electric shock in welding

jim hardy

that's it exactly.

I assure you, from personal experience, that
when you are out in the ocean soaked with salt spray
and your boat's battery terminal comes loose
you cannot hold onto the positive wingnut to tighten it
because your salt soaked skin allows the battery's measly 12 volts
to push curent into your fingers
and hand
and arm
and you even feel it in your feet where the current exits into the bilgewater.

Your hand and arm muscles contract and pull away from the battery.

And you "get the feel" of current stimulating your nerves.
Because it's painful, the lesson sticks.

old jim

Averagesupernova

We had a prof. in school that would refer to it as 'reach out and touch ya'.

Cyclix

There is a fallacy in this argumentation. You are involving variable resistance in the equation but not adjusting the other important parameter (voltage) at the same rate. If we lower the voltage with the same factor as the resistance we will see again that if the input voltage is not enough to push our given dangerous current, nothing happens. The power supply itself could be rated for 1000s of A but if its voltage is below a certain threshold for a given resistance then nothing will happen. The only way for some living thing to get harmed in this setup is by increasing the voltage.

Next time you hear "Amps kill you, Volts don't" ask that person what would he rather touch, 15A at 12V or 40mA at mains voltage. The latter supplies hundreds of times less "amps". Just don't let them try to disprove me by experiment.

Averagesupernova

I AM adjusting voltage. Pay attention already. Jim Hardy gave a pretty good example of how it happens at 12 volts. And I said:The amount of voltage it takes to do this varies widely on conditions of the body.
Is the above an attempt to disagree with me? It is in fact agreeing with what I am saying. I'll say it again: Voltage that it takes to injure/kill varies with body resistance. Halve the resistance and halving the voltage will accomplish the same thing. The current will not change.
This isn't really much of an argument. The amount of current required to injure/kill is pretty much constant. The voltage required varies with body conditions. Now that I've said it three times in this post is it sinking in yet? Oh yeah, given the question about a 12 volt 15 amp supply vs. 120 volt .04 amp supply. I've been across 120 volts hand to hand with virtually no sensation. Obviously my hands were dry and due to no sensation many times less than .04 amps in my body. I've also been across 12 volts with wet hands and a VERY noticeable current flowing but I can assure you it was nowhere near .04 amps to say nothing of 15 amps. Do you see what I mean yet?

sophiecentaur

Those are meaningless figures. The current flow depends upon the applied volts and the (possibly non-linear) resistance of a load (in conjunction with with the source resistance). You are being irrational in the way you are approaching this topic. It is such a well established field of theory and practice yet you seem to insist on your own view. Just do some serious reading and educate your crackpot ideas about it. You are arguing with people who know what they're talking about (mostly ).

Cyclix

And there you have it, increase the voltage, the danger goes up. Conversely, you can't increase the supplied current and somehow expect that it starts flowing where it didn't flow before. So given a non dangerous live circuit and a grounded organism touching it, we can only make it dangerous by increasing the voltage. In such a scenario the quote "it's not the voltage, it's the amps that get you" is just plain absurd since ramping up the volts is what WILL get you. :)

Yes adherents of the not-the-voltage-but-the-amps theory start to say these things sooner or later. If you look at similar discussions you will even see people who claim to have been struck by lightnings & stuff and nothing happened since 'it's not the volts'... Unverifiable claims at best.

You also seem to be mixing up current, voltage and feeling. We feel pain (and anything else physical) when neurons get excited. A neuron can get excited either when a chemical, aka neurotransmitter gets released near it or some voltage is applied directly to it. In either case, once a neuron is excited it releases the above chemical on its other end thus exciting the next neuron in the chain until the pain center of the brain is reached.

Now, even a single electron is enough to excite a neuron if the voltage is high enough. And current being a measure of the number of flowing electrons, or charges in general, we see that even a minuscule current (1 electron) can be painful.

If we have a voltage high enough to excite a neuron with a current of 1 electron, adding 20 more electrons to the equation will not make the experience more painful, it will stay the same. On the other hand, if we don't have a high enough voltage to excite the neuron, even a current of 2000 flowing electrons won't make it fire.

Now hopefully you see that you can't judge the level of current on what you feel.

Similarly with the heart: it has a neuron bundle that is easy to whack out of sync with a high enough voltage. A current of 1 flowing electron will be enough. So yeah, here too it will be the volts that get you irrespective of the magnitude of the current.

This is entry level physiology of electricity; funny how people are sending me to do 'my own research' but
1. This is a science forum and not yahoo answers

2. Most things are fundamentally simple, even Special relativity: nothing can move faster than light in vacuum. So present simple, exemplified arguments (not memories).

3. Whoever thinks that a low current high voltage system is safer than a high current low voltage system is most likely neither an electrical engineer nor a physician, never asked himself why instruments and installations are safety rated for volts and not amps, and why there are placards saying "danger! high voltage" and none with "danger! high amperage".

I mean I can only try so hard

davenn

its not that I think that its that we all know and you dont seem to understand that.

I have suffered many low current and extremely high voltage shocks over the years and the only reason I can write this is BECAUSE they were LOW CURRENT ! ;)

Again I say .... Volts Jolts, Current Kills and that ol' saying has well stood the test of time!

Dave

Averagesupernova

Cyclix, I can tell you are certainly not an electrical engineer:
Not sure what you are saying here. It really makes no sense. Are you saying that increasing available current will not make it flow where it wasn't flowing?
Ohms law tells us that if we do not change the voltage and wish to change the current the only thing we can do is to lower the resistance. In this case that would equate to wetting the hands. The converse is true of course which you have been arguing all along No problem with increasing voltage to increase the amount of current as long as the source is able to supply the current. YOU seem to be the one getting current and voltage confused here.

So will wetting the hands.
-
In old CRT type televisions high voltages existed in many places. Voltages as high as 30,000 volts were present. Guess what was generally considered the most dangerous voltage in the TV set? It was the 120 volt line voltage. The only reason other voltages were considered dangerous was because when you got hit with one your arm would jerk out of the set so quickly that you would likely cut yourself on part of the chassis. So cyclix, have you worked on alot of CRT type televisions?

davenn

yeah thats one of the places I was thinking :), have had a good few 20 - 26kV belts over the years from tubes that weren totally discharged. Wakes ya up ;)

the other thing I was think was the multiple kV, often well in excess of 10kV every time I walk across the carpet on a dry day and discharge on a door knob, my workshop anti-static mat etc

cheers
Dave

Cyclix

Nope I haven't done that at all. But I can still tell you why the mains voltage had to be considered the most dangerous one :)

It is because those 30,000 volts are only supplied for a very, very short time; probably a microsecond or so. Make that pulse a whole second long and the mains voltage suddenly becomes a far distant second level safety priority. Or maybe I am misunderstanding you and you are claiming that, in a powered CRT, the mains voltage is more dangerous than what comes out of the flyback?

The tens of kVs potential that builds up when walking on carpets with rubber soles etc - it's again too short of a pulse.


Hm, if we go back to the physiological aspect of electrical danger and I am to say that the structures responsible for activating / confusing the neurons, in the presence of electricity, are called 'voltage dependent sodium channels' and not 'current dependent sodium channels' it still won't make you believe that it's the volts and not the amps that get us, would it?

jim hardy

what is the potential of those sodium channels?

I believe biological electricity takes place at tenths of a volt.

That's why a copper wire "feels" so peculiar when it works its way into a cut.

old jim

Averagesupernova

Cyclix, there is something called source impedance that you don't understand, or for that matter, ohms law in general. The line voltage coming into the TV set has a very low source impedance compared to the higher voltage supplies in the TV. This means that the a source with a higher impedance is not able to supply the current. Part of the voltage is lost across the internal impedance of the source and the output voltage falls. For instance, we have a 1000 volt supply with an internal source impedance of 20000 ohms. This means that if we put a load on it of 20000 ohms only half of the full voltage will make it to the load and the other half is lost across the internal supply impedance. So doing the math, .025 amps will flow in this circuit. If a human being with an approximate resistance from hand to hand is 20000 ohms, this is what will happen. If the resistance hand to hand drops to zero (hypothetically), the most current that can flow is .05 amps, and the voltage from hand to hand will be zero with ALL the voltage being lost across the internal impedance of the source. So what has happened here is a reduced body resistance causes more current to flow and less voltage appears across the body. So which do you consider the most harmfull shock? The shock received with dry hands and 500 volts across the body carrying .025 amps or the shock received with very wet hands causing less voltage to be dropped across the body but much more current?