It's not the volt that will kills you, It's the amp ??????

[RivaultUser]

Okay now, a lot of people say that, but after watching Afrotechmod video in YouTube, I know that the voltage also takes in part in making the current flow

Now we know that I = V/R

In schools, it was so easy that we just use the formula up there and determine how much current flowing through a circuit.....

But in a REAL WORLD, we usually end up with LOAD and not with RESISTOR

Lets say that I have a fan that draws 12 V & 2A , we all know that current around 1 A can cause death.

What will happened to me if I touched the connection (positive & negative ) of the fan that connected with the Power Supply ( obviously I scared to try -.-)

Through my experience, I've experience an electric shock with my metal flashdisk that connected through my computer, when I touched it I felt an electric shock ( well, I can withstand it ). BUT WHEN I SIT ON THE CHAIR & MY FEET DON'T TOUCH THE FLOOR AGAIN, I don't feel the shock again

My hypothesis :
1. We can feel electrical shock through our body if we touched the ground & if we wear some kind of isolator ( like sandals ) the current will not pass through our body to the ground, so we won't get electrical shock

But what happened If I touched a cable that has different potential ?

We all know how to use a multimeter to measure current & voltage

Imagine that I hold the wire in a position to measure a current with multimeter ( substitute multimeter leads with my hands ) & 200 mA current flows & I wear sandals ( will it make different if I don't wear any? ). Will I feel electrical shock ?

This time with the position to measure voltage ( again, substitute multimeter leads with my hand ), but this time, lets say that I place my hand in besides my fan 12 V & 2 A ( like my right hand hold the positive cable & my left hand touched the negative cable ) & I wear sandals ( will it make different when I don't wear any? )

What will happened to me ?

[TMM]

The resistance of your skin is many orders of magnitude higher than a 12volt fan. As such you need a much higher voltage to draw the same amount of current through your body.

The shock you felt from your computer would have been from static electricity, not from the DC power supply in the computer.

[olsenn]

He makes a good point tho; why does everyone always say it is current that is harmful to humans and not voltage? Assuming our bodies have a fixed resistance, to pass the minimum harmful current through our bodies a minimum voltage (as per ohm's law) is required. If we touch a source of voltage that is strong (bolt of lightning for instance) that is incapable of delivering enough current to our bodies to satisft Ohm's Law, then said voltage won't actually be applied to our body! The voltage that our body will see in this case is the max current that source can apply * our body's resistance.

As for the 12V terminals on your fan... don't worry about it, haha. Touching them won't do anything, but if you lick the terminals you'll feel a slight sting (think 9V battery only 12V instead). You won't die!

[Psi]

He makes a good point tho; why does everyone always say it is current that is harmful to humans and not voltage?

Because it's true, you need current to do bad things like burns or to stop your heart.
Also, engineers get annoyed when we hear people say things like.. "ten thousand volts! damn, that would kill you instantly"

Assuming our bodies have a fixed resistance, to pass the minimum harmful current through our bodies a minimum voltage (as per ohm's law) is required.

Body resistance changes a lot between people and it depends on many environmental factors. Exposure to salt water for example can make it possible to die from 24V. Other times you maybe able to touch 90V safely.
It's impossible to give a useful answer to the question.. how many volts can kill

If we touch a source of voltage that is strong (bolt of lightning for instance) that is incapable of delivering enough current to our bodies to satisft Ohm's Law, then said voltage won't actually be applied to our body! The voltage that our body will see in this case is the max current that source can apply * our body's resistance.

Lightning is a bad example, they have millions of volts and millions of amps.

[LEECH666]

[alanb]

Excelent video Leech666.

I would like to add that its worth getting into the habit of working with one hand on anything where more than about 50 volts is involved. The habit could be a life saver.

[saturation]

http://en.wikipedia.org/wiki/Electric_shock

You get shocked with AC if you hold both terminals, or the live terminal, since AC is earth ground referenced.  If its fully floating, you won't, as with DC.  With both, you need to raise the voltage enough to overcome the resistance then dielectric effect of the skin, if you pass the voltages through a cut or none skin surfaces like mucosa: tongue, inner lips, eye balls, etc., you'll feel the effects very easily: just touch your tongue to a 9V battery.

The body is not a wire, more like a liquid surrounded by an insulator.  When you measure the current flow on the skin, you are measuring the surface flow.  If you break the insulator and enter the fluid core, you get different readings.  How its electrocutes you depends on the organ involved through volume flow of current: whether its muscles, nerves, heart or brain.  Whether it kills you or not depends on the most sensitive organ to current, i.e., heart.

[helloworld922]

I think this is a silly quote. How do you get a current without a potential difference? You can't. However, there's very little actual current that needs to be sourced through vital organs before you could potentially die (I believe about 40mA through the heart is potentially lethal, though don't quote me on this), so if someone stuck electrodes on both sides of your heart they wouldn't need a very large potential difference to provide a potentially lethal shock. This is probably where the origin of the saying comes from.

I'm assuming you're touching the fan with the skin on your fingers/hands, in which case unless you have something like a cut or other low-resistance flow path through your skin to the more conductive liquids inside, you'll be fine. I've played around with car batteries which can source a hundred amps or so if shorted, but because the 12V potential difference wasn't high enough to breakdown the dielectric of my skin, I was fine (still always slightly daunting whenever I worked with these, though).

I've always been more a fan of the saying that power kills. Works for tons of different sources of death, not just V*I.

[FenderBender]

Yeah the problem is that the resistance of your skin/body is very high, in the order of megaohms. To have a significant current flow across your body, you need a significant voltage.

[Psi]

Yeah the problem is that the resistance of your skin/body is very high, in the order of megaohms. To have a significant current flow across your body, you need a significant voltage.

It's way less than megaohms, normally between 1K and 100K for skin resistance.

It can even be lower than 1k if your skin is saturated with salt water.
24V lead acid boat batteries have been known to kill people out at sea if they've been standing/swimming in salt water for a while.

(Note: Ya cant measure your skin resistance with a DMM)

[FenderBender]

Yeah the problem is that the resistance of your skin/body is very high, in the order of megaohms. To have a significant current flow across your body, you need a significant voltage.

It's way less than megaohms, normally between 1K and 100K for skin resistance.

It can even be lower than 1k if your skin is saturated with salt water.
24V lead acid boat batteries have been known to kill people out at sea if they've been standing/swimming in salt water for a while.

(Note: Ya cant measure your skin resistance with a DMM)

Really? Well my apologies for false information. I had been told that like a foot of skin was like a megaohm or something....Guess I'll delete that from my head.  Whoops

[TriodeTiger]

Yeah the problem is that the resistance of your skin/body is very high, in the order of megaohms. To have a significant current flow across your body, you need a significant voltage.

It's way less than megaohms, normally between 1K and 100K for skin resistance.
[...]
(Note: Ya cant measure your skin resistance with a DMM)

Really? Well my apologies for false information. I had been told that like a foot of skin was like a megaohm or something....Guess I'll delete that from my head.  Whoops

Oh?

(If you're comfortable putting small constant current, maybe ~<1mA between your hands)

It's this high because my contact is poor. If I lick my fingers and grip with the leads in (greater surface area), of course I get lower resistance, so don't do that with live wires, more importantly don't assume doing not saves you (but people's "I touched mains and lived when doing xxxx" may be because of the above reason, little contact and non-lethal path.)

It's winter here so its awfully dry. I remember reading a tale about a sailor reading his skin's resistance and killing himself by accident, pressing a little too hard, but I suppose (a reasonable) multimeter limits current to <1mA for such reasons.

Of course blood is pure electrolytic goodies if you happen to have a cut, that's what you would "break down" in to.

lightening

Power will kill you likely in such high voltages, if you've ever seen unfortunate accidents regarding high tension lines or high voltage supplies, sometimes only milliamps are drawn - yet enough power (Watts) to power an oven flow through them! Voltage = force as well.

if we wear some kind of isolator ( like sandals )

sometimes this is not enough. If you wear rubber shoes and walk near a downed power line, the current may flow through metres of concrete, dirt, everything you think of as non-conductive and your (non-rated) rubber boots to get to ground, if it happens to be a lower resistance path.

