Question About Electrical Safety

[alexibm]

Hi !
I found a really nice website "Lessons In Electric Circuits"

My question, looking at the picture, why both people will not get electricuted ?
Obiviosly person at the top is electrically common with the tree and person on the buttom is not........and he is shocked.

However both people are not electrically common and both have ground connection ?

That is the website: http://www.ibiblio.org/obp/electricCircuits/DC/DC_3.html , where I found information.
Thank you

 

[PRR]

Put some numbers on that thing.

Assume ground is zero volts, the tree has negligible resistance (true-enough compared to a human body).

Top person has zero volts on the hand, zero volts on the foot, zero minus zero is zero, that's the shock she feels.

Bottom person has 1,000V on the hand, zero volts on the foot, 1,000V minus zero is 1,000 volts,a big buzz.

Bird has zero volts on the foot, no other good connection to anything. Even if the bird stood on the 1,000V line, the fact that the bird has no good second connection means that no current will flow. (OK, if we raised the voltage to 1,000,000 volts, electrons start jumping off into the air, perhaps enough current to fry the bird.)

 

[Butterylicious]

1st off, the word electrocution is defined as to kill with electricity. Shock is defined as The sensation and muscular spasm caused by an electric current passing through the body or a body part. As an electrical contractor, this misuse of the word is a pet peeve.

In the senario pictured, one of the phase conductors is connected to earth via the tree. Just like with little transformers in the gadgets we mess with, you can earth 1 lead of a secondary. Same thing holds true for even the big trannies. Once upon a time, I worked on a 480 volt 3 phase delta system that was corner grounded. Scared me to death. It was an ancient brick factory out in the country. Really old, really not right on a modern standard. We upgraded the system to a 277/480V wye system which required adding a 3rd phase conductor to most of the branch circuits since their 3 phase equipment was using pipe as the corner grounded leg and 2 hots inside. Like I said, it was BAD NEWS.

 

[alexibm]

Hi !
Thanks for reply.

Picture with the numbers made a lot of sense.
I would have to rework my basics than.

My idea was that one of the guy is getting zapped the other should be getting zapped too, since they are not electrically common and both standing on the ground.

 

[Ethan]

What about the wrist grounding strap... By using one you protect ESD sensitive parts but basically make yourself a conduit to ground. I suppose the idea is that most ESD sensitive parts operate at low voltage and low current, that it doesn't pose a huge danger in those instances (unless you're at the AC power inlet).

Does anyone have any other explanation for that one? I've just always figured the strap isn't to protect your life in the slightest, just the life of the cheap $0.50 IC... :roll:

 

[PRR]

> I've just always figured the strap isn't to protect your life in the slightest, just the life of the cheap $0.50 IC...

We know what is important.

However, for the chip's protection (and incidentally yours), any proper tech-ground has a resistance in there. Static charge is small, 100K or even 1Meg will bleed it off pretty quick. If you stick your finger in the light socket, only 120V/100K= 1.2mA will flow: not enough to roast flesh, not enough to zap your heart as long as the 1mA is spread over your whole chest. (If you do open-heart surgery, you have to be careful about the mA.)


Anyway, as the birdie shows: if you must grab live wires, the best place to be is NOT on ground or connected to ground. The hard-grounding strap goes on the case, conduit, building structure, to divert stray current BEFORE it reaches a person. (This is one of 99 reasons Buttery was upset to find "pipe" conduit used as a current carrying conductor.)

> isn't to protect your life in the slightest, just the life of the cheap $0.50 IC...

Well, the failed IC has to be found and replaced, which can easily cost more than the grounding straps. It can also fail "later": the oxide is weak but passes factory tests, fails in the customer's hands, which is a bigger headache than a few straps. And arguably it could wait to fail until the Enemy attacks, putting someone's life at increased risk. Awful lot of chips in radar systems, both military and the next airplane or ship you ride in. (Except in NYC) subways are chip controlled: there is a train stalled ahead of yours, but the chip-input reading that section is stuck-low because of poor chip workstation grounding.

 

[NewYorkDave]

like over 1m

1m won't do much to limit the current, but 1M will. :razz: :razz:

 

[AnalogPackrat]

[quote author="Chae H Ham"]I danced around on the carpet shocking him repeatedly until he figured it out :grin:[/quote]

Nothing like a little reality check every now and then, is there? Love it! :grin:

A P

p.s. Anyone remember the old psych experiment... "Wrong! 280 Volts!"

Edit: found it Milgram's Obedience Experiment.

 

[CJ]

<the tree has negligible resistance (true-enough compared to a human body)>.


I don't know, I just ohmed out a pine tree outside and I couldn't get anything but a couple of strange looks from some folks outside.

I think wood is a pretty good insulator compared to the body, which is 90 percent salt water. Especialyy if the tree has thick bark like a redwood.
True, once you arc thru to the tree center with HV, everything changes.

E-meters at the Scientology center are calibrated for 8000 ohms men, and 12,500 women, if I remember correctly.

It's funny, when you are ill, this reading goes up.

This is grabbing lemonade cans, standard electrodues for the poor victims of this hideous cult! :razz:

 

[PRR]

> I think wood is a pretty good insulator compared to the body

You may be right. I was going by the apparent point of the diagram, which seems to assume that the tree conducts well enough to shock you.

Both the tree and the person are watery blobs covered in "bark". Dry people skin is around 100K for a small contact; the E-meter has large contacts and newbies are sweaty. Dry pine bark might be well over that figure, but I know I have been zapped through damp wood.

Also: if you have 13,000V (a typical figure for the high voltage on a streetpole), you conduct 10 times better than the tree, the voltage divides 12,000V for the tree, 1,200V for you. Ouch!

Not to mention the damage I've seen from lightning hitting a tree. Power lines are not so powerful, but they sure can pass enough current into a tree to do big damage. You don't want to be part of that circuit.

Hmmm... wood poles have been used to do hot-fixes on high voltage lines. But they have to be dry wood. The linemen stored them in a hot closet, tested them every morning and several times a shift. One of the first commercial uses of FiberGlass was to replace wood hot-sticks: with the right resin, dampness is far less of a problem. They can also be tougher and lighter, but the damp-factor was the killer reason wood hot-sticks vanished. Too much work keeping wood dry.

 

[CJ]

Yes, I had a tree blow up about 20 yds from my head one morning.
I thought it was another 911. Looked out the window and water was coming out of the 6 foot high stump.

What the.....

The electricity went thru the tree and into a water pipe which ran underneath the trunk.
The severed pipe was letting the water into the now hollowed out trunk.
'
I got a video of that, pretty weird.
Waterfall coming out of a tree.
Yes, it was loud!

 

[Larrchild]

Here's a guy that needed to read this: http://members.tripod.com/~StormTrooper_2/index2.htm


Is the voltage drop through the tree the square of the root?

 

[CJ]

That was almost as good as the "bus compression" joke I saw pertaining to the bus who smashed thru a studio recently.
:grin:

 

[The Kid]

Bottom person has 1,000V on the hand, zero volts on the foot, 1,000V minus zero is 1,000 volts,a big buzz.

Well, technically, wouldn't it be 0 minus 1000V? As drawn, the negative end of the voltage source (battery) is the bottom. Still a big buzz, though!
Also, the minute the tree branch is removed the top person is in trouble! :idea: