Seven steps to electrical safety

A natural fiber rope wouldn't be a good choice. Polypropylene rope doesn't absorb water and would be a better starting point.

Last night Jim and I were talking about this issue. Something that we talked about was soil impedence as a matter of fact. Since there is no way to know what to expect unless there were lots of testing we came up with a visual to help understand how to think about what's going on underground.

What do you think of this?

The bounce of the tennis ball is used to illustrate soil impedence.

If a tennis ball is dropped onto a hard surface the frequency of the rebound will diminish uniformly. It would look like the graph in the OSHA document.

Now, drop the tennis ball onto a hard surface and let the second bounce go to turf or sand. The soft surface has more resistance and absorbs the energy.

If the tennis ball hit a hard, then soft, then hard surface the graph would look different too. And on and on....

What I'm trying to do is get some visual models to explain what's going on.

Jim said that the 10 meter radius is only for 60 hz electricty like we see in the US and some other parts of the world. Where power comes in other packages the radius would be different I think.
 
Where power makes contact it creates a zone of influence that will kill you.
The closer you are to the contact point within the zone of influence the higher your chances of getting electrocuted.
People should be able to understand something as simple as that without drawing it out for them.
Stay out of the zone of influence.
 
I think what we are trying to get a handle on is this zone of influence.

I was setting up a perimeter to drop a hazard tree today. I was the person responsible to establish the zone of influence. My contentions were the power of gravity as well as the power of students, like masses in motion, tend to stay in motion, will not deviate unless you park a big truck in front of them.

Through experience I have a tangible way of establishing that perimeter, estimating vertical falls, swinging, bouncing, et cetera. And then I can add a sufficient margin for error.

But, with electrical hazards, the picture is different. We are trying to get a grasp on what dimensions and the size of the zone of influence and the processes to find that out. We may be the ones responsible for establishing the bounds for influence and more importantly, our behavior is much different when we have to operate inside of those bounds.

Filling in the gaps about my conversation with Tom, I figure the electrical field would be _different_ in 60Hz vs. 50Hz but which way.

Initially we need to understand the soil impedence model and then correctly communicate it boots and jeans terms. I am very excited that we are questioning the various models through a forum rather than in field or live.

With tennis ball bouncing analogy, we should note that the peak of the successive bounces diminishes in a logarithmic curve-- like those in the OSHA documents. Somewhere, someone [how and who are these people is the $1.387 question] has benched 33 feet as a good radius where our invisible bouncing ball called electrical potential a bound.

Would someone locate the references of the ABB Group article: "Reliable detection of high-impedance faults caused by downed conductors" [see link above]

Sounds like a good one:
Downed power lines: Why they can’t always be detected. IEEE Power Engineering Society, New York, Feb 1989.

Rocks are hard.
Water is wet.
Fire is hot.
Gravity sucks.

and I must now add to these,
Electricity is fast.
 
The zone of influence will primarily be governed by the amount of voltage and the impedance properties of the earth within that zone.
My work boots are rated for 20,000 volts but I'm not willing to test them in any zone of influence.
I was working off a strand one windy day when a tree brought the primary down just up the road from me on the same pole lead.
I don't work joint use plant any more on windy days.
Many tree workers are way too complacent around power lines.
Awareness is a good tool to have and use.
 
I saw something that said to use natural fiber rope for moving a downed line. I'd think natural would hold more moisture but maybe it's better to have it absorbed rather than sitting on surface??
Randy

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We performed tests on different ropes to settle a dispute between our Line Department and Forestry and found that a clean, dry polypropylene rope tested well within this utility's guidelines. Dirty dry rope tested even better. As for natural fiber - no way. They tend to form carbon trails quickly, causing phase to phase outages.

Even with this testing to back us up, we erred on the side of good judgement and required our tree trimmers to wear class 2 gloves and 15kv booties while tying down primaries.
 
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Sounds like a good one:
Downed power lines: Why they can’t always be detected. IEEE Power Engineering Society, New York, Feb 1989.