[peter.mitchell]

sometimes this is not enough. If you wear rubber shoes and walk near a downed power line, the current may flow through metres of concrete, dirt, everything you think of as non-conductive and your (non-rated) rubber boots to get to ground, if it happens to be a lower resistance path.

http://en.wikipedia.org/wiki/Earth_potential_rise worthy of a quick skim

[Psi]

I think the reason you get megohms using a DMM on your skin is because the DMM is sourcing an ultra low voltage, maybe 4V.
You need more than that to get a useful reading on body resistance because the resistance changes with applied voltage somewhat and its pretty high at 4V

[EEVblog]

Lets say that I have a fan that draws 12 V & 2A , we all know that current around 1 A can cause death.

Stop right there!
As little as 30mA can cause death!
Why do you think Earth Leakage Circuit Breakers (ELCB, but have other names like "safety switch") have a trip current of typically 30mA?

How much current is drawn through your body from a voltage source will depend upon:
a) The impedance of your body (vary massively!)
and
b) How much current the voltage source is capable of sourcing.

e.g. a 10kV spark generator from a cigarette lighter can never generate enough current to kill you, but a 240V or 110V mains outlet easily can.

Dave.

[FenderBender]

Of course Dave is spot-on.

Not only does the voltage need to be high enough in order to allow current flow through your body (which is high resistance), but the source itself needs to be capable of sourcing sufficient current to do so.

As Dave said, you can have a 1kV source, but if it can't source enough current, it's not an issue. Your wall outlet on the other hand, can provide a huge amount of current, and that's partially what can makes it dangerous. There are many sources which are significantly higher in voltage, (thousands of volts), but because they can't provide very much current, they aren't dangerous enough to kill you.

If you rubbed your socks on a rug, you will build up a tremendous charge on your body. Typically greater than 10kV (10,000V)! However, even though you might have a giant charge on you, if you were to touch say a doorknob at a low potential, you'll feel a shock, but it wont kill you because the amount of current will be very low. Not even close to being able to kill you. Probably in the micro-amp range, though I'm not sure.

[TriodeTiger]

I think the reason you get megohms using a DMM on your skin is because the DMM is outputting an ultra low voltage, like 4V.

I believe it is a constant current source, putting up to its battery voltage to put 100uA or so through the resistance to measure voltage drop and derive resistance (which otherwise would be "short battery to resistor, measure current" kind of measurement, and that is no good)

You need more than that to get a useful reading on body resistance because the resistance changes with applied voltage somewhat and its pretty bad at 4V

A misconception? I have just tested a small watch battery to pass 3uA of current at 1.3V hand to hand = ~500kOhm, as agreed with my multimeter using the same testing surface (borrowing battery's top as a surface again so area does not change much) and the resistances are the same.

I've passed 45V through a 1 megohm resistor, right hand to foot, through earth and back and the resistances seemed to agree with my DMM as well, taking the resistor out of the equation.

Why would 4V "see" resistance significantly different from 40 or 120?

Maybe nAs are passing through my heart, and only when voltages high enough occur those levels raise? skin sees much higher currents than that fatal 10mA? I just can't picture skin as a "dialectric" with "breakdown"! (unless skin is converted in to something more conductive from high current or power)

(my thinking is curious and strange, but, do not see much except vague or limited textbook material out there, so maybe my curious thinking style is helpful.)

[BravoV]

It's not the volt that will kills you, It's the amp

Change your paradigm and don't think only amp (current) nor only volt, it needs "BOTH".

[amyk]

If you rubbed your socks on a rug, you will build up a tremendous charge on your body. Typically greater than 10kV (10,000V)! However, even though you might have a giant charge on you, if you were to touch say a doorknob at a low potential, you'll feel a shock, but it wont kill you because the amount of current will be very low. Not even close to being able to kill you. Probably in the micro-amp range, though I'm not sure.

It doesn't matter where that 10kV is from. 10kV is 10kV. In theory if you apply 10kV from any source across your body, the same current will flow.

