O.T. Step Potential ...

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

that

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

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

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

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

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 news:fu1st2$i9g$ snipped-for-privacy@registered.motzarella.org:

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.

"Arfa Daily" wrote in news:lt5Nj.45983$ snipped-for-privacy@newsfe5-win.ntli.net:

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.

the

know

sparking

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

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:

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

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

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

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

Arfa