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Safe current limit for human contact?

From what I've heard:

  • 110 V (or 220 V; household voltage pretty much) is dangerous (i.e. can kill you) I think there's consensus on this, no need to try :)

  • 60 V (old telephone lines) is supposedly dangerous (never tried, only heard it once... probably won't try)

From what I know first-hand:

  • 9 V is not dangerous (I've put a 9-V battery on my tongue, nbd... actually it kinda hurt!)

  • 1.5 V can indeed be quite shocking with enough current (fell for one of those "Do you want some gum?" tricks back in high school...)

So I guess there's two parameters here, voltage and current... but are there rough numbers on how much of each (or in combination, which I guess would be power) can be considered hazardous?

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I'll let more experienced users write the whole story, but basically it's power that kills, or better yet, current through vital organs which depends on the current capacity of the source and its voltage (and the needed voltage depends on the resistance through the body which again depends on the skin condition and so on). That's why you don't get killed by static electricity discharges that can go into tens of kilovolts easily and why it's dangerous to touch both terminals of a 12 V car battery. –  AndrejaKo Sep 5 '11 at 19:20
    
@AndrejaKo: Ah, interesting; I only thought messing with a car battery is dangerous because of the hydrogen, not because of the current as well! :) –  Mehrdad Sep 5 '11 at 19:35
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@tcrosley - agreed, it's basically the same question. –  JustJeff Sep 5 '11 at 20:58
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Note that the 1.5V shock devices will use the single cell to power a mechanical vibrator in older designs or a flyback boost converter in more modern deigns to produce high voltages - probably in the 100V - 200V range. Current will be purposefully limited and they will aim to kill approximately zero customers per year. –  Russell McMahon Sep 6 '11 at 6:34
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I have always been told the primary concern with high power sources is that a tool will short across it and the metal will vaporize causing an explosion that actually does the damage. –  Kortuk Sep 9 '11 at 20:20

8 Answers 8

up vote 24 down vote accepted

How much voltage is dangerous is not really a static number as it depends on your body resistance, time of exposure and source "stiffness" (i.e. how much current it can supply). You get figures like 60V (or as low as 30V) which are an attempt at an average figure above which "caution should be taken".
However, depending on how "conductive" you are at any one time, sometimes e.g. 50V might be quite safe and other times it may kill you.
DC or AC (and what frequency) seem to make a difference too, female or male, etc - this table is very instructive:

Table1

Figures as low as 20mA across the heart are given as possibly capable of inducing fibrillation - here is another table from the same source that gives body resistance based on different situations:

Table2

You can see that as low as 20V may be dangerous given the right conditions.

Here is the reference the tables came from, I think it is quite accurate based on some experiments I have done myself measuring body resistances. The rest of the site seems to be generally very well informed and presented from the bits I have read, so I think this may be quite a trustworthy source.

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Your reference actually references other references: The MIT safety group and a Bussman publication Deleterious Effects of Electric Shock - allaboutcircuits.com/vol_1/chpt_3/10.html, first paragraph. –  Kevin Vermeer Sep 6 '11 at 16:58
    
Ah yes, thanks. I should have said "page" rather than "reference". –  Oli Glaser Sep 6 '11 at 17:17

It's not the voltage but the current that kills.

About 60V is considered the level at which you can start getting an electric shock.

According to Joseph J Carr's. "Safety for electronic hobbyists. Popular Electronics." October 1997:

In general, for limb-contact electrical shocks, accepted rules of thumb are: 1-5 mA is the level of perception; 10 mA is the level where pain is sensed; at 100 mA severe muscular contraction occurs, and at 100-300 mA electrocution occurs.

Electrocution becomes fatal when the current passes through the heart and causes fibrillation - the current causes the heart's beat to get out of sync and it can't pump blood any more.