Then why is a 10kV mains transmission line lethal and a static shock isn't? The latter cannot deliver very much energy, while the former can. With static, the source of energy is stored charge and that charge dissipates quickly (acts like a capacitor), while the mains can provide energy continuously. When you receive a static shock, the instantaneous current would be lethal if it continued, but it doesn't; it decays to 0 before it does any harm. The skin is also not ohmic, and decreases resistance with higher voltage, further increasing the current when the source can sustain that high voltage.

In short (no pun intended), it's volts * amps * time that kills you.

[BravoV]

In short (no pun intended), it's volts * amps * time that kills you.

This, even better.

[FenderBender]

Correction: The same initial current will flow, but it will so quickly drop off that it doesn't really affect the human body.

A 10kV transmission line as you take for example, can supply lots of current over time, whilst that static shock can only provide a pulse of high current.

Edit: Seems I just repeated what you said.

[Psi]

Why would 4V "see" resistance significantly different from 40 or 120?

Yeah, if the body was a true "resistor" it wouldn't matter.
But i was under the impression that the human body is a bit more complex than that, and its resistance varied depending on the voltage you apply.

Dont ask me to explain why but i've been told it on a few occasions.
Afrotechmods even mentions it briefly in the video on page one.

[HackedFridgeMagnet]

I'll get my mega out, and see what my resistance is at 500V.

Ouch.

[dr_p]

It's not the volt that will kills you, It's the amps.
 

Technically it's true, since the skin is presented as a resistance, so the higher the voltage, the higher the current.

But there's more:
If you measure your skin resistance with a DMM, you'll get something in the area of 100K-1M. But that's at low DC voltage (~4V, like someone else said). A very important thing to understand is that inner body resistance is 300-1K, but skin resistance is comparatively high (1K-100K). Since current goes through the skin, then body, then skin again to exit, measured resistance is twice the skin resistance + inner body resistance. As an example, touch both a battery's terminals with your tongue - it stings. It doesn't sting though when touching it with your fingers(skin).

Also, skin resistance drops when wet (obvious) and burnt (not so obvious). So High voltage electrocution is a cascading effect.

On top of that, skin acts somewhat like a non-ideal capacitor:mains wire(conductive) -- skin (dielectric) -- inner body (conductive). It nas no (negative) effect on DC, but some AC passes through, therefore AC electrocution is worse.

Furthermore, the skin dielectric breakdown takes place at high voltages (400-600V), so thins really get nasty.

And it's also a good idea to note that 220V mains is RMS voltage, meaning a sinewave with 220*1.41=310V peak, therefore a Peak-to-Peak value of 620V. Taking all the above into consideration, the equivalent skin resistance in AC mains electrocution is somewhere way below 1K.

To sum it all, the current requred to kill a person in less than 1mA TO THE HEART, so the closer to it you pass a current, the worst it is. Higher voltage AC exponentially increases current.

So yes, it's the current that kills you, but voltage creates current, so one could say the voltage kills you. Or both, or whatever.

[Psi]

Things get more interesting at higher AC frequencies.

At ~100+khz you start to run into "skin effect" where most of the energy flows on the outer edge of the conductor.

In the case of a human that's your skin and a few mm below. Your deep tissue and internal organs get less and less of the energy as the frequency goes up.
So high frequency AC, like a telsa coil output, is somewhat safer than the same voltage and current at a lower frequency.
(This doesn't mean you should run around touching Tesla coils, they're still dangerous. Especially the primary coil)

[TriodeTiger]

At ~100+khz you start to run into "skin effect" where most of the energy flows on the outer edge of the conductor.

I believe surgeons use high frequency AC to cauterize wounds or veins very accurately sometimes, or alike.

but some AC passes through, therefore AC electrocution is worse.

Well, I'm not so sure about that, but the capacitance lowering impedance is definitely something I've never seen in "why AC may be more dangerous". This is very interesting. I've found numerous links to bio-impedence papers and interesting things! i.e. http://www.mn.uio.no/fysikk/english/research/projects/bioimpedance/publications/papers/osmose.pdf

I was tempted to hook myself up to limited 1.3VRMS AC and calculate impedence vs my 1.3VDC but my DAC seems to not be working, and have no function gen, but I'll leave that to the more adventurous having found at least one reason above.

