on the ground, the second half way up the ladder and I was still

on the
roof lightning struck a railway engine, only a few yards away from us. I
happened to be looking in that direction and was momentarily blinded.


Was there a bang? I once saw lightning strike a nearby pole mounted
YEB sub-station only about 30-40ft away from my rear window. It burnt
out one of the cables which dropped to the ground setting poles and
remaining cables shaking violently all down the field. The only noise was
a slight 'fizzing' sound and no bang. Duncan please note: it was raining
at the time.


I was driving home many years ago, through Moss Side, listening to R4 LW
when the radio suddenly started a loud screaming noise. After a few
seconds a huge lightning flash hit the ground about 50 yards away.
Normal reception then. I assume the screaming noise was corona from the
whip aerial.

Mike
--
M.J.Powell
  #9  
Old July 25th 03, 11:35 AM posted to uk.tech.digital-tv
M. J. Powell
external usenet poster
 
Posts: 103
Default Lightning and aerials - LONG POST

In message , Les Hellawell
writes

"M. J. Powell" wrote in message

In message , Les Hellawell
writes

"Duncan Ross" wrote in message

Interesting post - if I can add a few points;

Lightning ground strikes occur when the potential difference between

the
cloud and earth is allowed to build up to such a voltage that the
resistance of the air breaks down and the air is ionised. To do this

you
need lots of volts just to get there and enough power (volts x amps) to
actually get the work done.

Right. It is the quantity of electricity that determines the potential
difference between points (measured in Coulombs?) Big capacitors
carry higher volts than small ones. Remember this is static or stored
electricity not dynamic electricity as produced by a battery or
generator.


Well, no. For a given quantity of charge (in Coulombs) a small capacitor
will show a higher voltage than a small one.


Looking it up v = q/c (Nelkon again :-)

Thinks: Halve c and you double v.
Yes you are right, I stand corrected.

but voltage is proportional to quantity which is what I was trying to get
at.

I am afraid my electrical theory has got a little rusty down the years :-(


Right! And nobody noticed that my second 'small' should have been
'large'!

Mike
--
M.J.Powell
  #10  
Old July 25th 03, 01:04 PM posted to uk.tech.digital-tv
Les Hellawell
external usenet poster
 
Posts: 20
Default Lightning and aerials - LONG POST


"Duncan Ross" wrote in message
...
From: "Les Hellawell"
Date: 24/07/2003 20:57 GMT Daylight Time
Message-id:



Because of this, strikes through rainfall are almost unheard of -
cloud to ground lightning almost always occurs around the edges
of the cloud where air resistance is greatest.


You seem to be arguing that the discharge will take a longer and harder
path ignoring a shorter path of lower resistance?


No, the ground to cloud potential difference in the area of rainfall is
much lower than the potential difference around it. In addition wet air
conducts, and a spark cannot pass through a conductor, only across
an insulator (dry air).


You are restating your argument not answering the question. How
can a voltage build in one part of the cloud sufficient to discharge a
long way to earth if an adjacent area of cloud has (according to your
argument) a low resistance path to earth? Surely, in these
circumstance the discharge would be between these areas of cloud -
the shortest/easiest path in this case?

Yes the wetter path to earth may have a lower insulation resistance
but it wont be zero and I doubt if it will be significantly less than the
25,000V figure quoted. All this means is that a discharge can occur
at a lower voltage in a wet zone than in a dry one, so a 'dry' discharge
will have more potential energy than a 'wet' one and be more
dangerous.

As to whether the leakage is enough to prevent a strike, who really
knows? I don't. it all depends on the circumstances and severity of
the storm. The stormclouds can reach very high into the sky. I have
seen the hammerheads from aircraft and once flew into Venice Airport
near one. It was fantastic descending between great towers of solid
white. You could hardly see the ground above the airport as it was like
looking into a murky pond. Getting off the aircraft was like stepping into
a Turkish Bath. The slow moving storm reached the coast that same
night about 0100 and was a real humdinger.

suddenly get enough current from the cloud (which
has finally managed to reduce the resistance of the surrounding air to
almost zero) to zap all the televisions in one go!


The short circuit will only be in the narrow corridor where the
lightning discharge occurs, the surrounding air will still have the
same resistance surely?


This is from inductance, a large current flowing down a nearby conductor
(the lightning channel) will induce a smaller current flow in the aerial
conductor - this is the current flow that it was waiting for to kick
things off.


Inductance induces voltage not current. Current is proportional
to the voltage induced if electrical resistance is not infinite.

So you introduce a new circumstance by introducing the
electromagnetic pulse caused by a lightning discharge? OK.
Yes this will indeed induce a voltage spike in adjacent conductors.
The magnitude will reduce in proportion to the square of the distance
from the lightning. I doubt if this will be very large (enough
perhaps to induce a crackle on a radio?) but I simply do not know.
This momentary increase in p.d. induced in the conductor may
indeed be enough to cause a flash if the existing voltage is just
on or below the threshold (and has not yet started to collapse
due to this same adjacent short circuit in the lightning path) but
current will only start to flow ONCE the breakdown has occured
and it has a path to flow through.

Let's just consider an unplugged aerial lead here which is
insulated at the bottom by air between the nearest ground point
and its plug and, therefore, has a path of electrical resistance
approaching infinity - there will be ionisation leakage as you
mention to ground. (Note: The insulation resistance of air is a
different thing entirely from its almost infinite electrical
resistance.)

Insulation resistance of an insulator (whether air, plastic
mica etc) is measured purely in term of the potential difference
or electrical stress that it can withstand. As already stated air
can withstand about 25,000v/inch. Insulation breakdown occurs
when the voltage limit is reached. (that's a full stop there) Only
AFTER breakdown has occurred does a discharge current start
to flow though it. How long this current lasts, depends on the
quantity of charge stored (in static or stored electricity) and this is
proportional to the voltage before the breakdown occured.
Putting it the other way round: For a given situation a doubling of
the voltage means a doubling of the charge stored (in the cable
in this case*). So, the discharge current does not have to come
from (your zero resistant) air above the aerial as you suggest but
is already stored in the cable.

*I have a feeling this is not quite so simple as the potential
difference between ground and aerial top will still exist in this
moment in time even though the flash has short circuited the
bottom of the cable. Strewth who knows what happens under
these circumstances!

ps thanks for the interesting debate forcing me to clarify my
thinking here. I hope others are not finding it too boring :-)

--
Les Hellawell

Greetings from :
YORKSHIRE - The White Rose County











 




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