Sorry all. O.T. as a repair issue. Just something I read about today that
I'm not sure I understand.

Anybody come across the term "step potential" or its effect in regard to a
downed live power line in contact with the ground ?

Arfa

Arfa Daily wrote:

Sorry all. O.T. as a repair issue. Just something I read about today that
I'm not sure I understand.

Anybody come across the term "step potential" or its effect in regard to a
downed live power line in contact with the ground ?


It is the voltage drop across the surface of the soil or concrete,
and if it is high enough simply taking a step will electrocute you.
There was a story about this on an Orlando TV station a few years ago
about a police horse being electrocuted in the downtown area. An old
outdoor sign had been removed, and the wires abandoned. Apparently,
someone turned the breaker back on, and it didn't trip. The officer was
on patrol when his horse crossed the path, and was killed. The shorter
the path to a good ground, the higher the available voltage is, per
step.


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"Michael A. Terrell" wrote in message
m...

Arfa Daily wrote:

Sorry all. O.T. as a repair issue. Just something I read about today
that
I'm not sure I understand.

Anybody come across the term "step potential" or its effect in regard to
a
downed live power line in contact with the ground ?


It is the voltage drop across the surface of the soil or concrete,
and if it is high enough simply taking a step will electrocute you.
There was a story about this on an Orlando TV station a few years ago
about a police horse being electrocuted in the downtown area. An old
outdoor sign had been removed, and the wires abandoned. Apparently,
someone turned the breaker back on, and it didn't trip. The officer was
on patrol when his horse crossed the path, and was killed. The shorter
the path to a good ground, the higher the available voltage is, per
step.



Ok Michael. This is kind of the way this article describes it, except it
states thus :-

"When a power line comes in contact with the ground, electrical energy
spreads out through the surrounding earth - like the ripples in a pond from
a tossed stone. Each expanding concentric circle has a different electrical
potential. Under these circumstances, taking a step could place your feet in
areas of different voltages, allowing electricity to course through your
body, entering one foot, and exiting through the other. The consequences can
be fatal."

There is a drawing of a power line, and someone trying to leave the car that
brought it down, and standing on two of these neatly drawn circles.

Now, I can see what the article (and you) are saying, but what I didn't
understand was the mechanism that caused these "expanding concentric
circles" of potential difference, which would be close enough together -
what's the length of a step, 45cm (18"") perhaps? - to allow you to step on
two of them together. At what speed are they expanding ? Presumably, slowly
enough to allow you to step on two for long enough to fry you ? Or has the
person writing the article, or the artist, got it basically wrong, or are
attempting to over-simplify or create a non-realistic model that might
better allow a semi-layman to understand the concept ? I can see, maybe, how
you might get a travelling wave radiating out from a high voltage point, but
with a wavelength equivalent of several hundred kilometers at 50 or 60Hz,
and I might be able to see how you could possibly get an interference
pattern between multiple downed phases, but I'm struggling with the basic
electrical physics of this. Any other thoughts, Michael ? Anyone ?

The article was a H&S publication from 2003 by National Grid, who apparently
own a number of large utility companies your side of the pond.

Arfa

Arfa Daily wrote in message
...
Sorry all. O.T. as a repair issue. Just something I read about today
that
I'm not sure I understand.

Anybody come across the term "step potential" or its effect in regard to a
downed live power line in contact with the ground ?

Arfa



Hollywood versions show the severed end of the cable, in intermittant
contact with the ground, snaking about like an unsupported hosepipe issuing
a water jet, - true, I don't know ?

The advice , if caught in a thunderstorm is to stand, or preferably crouch ,
with legs together, to avoid a ground distribution potential difference if
there is a nearby lightning strike. A direct hit, then likely a gonner, like
Cardiff boy last week.


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

Arfa Daily wrote:
snip
Now, I can see what the article (and you) are saying, but what I didn't
understand was the mechanism that caused these "expanding concentric
circles" of potential difference,

Does the article imply or depict a non-linear potential gradient?

which would be close enough together -
what's the length of a step, 45cm (18"") perhaps? - to allow you to step on
two of them together. At what speed are they expanding ? Presumably, slowly
enough to allow you to step on two for long enough to fry you ? Or has the
person writing the article, or the artist, got it basically wrong, or are
attempting to over-simplify or create a non-realistic model that might
better allow a semi-layman to understand the concept?