[FJV]

If I'm not mistaken it's the Joules that kill you.

[G7PSK]

If you watch this video it requires at least 1,25 amps to kill a pig. A pig's physiology is much the same as a humans so I would guess that it requires much the same current to kill a human. There has been a lot of scientific testing to come up with these figures.

http://youtu.be/-FEUfkmJQuA

[saturation]

A joule is also a measure of physical force, and thus is ambiguous in this context.  Its electrical equivalent is dependent on time; so you can have high joule rating by having a low threshold voltage x current but over long periods of time; thus touching a 9V battery to your intact skin for hours on end will do nothing compared to a microsecond of 500V at 1mA.

For electric shock, you need a threshold voltage and current, depending on the state of the skin.

http://www.afence.com/Electric_Fence/how_to_elecfence/Joulerelation.htm

If I'm not mistaken it's the Joules that kill you.

[Praxis]

Why would 4V "see" resistance significantly different from 40 or 120?

Yeah, if the body was a true "resistor" it wouldn't matter.
But i was under the impression that the human body is a bit more complex than that, and its resistance varied depending on the voltage you apply.

Yes and no.  The resistance doesn't really change much based on the voltage until you start applying enough power to boil things off, in which case you're already past caring.

On the other hand, very high voltages from static or tesla coils tend to be high frequency -- their tiny charge is quickly transferred and they stop conducting very quickly -- and high-frequency, high-voltage sources tend to bring out 'skin effects' of the conductors that they're using.  That is, the conductor -- be it a wire or a body -- will tend to have the current flow around the outermost portions -- literally the skin in the case of a person. 

You can think of these kinds of sources like loading a catapult with a pea -- sure, it can toss that pea really far -- in a vacuum, just as far as a hundred-pound boulder -- but it's not going to smash anything like a small boulder would, because it lacks the mass to have gained damaging inertia from the launch.  The high-voltage sources that are commonly used safely are similar; they have a huge electrical potential -- analogous to the kinetic energy stored in the catapult's spring -- but they can only hold a very small payload of electrons in this state, and amperes are just a shorthand for the number of electrons passing through a point at any moment.  Then, when the payload hits the person, the skin effects mitigate it more.

...Touch a 10KV trunk line, on the other hand, and it's likely that the rest of your day will be short indeed.

[RivaultUser]

Okay then, I've post my water analogy
THE HEIGHT represent POTENTIAL DIFFERENCE
THE WIDTH OF THE TUBE represent the CURRENT


So it does matter about the voltage, because the voltage also determine how many current that will flow through our body

However, even though if the voltage is 10kv, but if the power supply only give you maximum current of 10 mA, it's not going to kill you.......

So, lets say that I have 5 V & 800mA ( standard for USB charger ). We know that 800mA is pretty dangerous, but 800mA is the maximum current that the charger can provide. And he current that will flow through our body ( if we touched it ) is determine by I = V/R. In this instance, the Voltage DOES MATTER

Am I correct?

[IanB]

Okay then, I've post my water analogy
THE HEIGHT represent POTENTIAL DIFFERENCE
THE WIDTH OF THE TUBE represent the CURRENT

Almost. Actually the width of the tube represents the resistance to flow, and the rate of flow (e.g. liters per second) represents the current.

So for instance you could have a great height and a very narrow tube and you would only get a small flow (high resistance), or you could have a low height and a wide tube and get a much larger flow (low resistance).

[Stonent]

Just thought I'd introduce a movie quote from the 80's

Running Scared (1986)

The scene is they have a car driving on train tracks. And for whatever reason which I can't remember, they have a Nun in the back seat.

Danny: Try not to scrape the third rail, okay? There’s about 600 volts in there.
Ray: It’s not the voltage that gets you. It’s the amps.
Nun: How many amps are there?
Ray: Enough to push a train, and shut up!