Seven steps to electrical safety

Here are my thoughts on this misunderstood subject.

The step potential is created by you taking a step to a higher voltage, not a sudden change in the voltage potential. I like the topographic representation since topography like the voltage does not usually change instantaneously. The rings are just lines of equal elevation/potential on a continuous function You can get sever distortions from the idea circle by local effects (buried conductors, soil type, moisture content, etc.). The problem with hopping is falling and getting a large potential across your total body. And no Glen, a birds feet do not tingle, since they have a very small voltage differential between them.

I don't think the frequency has much to do with the resistance since the reactive part of the impedance will not change that much relative to the resistance. I also don't think skin depth is an issue since we are talking about a distributed system. It is also probably safe to ignore transmission line effects. There are a number of sites where you can find the resistivity of the earth under different conditions, but you cannot accurately measure it with a conventional ohm meter (DMM).

As far as the zone of influence goes there are some unstated assumptions in the 33ft guideline. Line voltage and soil conditions/characteristics are the big ones, but it is probably not too bad. What I would worry about is that anything that touches the interior of the zone can potentially increase the zone in weird ways. If you have a tree that is not contacting the line but was uprooted next to the fallen line can you safely cut the top out if it is 100' away? This is well outside the 33' zone, but given that the roots/stump are at an elevated potential I would make the assumption the whole tree was. The likely situation is that the top will be at a lower potential since it will likely be somewhat grounded by all the limbs touching/in the ground, but what happened when you finish the cut and the top is no longer connected to the tree but the saw still is? You always have to consider the sneak paths!

Thinking that the voltage is being entirely dissipated in the zone isn't really correct either! To me the zone represents the point where the differential voltage has not decayed to a safe level. The fact of electricity is that the current created by the downed line must be returned to the source. This could be the primary generation site or a distribution center. In actuality there are two zones! One where the current goes into the ground and one where it returns to the source. To me this is best represented as the contact resistance to/from the earth. The effects in the zone are nonlinear because of the half spherical nature of the contact. There is a mostly linear decay between the fault and the source. The correct way to explain the contact resistance is to setup and solve an advanced calculus equation, but I don't think many here would appreciate that so here is a mental way to think about it.

The line has a certain current flowing in it which is going into the ground. The ground isn't a very good conductor (it has a high unit resistance). From basic electronics the voltage drop is equal to the current times the resistance. Since the point that the line contacts the ground represents a small area (has high resistance) the incremental voltage drop is high in that local area. As we move farther from the contact point the area the current flow across increases as the square of the distance (½ the surface area of a sphere) so the incremental voltage drop is reduced by that same amount. At some point the incremental voltage drop is no longer a safety issue which is the edge of the safe zone.

Here are a couple of last things. The voltage isn't what is dangerous! It is the current that actually kills you. Here is a link to more information. One extreme example of this is that Tesla used to grab coils that produced half a million volts without harming himself. The other thing is that the power system does not use the earth as the return for the hot lines! Most power systems are balanced three phase either Y or delta and what we call a neutral in residential or Y circuits is locally created and connected.

That's enough for now!

Cary
 
You are welcome Tom,

I used more field specific terms than I would have liked to, but people can always ask for clarification if they need it. My limitations are certainly more along the time axis than the feel axis. I realize that probably sounds strange coming from someone who spends most of his days working/hanging around the house.

Cary
 
Our teacher here said the same. He said that you should shuffle and be extremely careful not to fall down! He said that this was the practice when you step into a ground radiant situation and you start to tingle. That's a real good time to shuffle!
 
When I was working for Davey your throwline was your best friend not only for setting ropes but for pulling energized lines out and away from a tree closer to a tree or spread them to let limbs and wood fall in between them . I have used throwline on 12kv 21kv 60kv and 230kv , The 230 was a little nerve racking the first time and will not recomend do so . We also used climbe line for the same purpose . I recommend slick line instead of climbing line due to moisture and dirt absorbtion . During storm damage in the mountains a few years ago I had to climb a cedar and basically make a pole out of it to temporarily replace a broken one , the line was tied to the tree using "hot rope" which was similar to my throwline but much larger diameter . I also used to hang my pruner from lines (12kv) on a regular basis . We would also use our pruner if need be to cut the line if there was an emergency , however I can only see this working on older copper or steel line. I have been tickled before ,from indirect contacts with limbs . Be careful out there . If you are using a lift truck and it becomes energized DO NOT make contact with ground and truck/chipper at same time. Many have been killed from this senerio. Heard stories of peoples legs and hands getting blown off , chests exploding , all kinds of sad stuff
 
No offense daveyclimber but spreading the phases to let limbs fall through???? I've put my throwline over 7200, 12 kv delta, 14.4, and 34.5 delta with no problems just to install ropes into trees. I've even heard of guys putting it over 115 kv.
When you hang your pruner from 12 kv is it di-electrically tested every year?
 
Not whole limbs just pieces , it is amazing what you will do when the power co refuses a line drop. You can get close to a ten foot spread on two phase , three phase was not as accomidating Pruners were
replaced on regular basis due to breakage with poly rope and never on a rainy day . Practices were not Davey Approved but as alot of you know when you work line clearance you must make compromises and to the impossible with the least amount of resourses possible . That is why the statistics are so high for line clearance workers
 
maybe those stats should be reason enough to do something different! If you dont need to put yourself at risk you shouldnt. How much were you paid????was it worth the chance of not collecting the check??
 
When I left I was making a measley $16.58 an hour as journeyman climber ,when I was forman it was around $17.00 hr. I must say it was the most exciting job at times , scary as well . But it formed me to be the best that I can be at what I do , thats why I am still alive ,skill with alittle luck tossed in
 
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