I would expect that all of this would be highly dependent on soil
conductivity and the underlying geology and that models must exist for
a variety of soil types, locations and conditions as this must be
a topic of importance to utility companies and insurers. In
rural areas where "stray voltage" complaints are encountered, the
utilities must have some good data. I suspect that natural resource
exploration businesses also have data and instrumentation.

The essential concern is for a high radial potential gradient from the
point of the downed line; I wonder if there are electrolysis effects
that would decrease soil resistance over time that could cause
an "expanding" gradient.

Michael

Arfa Daily wrote:

"Michael A. Terrell" wrote in message
m...

Arfa Daily wrote:

Sorry all. O.T. as a repair issue. Just something I read about today
that
I'm not sure I understand.

Anybody come across the term "step potential" or its effect in regard to
a
downed live power line in contact with the ground ?


It is the voltage drop across the surface of the soil or concrete,
and if it is high enough simply taking a step will electrocute you.
There was a story about this on an Orlando TV station a few years ago
about a police horse being electrocuted in the downtown area. An old
outdoor sign had been removed, and the wires abandoned. Apparently,
someone turned the breaker back on, and it didn't trip. The officer was
on patrol when his horse crossed the path, and was killed. The shorter
the path to a good ground, the higher the available voltage is, per
step.



Ok Michael. This is kind of the way this article describes it, except it
states thus :-

"When a power line comes in contact with the ground, electrical energy
spreads out through the surrounding earth - like the ripples in a pond from
a tossed stone. Each expanding concentric circle has a different electrical
potential. Under these circumstances, taking a step could place your feet in
areas of different voltages, allowing electricity to course through your
body, entering one foot, and exiting through the other. The consequences can
be fatal."

There is a drawing of a power line, and someone trying to leave the car that
brought it down, and standing on two of these neatly drawn circles.

Now, I can see what the article (and you) are saying, but what I didn't
understand was the mechanism that caused these "expanding concentric
circles" of potential difference, which would be close enough together -
what's the length of a step, 45cm (18"") perhaps? - to allow you to step on
two of them together. At what speed are they expanding ? Presumably, slowly
enough to allow you to step on two for long enough to fry you ? Or has the
person writing the article, or the artist, got it basically wrong, or are
attempting to over-simplify or create a non-realistic model that might
better allow a semi-layman to understand the concept?


That description is more than simplistic, it is completely wrong.
There are usually more that one good ground path, so you can have a
gradient even if you walked in perfect circles. It all boils down to
Ohm's law, and the current path to every point that is well grounded.
If you have no other choice, you want to take as small of a step as
possible, but a pair of dry shoes add a lot of protection. It's possible
that the ground might only be 10 feet from where the wire touches the
ground. If that is a 7200 volt line, that would be 720 volts/foot. In
that case you would get a shock, even if you are standing on one foot.


I can see, maybe, how
you might get a travelling wave radiating out from a high voltage point, but
with a wavelength equivalent of several hundred kilometers at 50 or 60Hz,
and I might be able to see how you could possibly get an interference
pattern between multiple downed phases, but I'm struggling with the basic
electrical physics of this. Any other thoughts, Michael ? Anyone ?

The article was a H&S publication from 2003 by National Grid, who apparently
own a number of large utility companies your side of the pond.


It was probably written by the marketing department. A good technical
paper could run 100 pages or more, and include actual case studies. No
one outside the industry would read that, so the dumb it down to the
point most people will at least take a quick look before tossing it into
the trash.


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In article ,
"Michael A. Terrell" wrote:

It all boils down to
Ohm's law, and the current path to every point that is well grounded.
If you have no other choice, you want to take as small of a step as
possible, but a pair of dry shoes add a lot of protection. It's possible
that the ground might only be 10 feet from where the wire touches the
ground. If that is a 7200 volt line, that would be 720 volts/foot. In
that case you would get a shock, even if you are standing on one foot.

I'm envisioning an episode of Man vs. Wild here...

"I'm being dropped down into a veritable electrical minefield, with
hundreds of writhing high tension lines covering the ground for as far
as the eye can see. I'll show you how to thread your way through this
potentially (sorry) lethal maze by using your pocketknife to fashion a
crude voltmeter from an eyeglass hinge, a bootlace grommet, and some
carefully woven root fibers from the native weeds."

"Smitty Two" wrote in message
news
In article ,
"Michael A. Terrell" wrote:

It all boils down to
Ohm's law, and the current path to every point that is well grounded.
If you have no other choice, you want to take as small of a step as
possible, but a pair of dry shoes add a lot of protection. It's possible
that the ground might only be 10 feet from where the wire touches the
ground. If that is a 7200 volt line, that would be 720 volts/foot. In
that case you would get a shock, even if you are standing on one foot.

I'm envisioning an episode of Man vs. Wild here...

"I'm being dropped down into a veritable electrical minefield, with
hundreds of writhing high tension lines covering the ground for as far
as the eye can see. I'll show you how to thread your way through this
potentially (sorry) lethal maze by using your pocketknife to fashion a
crude voltmeter from an eyeglass hinge, a bootlace grommet, and some
carefully woven root fibers from the native weeds."

Bear Grylls could do it too ! Do you get his TV shows there ? He's an ex
British Special Services survival specialist, and he's a sort of cross
between an escaped mental patient, and Steve Irwin ... Totally mad, but full
of *really* useful and practical nuggets of survival technique. He really is
the sort of bloke that you would want with you, if you found yourself
somewhere really inhospitable ...

Going back to the step potential question, thanks all for your thoughts. You
all seem to understand it pretty much as I (thought I) did. I think that the
concentric circles thing is just an attempt at simplification so that it
makes for an 'interesting' and understandable in principle article in a
company mag for general reading by everyone i.e. office staff, managers,
those in the field and so on. I think we'll settle on there being a
potential gradient between the downed cable's point of contact with the
ground, and the points around it where the land sinks back to inert ground
potential. We'll also take it that this could be largely round, and very
gentle if you were in an open field, and might be far from round and steep
in particular directions towards nearby 'good' grounds in an urban area.
Further, the gradient might be linear, and might not, depending on ground
conductivity in any direction between the point of contact, and inert ground
potential. Sound a reasonable analysis ?

Arfa

Arfa Daily wrote:

snip


Going back to the step potential question, thanks all for your thoughts. You
all seem to understand it pretty much as I (thought I) did. I think that the
concentric circles thing is just an attempt at simplification so that it
makes for an 'interesting' and understandable in principle article in a
company mag for general reading by everyone i.e. office staff, managers,
those in the field and so on. I think we'll settle on there being a
potential gradient between the downed cable's point of contact with the
ground, and the points around it where the land sinks back to inert ground
potential.

snip

Geoff,

You didn't answer my question from my previous reply: does the article
imply or depict "expanding" equipotential rings around the downed
cable over time, thus implying some electro-chemical effect that
makes the area more dangerous over time?

Michael

"N_Cook" wrote in
:

Arfa Daily wrote in message
...
Sorry all. O.T. as a repair issue. Just something I read about today
that
I'm not sure I understand.

Anybody come across the term "step potential" or its effect in regard
to a downed live power line in contact with the ground ?

Arfa



Hollywood versions show the severed end of the cable, in intermittant
contact with the ground, snaking about like an unsupported hosepipe
issuing a water jet, - true, I don't know ?

Seen it. I was at a gas station north of LSU one day when linemen were
working to straighten a power pole.
They were tightening a guy wire when the winch slipped.

The pole snapped back to 'relaxed position' (about a 3 degree lean) and the
impulse traveled down the power lines.

The power lines started swinging to-and-fro. The linemen ran. I didn't know
why.

The wires touched on a swing and arced. Bzzzzzt. A loud, rough, 60 Hz
note.

The arc cut the wire.
The hot wire dropped onto the pavement and started jumping around, sparking
and buzzing at each contact.

I am standing about 50 feet away, putting gas in my car.
Cars on the street are slamming on their brakes and dodging the hot wire.

Finally, the circuit breaker tripped, killing the power.

THEN I had time to be scared.

I suspect the jumping is due to the magnetic field caused by the current
flow.
One section of the wire attracts or repels another, nearby section of the
same wire.



--
bz 73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

remove ch100-5 to avoid spam trap

"Arfa Daily" wrote in
:

We'll also take it that this could be largely round, and very
gentle if you were in an open field, and might be far from round and
steep in particular directions towards nearby 'good' grounds in an urban
area. Further, the gradient might be linear, and might not, depending on
ground conductivity in any direction between the point of contact, and
inert ground potential. Sound a reasonable analysis ?