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I think this is a way to get it. Couldn't find actual article...

Downed Power Lines and Why They Can’t be Detected
In 1989, the Power Engineering Society (PES) of the Institute of Electrical and Electronic
Engineers (IEEE) published a paper entitled “Downed Power Lines: Why They Can’t Always be
Detected.” It is an excellent layman’s manual on the what, why, and wherefore’s of downed
power lines. It discusses the basic concepts of electric distribution and the causes of system
faults. Its main point is the discussion on the limitations distribution utilities have in the detection
of and protection system response to high resistance ground faults. It is an excellent booklet for
the education of the public and those individuals that deal with such matters. It is also a
significant resource for safety presentations and meetings.

Currently the booklet is out of print and the IEEE-PES has no plans to reprint. The NRECA T&D System Protection Subcommittee feels that the booklet is an excellent resource for rural electric cooperatives and has secured the rights for reprinting. If you are interested in obtaining one or more or copies of this paper, please contact:
NRECA T&D System Protection Subcommittee
c/o Patterson & Dewar Engineers, Inc.
P.O. Box 1048
Decatur, GA 30031
Telephone: 404-296-5990
Fax: 404-299-3542

There will be a nominal charge for production, shipping, and handling. If you would like more information or have any questions, please call Harvey Bowles, Chief,
Distribution Branch, at (202) 720-5082.
 
Even with this testing to back us up, we erred on the side of good judgement and required our tree trimmers to wear class 2 gloves and 15kv booties while tying down primaries.

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I wonder what they might find for working on secondaries or house drops? This might settle the "clean, dry rope" issue.
 
These guys do some interesting work...

http://www.ground-it.com/

"Grounding safety from step and touch potential hazards. We can calculate potentials that develop in the soil around a ground grid and from that, the step and touch potentials at the soil surface and can compare these against the tolerable levels as defined by the IEEE. We have developed a rational method for calculating step potentials from soil potential information."
 
Outstanding contributions to this thread!!! There are many things that I've learned from reading the links.

Tomorrow I'm doing an EHAP training and a couple of the illustrations will be included.

Thanks :)
 
Well now that I know that the dielectric capacity of Martian soils can be guessed I am holding my breath to see if they can find a way to measure the soil here on earth. How can they extrapolate the soil characteristics of a whole planet from a few tiny samples? How does the soil in Maple Grove relate to the soil in Denver?
 
Step and touch potential can sometimes be difficult to understand for some people. Its good to see a lot more resources on the subject from this discussion.

However, some people just don't get it. The last wind storm we had brought down primary distribution lines everywhere. Before we got to one spot a logger had took it upon himself to clear the tree off the road that was blocking his truck. Then he used a metal rod to push the lines out of his way to get his logging truck through /forum/images/graemlins/confused.gif
He had no concept that the power could have still been on because fuses and reclosers don't always trip...but there was also two 230KV transmission lines just 2 kms down the road that could have created enough induction to charge the line. Some people are luckier than others.
 
Here's a hypothetical that may help some of us understand the voltage gradient using made up numbers.

Suppose a 10,000 volt line is laying on the ground. Right at the point of contact there is 10k volts. Lets say the soil is of a type with an electrical resistance such that at 20 feet distance the voltage is 0 volts. This makes the voltage gradient about 500 volts per foot.(it's never linear)

If one of your feet is lets say 10ft.(5,000 volts) from the wire.....and your other foot is 11 ft.(4,500 volts) from the wire, then you now have 500 volts of potential between your feet.

This 500 volts goes into one foot, through your body and out the other foot. This causes you to fall to the ground. Now you are on the ground and the same voltage gradient is getting you where ever you touch the ground.

If both feet are at the same place there is no difference in the voltage between them and therefore no shock. If the voltage is high enough then the difference between your heel and your toe is going to be enough to bring you down. Once down you're in big trouble.

If you are six feet tall and fall into the above voltage gradient your chin and toes will be six feet apart(worst case). Six feet at 500 volts per foot means you are now getting shocked by 3,000 volts.

I think they picked 33 meters because in the worst case that distance should be safe.

Dan
EE
 
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Take a look at these drawings. I know they're only the beginning but might they help tell the story...

http://www.allaboutcircuits.com/vol_1/chpt_3/7.html

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The drawings seem to contradict some of what has been said in this thread. Several people have described the ground immediately under/next to a downed power line as having about the same or just slightly less electrical charge as the downed line itself. The charge is then said to 'dissipate' or gradually disappear along a gradient, creating a zone of influence, somewhat similar to the way a pebble tossed in the water creates ripples that gradually get smaller as they move away from the rock, or like a line of bright color that gradually gets lighter.

But the article in the link says:

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The earth, being a conductor (if only a poor one), will conduct current between the downed line and the nearest system ground point, which will be some kind of conductor buried in the ground

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and the drawings in the link show the voltage being exactly the same along the entire distance from the downed conductor to the system ground point.

How far apart might the downed line and the system ground point be and still have electricity flow between them?

What do they mean by "...voltage dropped..." in the following:

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there will be substantial voltage dropped between the point of cable contact with the ground and the grounding conductor,

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Does the voltage gradually disappear or is there a steady voltage from the downed line to a system ground point??


/forum/images/graemlins/confused.gif
 
What they are saying is the current wants to flow back to the system ground which completes the circuit.
It will take any acceptable ground to make the easiest and shortest path to ground.
The wire they seem to be referring to is the system ground or neutral which makes sense only if the MGN is close to the source of the primary conductor that happens to be looking for the path back through the system ground.
Where the MGN isn't present it will take the easiest path to ground which might be a vehicle, metal conduit, tree, rope, person etc.
If you drop a primary in a large open field the current will flow out through the ground until it dissipates it won't seek a buried wire because there are none.
 
Thanks Kevin.

What is MGN? Is that the same as what they are calling the 'system ground point'?

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If you drop a primary in a large open field the current will flow out through the ground until it dissipates it won't seek a buried wire because there are none.

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This has always been my understanding of downed lines, and it's consistsent with what has been said in the rest of this thread. But this isn't what is indicated by the drawings. The drawings show a steady current between the downed line and "...the nearest system ground point."

I have another question about these drawings. I have a colleague who is a volunteer fireman and he says that they have been taught that, if they encounter a downed line, they should maintain a distance from the line equal to or greater than the span of one pole. The drawings show current flowing a distance greater than the span between four poles.

The drawings show something that is different from the general picture drawn by the discussion in the rest of this thread. Although the drawings show something that might occur in some (rare?) situations, isn't it much more likely that the electricity will dissipate as it moves farther from the downed line?


If this is so, then the drawings should be shown as something that could occur if the system ground is nearby. But it is much more likely that the current will dissipate over some (uncertain) distance from the downed line.
 
This just doesn't sound right:

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If the power disconnect switch cannot be located quickly enough, it may be possible to dislodge the victim from the circuit they're frozen on to by prying them or hitting them away with a dry wooden board or piece of nonmetallic conduit, common items to be found in industrial construction scenes. Another item that could be used to safely drag a "frozen" victim away from contact with power is an extension cord. By looping a cord around their torso and using it as a rope to pull them away from the circuit...

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An extension cord? The quote comes from a different page of the same website that had the drawings.

emergency response

This adds to my doubt about some of the information that this page provides...
 
Hey Mahk,

Ground is ground the world around. The power companies only run the hot lines because they can pick up ground to complete their circuts almost anywhere......and that saves them 50% of the cost of the lines. A ground rod for a house is 8ft. of copper driven into the ground at each service entrance. Each transformer on the pole also has a ground rod.

The current flow would only reach the system ground if it was the closest ground. The current from a down power line will flow in many parallel paths seeking ground. The paths with the least resistance will get the most current flow.
The drawing of the poles and the downed line seeking system ground is not accurate IMO.

Using an extension cord to pull someone off a power line would be a very poor choice as the cord is a conductor.
 
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