Code related Theory question

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Re: Code related Theory question

I did the math alredy once using a realistic resistance for the human and got some number of nanoamps.
 
Re: Code related Theory question

It was stated that the remaining Phase A conductor had a voltage drop of 2.7 volts, with a load of 200 amps. That gives 0.0135 ohms per conductor.

Let's say our guy is a bit sweaty, so we take him at 20,000 ohms.

The total A phase resistance would be -

0.0135 x 20,000 divided by 0.0135 + 20,000
270 divided by 20,000.0135 = 0.0134999 ohms

Ed
 
Re: Code related Theory question

Taking the man's body resistance at (a low) 20,000 ohms, the 2.7 volts would cause the current through him to be .000135 amps, or .135 milliamps.

Ed

Safety9.gif
 
Re: Code related Theory question

Ed's diagram is perfect.

The point being missed is that the entire circuit is a series circuit which includes A phase, B phase, and the load. The vast majority of the 480 volts is dropped across the load. Very little voltage drop occurs on the phase conductors themselves. And if the guy is in parallel with only one of the phase conductors, he is only getting zapped by the voltage drop that is across the conductor. And that is very little voltage.
 
Re: Code related Theory question

I don't agree you are going to get 20k under even the most insanely optimum conditions.
 
Re: Code related Theory question

Ed, I contend that the precision of our numbers is only good to 3 significant figures, therefore we cannot reliably compute the resistance of 20K and 0.0135 Ohms in parallel. True the parallel resistance will be reduced a smidgen, but that is inconsequential in this problem.

Using Ed's numbers, the problem boils down to:

I = 2.7V/20K = 135 microAmps

If we assume R to be 150K, then:

I = 18 microAmps

If the man feels a tingle, it would be from the 2.7V, not from the 480.

Now if he is barefoot on a damp concrete floor, the current is:

I =480/20K = 24mA

But, that is line to line voltage; line to ground voltage should be less, but the poor guy would still be in a heap. I have read that currents in the order of a few milliAmps cause the body to throw itself away from the contact and possibly save the man's life.
 
Re: Code related Theory question

Edited out redundant post.

[ January 15, 2005, 12:43 PM: Message edited by: rattus ]
 
Re: Code related Theory question

I think I was using 1 or 2 megaohms because thats what I remember measuring squeezing both probes as hard as I possibly could with dry hands.

Edit: In the same fingers less than a quarter inch apart.

[ January 14, 2005, 10:54 PM: Message edited by: physis ]
 
Re: Code related Theory question

Resistance depends heavily on contact area. One the current spreads out in the body, the resistance drops dramatically.

Sqeezing probe tips is quite different from holding the metal handle of an obsolete power drill.

Of course, electricians have calloused fingers from twisting all that wire bare handed, and that raises R considerably.

We all have known smart Alecs who tested fuses with a thumb and a finger.
 
Re: Code related Theory question

Mr. MacLaren
I am impressed with your math but with the highest of respect I have to disagree because I come up with different numbers. The way that you came up with your numbers was you divided 2.7 volts by 20,000 ohms and came up with .000135. This is not the correct way to solve this problem.
Should we decide to short cut this and find an approximation of the amperage then we would divide 480 volts (the applied voltage) by the 20,000 ohms of resistance of the mans body and this would give us .024 or 24mA and by your chart this man has stopped breathing.
One law of electricity that cannot be changed by figuring a voltage drop across a conductor and that is the applied voltage is the applied voltage in a parallel circuit.
The person touches one conductor with his left hand (phase A) the other with his right hand (phase B) he is the difference in potential, current flows.
I have ran this on a simulator several times using 2.4 ohms of resistance to obtain the 200 amp load and the lowest amount of current I have got across human at 150k ohm, has been 13mA and as high as 169mA at 30k ohm for human.
I ain?t to brite but brite nough to know bird can?t stretch leg 100 ft. ifen he did he dead.
Said all I got to say about it and I am not coming back to this thread.
 