Yes.

Go out in your yard and drive two screwdrivers into the ground 1 foot
apart.
Measure the resistance between them.

Picture a 40 kV line touching earth at some point.
calculate the resistance of concentric rings.

Keep in mind the fact that 'ohms per square' is a useful concept:
A square resistor of ANY dimensions, made of the same material(and same
thickness), will have the same resistance.
This may seem odd, but if you picture putting more resistors in series
(making it longer) at the same time that you put more in parallel (making
it wider), you will see why it is true.

Anyway, you should be able to calculate the voltage gradient around that 40
kV grounding to YOUR soil and then see how far apart two bare feet would
need to be for you to get electrocuted at various distance from the
grounding.





--
bz

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

remove ch100-5 to avoid spam trap

bz wrote in message
98.139...
"N_Cook" wrote in
:

Arfa Daily wrote in message
...
Sorry all. O.T. as a repair issue. Just something I read about today
that
I'm not sure I understand.

Anybody come across the term "step potential" or its effect in regard
to a downed live power line in contact with the ground ?

Arfa



Hollywood versions show the severed end of the cable, in intermittant
contact with the ground, snaking about like an unsupported hosepipe
issuing a water jet, - true, I don't know ?

Seen it. I was at a gas station north of LSU one day when linemen were
working to straighten a power pole.
They were tightening a guy wire when the winch slipped.

The pole snapped back to 'relaxed position' (about a 3 degree lean) and
the
impulse traveled down the power lines.

The power lines started swinging to-and-fro. The linemen ran. I didn't
know
why.

The wires touched on a swing and arced. Bzzzzzt. A loud, rough, 60 Hz
note.

The arc cut the wire.
The hot wire dropped onto the pavement and started jumping around,
sparking
and buzzing at each contact.

I am standing about 50 feet away, putting gas in my car.
Cars on the street are slamming on their brakes and dodging the hot wire.

Finally, the circuit breaker tripped, killing the power.

THEN I had time to be scared.

I suspect the jumping is due to the magnetic field caused by the current
flow.
One section of the wire attracts or repels another, nearby section of the
same wire.



--
bz 73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

remove ch100-5 to avoid spam trap


Current inducing reaction to earth's magnetism, Fleming's rule fashion ?
As you mention poles, I assume that was no higher than 5 or 6KV

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

Current inducing reaction to earth's magnetism, Fleming's rule fashion ?
As you mention poles, I assume that was no higher than 5 or 6KV




It was quite possibly much higher. Lines through residential areas are
usually 7.2KV in respect to ground, but 35KV lines on wooden poles are
common too.

"James Sweet" wrote in news:ca9Nj.15971$vz2.12679
@trndny05:





Current inducing reaction to earth's magnetism, Fleming's rule fashion ?
As you mention poles, I assume that was no higher than 5 or 6KV




It was quite possibly much higher. Lines through residential areas are
usually 7.2KV in respect to ground, but 35KV lines on wooden poles are
common too.

I have no idea of the tension on the poles but things were quite tense on
the ground.





--
bz 73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

remove ch100-5 to avoid spam trap

Michael A. Terrell wrote:

That description is more than simplistic, it is completely wrong.
There are usually more that one good ground path, so you can have a
gradient even if you walked in perfect circles. It all boils down to
Ohm's law, and the current path to every point that is well grounded.
If you have no other choice, you want to take as small of a step as
possible, but a pair of dry shoes add a lot of protection. It's possible
that the ground might only be 10 feet from where the wire touches the
ground. If that is a 7200 volt line, that would be 720 volts/foot. In
that case you would get a shock, even if you are standing on one foot.

Shock as in muscular contraction in the foot?

Terryc wrote:

Michael A. Terrell wrote:

That description is more than simplistic, it is completely wrong.
There are usually more that one good ground path, so you can have a
gradient even if you walked in perfect circles. It all boils down to
Ohm's law, and the current path to every point that is well grounded.
If you have no other choice, you want to take as small of a step as
possible, but a pair of dry shoes add a lot of protection. It's possible
that the ground might only be 10 feet from where the wire touches the
ground. If that is a 7200 volt line, that would be 720 volts/foot. In
that case you would get a shock, even if you are standing on one foot.

Shock as in muscular contraction in the foot?


Yes. Enough to knock you to the ground and expose you to a higher
gradient. One that will be fatal.


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bz wrote:

I have no idea of the tension on the poles but things were quite tense on
the ground.


Everyone needed a round of new underware?


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In article ,
"Arfa Daily" wrote:

Bear Grylls could do it too ! Do you get his TV shows there ? He's an ex
British Special Services survival specialist, and he's a sort of cross
between an escaped mental patient, and Steve Irwin ... Totally mad, but full
of *really* useful and practical nuggets of survival technique. He really is
the sort of bloke that you would want with you, if you found yourself
somewhere really inhospitable ...

Yeah, he's here. Um, I know it's bad form to denigrate the dead, but are
you implying that Steve Irwin *wasn't* an escaped mental patient?

Maybe I'm delusional, but somehow I don't think I'm going to be walking
down an unfamiliar street in a foreign city, take a wrong turn, and find
myself in the middle of a simmering lava field... so I might never need
a friend like Bear.

Smitty Two wrote:

In article ,
"Arfa Daily" wrote:

Bear Grylls could do it too ! Do you get his TV shows there ? He's an ex
British Special Services survival specialist, and he's a sort of cross
between an escaped mental patient, and Steve Irwin ... Totally mad, but full
of *really* useful and practical nuggets of survival technique. He really is
the sort of bloke that you would want with you, if you found yourself
somewhere really inhospitable ...

Yeah, he's here. Um, I know it's bad form to denigrate the dead, but are
you implying that Steve Irwin *wasn't* an escaped mental patient?

Maybe I'm delusional, but somehow I don't think I'm going to be walking
down an unfamiliar street in a foreign city, take a wrong turn, and find
myself in the middle of a simmering lava field... so I might never need
a friend like Bear.


With friends like that you don't need enemies.


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"msg" wrote in message
news:KZqdnXI3Eb3odZnVnZ2dnUVZ_umlnZ2d@cpinternet.. .
Arfa Daily wrote:

snip


Going back to the step potential question, thanks all for your thoughts.
You all seem to understand it pretty much as I (thought I) did. I think
that the concentric circles thing is just an attempt at simplification so
that it makes for an 'interesting' and understandable in principle
article in a company mag for general reading by everyone i.e. office
staff, managers, those in the field and so on. I think we'll settle on
there being a potential gradient between the downed cable's point of
contact with the ground, and the points around it where the land sinks
back to inert ground potential.

snip

Geoff,

You didn't answer my question from my previous reply: does the article
imply or depict "expanding" equipotential rings around the downed
cable over time, thus implying some electro-chemical effect that
makes the area more dangerous over time?

Michael

Sorry, no. The paragraph that I copied to the second post, is the entire
'technical' content of the article. It's only a half-pager of three
paragraphs, the first being

"Have you ever heard of ? ......."

and the third being

"so, should you find yourself in a situation ...... ..... here's how to
........ "

Apart from this, just the cg piccy of the crash-test dummy walking away from
the car with his feet planted firmly on two of the illustrated 'rings'.

The line that prompted all this was

"Each expanding concentric circle ..." See post with whole paragraph
to place this in context.

Arfa

"Smitty Two" wrote in message
news
In article ,
"Arfa Daily" wrote:

Bear Grylls could do it too ! Do you get his TV shows there ? He's an ex
British Special Services survival specialist, and he's a sort of cross
between an escaped mental patient, and Steve Irwin ... Totally mad, but
full
of *really* useful and practical nuggets of survival technique. He really
is
the sort of bloke that you would want with you, if you found yourself
somewhere really inhospitable ...

Yeah, he's here. Um, I know it's bad form to denigrate the dead, but are
you implying that Steve Irwin *wasn't* an escaped mental patient?

Maybe I'm delusional, but somehow I don't think I'm going to be walking
down an unfamiliar street in a foreign city, take a wrong turn, and find
myself in the middle of a simmering lava field... so I might never need
a friend like Bear.

Ah, but one day, you might be taking a well earned vacation, and have your
plane come down in deepest Alaska in the winter (it got blown off course,
and couldn't spot its own navigational problems, because by then, lead-free
solder had crept into avionics, and a tin whisker had compromised the
primary navigation computer in such a way as not to be spotted by the pilot
:~} ) and only you and one other guy survive. Bet you'd be glad then that
you were sitting next to Bear ... !!

Arfa

"bz" wrote in message
98.139...
"N_Cook" wrote in
:

Arfa Daily wrote in message
...
Sorry all. O.T. as a repair issue. Just something I read about today
that
I'm not sure I understand.

Anybody come across the term "step potential" or its effect in regard
to a downed live power line in contact with the ground ?

Arfa



Hollywood versions show the severed end of the cable, in intermittant
contact with the ground, snaking about like an unsupported hosepipe
issuing a water jet, - true, I don't know ?

Seen it. I was at a gas station north of LSU one day when linemen were
working to straighten a power pole.
They were tightening a guy wire when the winch slipped.

The pole snapped back to 'relaxed position' (about a 3 degree lean) and
the
impulse traveled down the power lines.

The power lines started swinging to-and-fro. The linemen ran. I didn't
know
why.

The wires touched on a swing and arced. Bzzzzzt. A loud, rough, 60 Hz
note.

The arc cut the wire.
The hot wire dropped onto the pavement and started jumping around,
sparking
and buzzing at each contact.

I am standing about 50 feet away, putting gas in my car.
Cars on the street are slamming on their brakes and dodging the hot wire.

Finally, the circuit breaker tripped, killing the power.

THEN I had time to be scared.

I suspect the jumping is due to the magnetic field caused by the current
flow.
One section of the wire attracts or repels another, nearby section of the
same wire.


--
bz 73 de N5BZ k

I don't know about that. I think that it might be due to the force of the
mini-explosion - i.e. expanding air blast - each time it touches. The only
reason I say this is because I saw something a couple of weeks back, that in
35 years of service work, I had never seen before. I had a switch-mode power
supply on the bench. An internal one from an LCD TV. The design has a PFC
boost converter stage on the fron end. When working correctly, it shoud
produce 390v across the main filter cap for all input voltages from a
hundred or so, up. The filter cap is laid on its side, soldered to two pins.
When I powered this one, it was going "CRACK - CRACK - CRACK" very loud. It
took me by surprise, but I quickly turned off and checked for any sign of
arcing. There was none, so I covered my eyes, and turned back on. The main
filter cap should be stuck down to the board, but they have always broken
free on these boards, so are secured just by being soldered to the pins. On
this one, when I looked closer, the cap was bouncing with each "CRACK !" It
was in fact arcing internally with enough force to physically move it - and
it's not a small cap. The reason was that the regulation feedback on this
converter had failed, and there was over 700v across the cap ...

Arfa

Michael A. Terrell wrote:

Shock as in muscular contraction in the foot?

Yes. Enough to knock you to the ground and expose you to a higher
gradient. One that will be fatal.

All you can really do is wear non-conducing footwear and hope any effect
is minimal and that the safety equipment trips the line very quickly.

As a realistic worry, probably lightning strike is far more likely.

Terryc wrote:

Michael A. Terrell wrote:

Shock as in muscular contraction in the foot?

Yes. Enough to knock you to the ground and expose you to a higher
gradient. One that will be fatal.

All you can really do is wear non-conducing footwear and hope any effect
is minimal and that the safety equipment trips the line very quickly.

As a realistic worry, probably lightning strike is far more likely.


So, you've been to Florida durning one of our major storms? 11,000+
lightining strikes were reported in a half hour. a couple years ago.

OTOH, you have a choice most times, to not go near a downed HV power
line.


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Terryc wrote:
Michael A. Terrell wrote:

Shock as in muscular contraction in the foot?

Yes. Enough to knock you to the ground and expose you to a higher
gradient. One that will be fatal.

All you can really do is wear non-conducing footwear and hope any effect
is minimal and that the safety equipment trips the line very quickly.

As a realistic worry, probably lightning strike is far more likely.

Could be, but nonconductive shoes quickly turn conductive when wet.
IME, most power lines get downed in storms. Sometimes they get downed
when cars hit the utility pole; but again, most accidents happen during
storms as well.

Based on that, I'd venture to say that much of the time that a power
line comes down, the ground around it is wet. Consequently caution is
indeed called for.

jak