Re: Code related Theory question

Originally posted by jwelectric:
The person touches one conductor with his left hand (phase A) the other with his right hand (phase B) he is the difference in potential, current flows.
Click to expand...
This is not what is happening in our proposed scenario. The man is not contacting phase B. The man is contacting only phase A. 480 volts does not exist from phase A to phase A. The only voltage that exists from one end of the intact A phase wire to the other end of the A phase wire is whatever voltage is dropped on the conductor itself. This is the voltage that the man is in parallel with.

Please go back and review all the diagrams that have been posted on this thread.
 
Re: Code related Theory question

jw, you have missed something along the way. This is just like the bird on the wire. Full voltage does not, can not, will not appear across the cut as long as 0.0135 Ohms is in parallel with the cut.

This is somewhat like the guys who test hot fuses with their bare hands. If the fuse is open, they feel a shock, if the fuse has any continuity left, they feel nothing. Of course the guy wears his rubber soled Red Wings.

[ January 15, 2005, 12:43 AM: Message edited by: rattus ]
 
Re: Code related Theory question

JWelectric

Think of this another way. If you have worked on a grounded "B" phase system you would know that we can ground the "B" phase because there is no reference to earth before the grounding takes place. thus no complete path for current to return to source. In the given question the person is waring rubber shoes and is not in contact with any other "return path" that would create a complete circuit that would put 480 volts across his body. Since the hands are the only part of his body touching the conductor it is the voltage that is at each cut end of this conductor which also connect to the ends of the other parallel conductor that make up phase "A". We know that there would not be 480 volts dropped across this intact conductor or no current would be able to pass to the load (as if both conductors was cut) so only the voltage drop of this conductor will be present at the two cut ends. and it would not matter if you ran a #16 awg wire to each end connecting one to the load end on phase "A" and one to the line end (breaker) on phase "A" the voltage will still be the voltage drop of the intact 3/0 phase "A".

Another way is have you ever messed with a shunted amp meter like the ones they use in a car. it would be the same connection the meter in a millivolt rated meter with a shorted strap across it. the voltage drop across the strap is what drives the meter. this is the same voltage drop the man would have in each hand because he does not have a complete circuit to any return path to receive the full voltage that would only be present between two phases or phase "A" and a grounded source. so the voltage will be only 2.7 that would be dropped across his body from one hand to the other hand. as said look at the diagram of Ed's, there is no way that he will have 480 volts. So it can not be part of the equation.

[ January 15, 2005, 02:21 AM: Message edited by: hurk27 ]
 
Re: Code related Theory question

I am glad that my name isn?t Pinochino for if it was my nose would grow.
I have returned to apologize for being so stubborn and not opening my mind to those who, with out doubt have far more knowledge in this matter than me.
In a series circuit the voltage is divided according to the resistance or impedance it encounters. In a parallel circuit the current is divided according to the resistance or impedance it encounters.
In the scenario proposed in this thread we have 480 volts with approximately 2.5 ohms of impedance in ?series? with .0135 ohm of resistance from the conductor. Am I correct thus far?
This would equate to 478.65 volts across at 2.5 ohm load and a voltage of 1.35 across the conductor
My mistake: Where I have so stubbornly stood my ground: instead of applying the voltage drop across the 2.5 ohm load, I have dropped that part of the equation and used the 480 volts for my entire math and even left it out in a simulation.
Thanks to the patience of Rattus my eyes have been open, Thank you Rattus for taking the time to help me see where I was making my mistake. I would have gone through life thinking that I had fried the person in this scenario.
Moral of this story, one can?t hear anything unless one closes one?s mouth. Again thank you Rattus.
 
Re: Code related Theory question

missing from all this is the reality that the human body is not a fixed electrical component with predictable and measurable resistance, current/time characteristics.

human body does not pass current at the near speed of light. it is a chemical reaction with different time characteristics. There is a lag.

Since there is a lag, the potential on one side of the body will fluctuate wildly in relation to the sine wave lag. I was inadvertantly shocked this way and I had to figure it out.

If all you bright guys want to stand up and do a demonstration, please let me get insurance policies in order on

paul :D :p you that include arrogant bright guy tricks. My projected retirement income is a little thin.
 
Re: Code related Theory question

oops1 dumb guy trick, don't check post before sending, move name and icons out for paragraph continuation
paul :cool:
 
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