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Another thing that's sometimes omitted but is also extremely important is that electrocution also causes burns which themselves may be enough to cause death. Here are a few videos demonstrating how the system works: youtube.com/watch?v=ehHo_P4O3FA youtube.com/watch?v=u-IbdeZW2PQ youtube.com/watch?v=gMEDcvmoAfI youtube.com/watch?v=eyuT4B6ZZpk –  AndrejaKo Sep 5 '11 at 19:45
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see my answer to this question: electronics.stackexchange.com/questions/9222/… which is a pretty much a duplicate of this one. –  tcrosley Sep 5 '11 at 20:43
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@Matt, I really really hate people saying "its not the voltage, it is the current". Measure the 9V battery when on your toungue and you will find it is a lot less then 9V. Yes, we often rate things by their open circuit voltage, which does not tell you much, but it is the power that kills, that little 9V battery cannot deliver much. I have a 400 Amp 3V source at work, It will stay 3Vs up to 400A. This makes 3V dangerous because it is able to deliver high power. The 9V battery has a big series resistor, a 9V lead acid would be dangerous as it does not have as big a series resistor. –  Kortuk Sep 5 '11 at 22:17
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@Kortuk Knowing the voltage doesn't provides enough information to determine the chance of damage, knowing current does (as measured through the body). Now you can argue that if you know one, you know the other based on some model of the resistance of the human body. However that pretty much impossible to determine in the general case. The resistance varies extremely widely based on contact location, moisture conditions, duration of application, frequency, etc. Thus the only term which is a consistent measure of damage, is current or current at 'x' frequency/duration more accurately. –  Mark Sep 6 '11 at 6:29
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It's not the current, either. When your body builds up a static electric charge and discharges into a doorknob, there are thousands of volts driving several amps, but nothing bad happens, since the duration is only a fraction of a microsecond and the total energy built up is in the millijoules. –  endolith Sep 7 '11 at 16:05

FACT:

  • 12 VDC CAN kill and has killed people.

  • While 12V is almost always safe, worst case situations can and have lead to death.

  • Mechanism may be ventricular fibrillation BUT paralysis of the respiratory muscles occurs at about 20% of the current needed to introduce fibrillation.

  • See discussion and references at the end of this answer.

12 VDC applied across the chest has killed volunteers despite medical experts standing by !!!
(From memory - volunteer prisoners participating in medical research).

Carry a car battery with exposed terminals on a hot day when you are sweating and press the terminals to your body (as could happen worst case when lifting the battery etc) and you may end up repeating the experiment.

Once conduction into the body starts you get a very low impedance / resistance circuit into what is essentially a large bag of dilute saline solution.


There are two mains "what kills" issues.

  • One is general trauma - burns etc, and that is obviously very situation and person dependant. I've had shocks from 1200 VDC, 230 VAC, 50 VDC, RF and miscellaneous other sources. No major burns. I'm still alive

  • Enough current for long enough to stop your natural heart rhythm and throw it into fibrllation.

    At typical domestic voltage levels you are USUALLY safe if the current flows for well less than one ventricular heart valve cycle and at "low enough" current.

    Earth leak circuit breakers (ELCB) also called ground fault interrupters (GFI) and other names aim to trip at currents somewhere under 10 mA and from memory (references later - rushing) in about 10 mS = well short of a heart cycle.

    A shock from a circuit protected with an ELCB / GFI device will be felt but will USUALLY not be fatal.

A 9v battery on the tongue almost certainly won't kill.

A 9v battery across the chest with saline solution (or sweat) just might - probably not.

A 12V "car battery" or any high current source from a few volts up MAY kill in the very worst case. Hand to hand I havve never heard of shock occurring or being felt.

110 VDC (not AC) routinely killed Edison's linesmen.

50 VDC MAY not be felt with dry hands on a dry day. On a high humidity day brushing the back of the hand with terminal strips with 50 VDC on causes annoying minor shocks (as experienced in eg Telecom wiring frame jumper running (based on my long ago experience)

75 VAC imposed on 50 VDC gives a very nasty shock sometimes. Worst case this could kill.

High current 1200 VDC hand to body somewhere may not kill - I'm still alive.


Can 12 Volts kill?

Yes.

Probable? - no.
Possible? - yes.

Data point: Note that this is a completely true and non-fabricated account. I have a friend (still alive) who built a lamp to take flounder fishing. It used a 12V SLA battery and an Aluminum pole with the light at the top. Flounder fishing involves wading through shallow salt water. In the course of fishing he discovered that an electrical fault existed - in some manner he was exposed to 12 VDC between his hand holding the pole and the water he was standing in. He was completely unable to release his grip - the current flow exceeded his "let go" threshold. regardless of how "worst case" this may have been and what various tables and standards say, it was clearly possible to reach his personal can't-release level. The literature states that respiratory paralysis can occur at currents not significantly greater than the can't release level. If he'd been by himeself (never a wise idea with such activities) he may have found himself floundering :-). Note that this was a hand to leg current path. Chest to chest worst case can be reasonably expected to be potentially higher.

The table below is from this page.

this is not a primary reference source but the figures used have been obtained from an "official" source. See above page.

Note that for 60 Hz Ac ventricular fibrillation is stated as occurring at 100 mA but paralysis of respiratory muscles occurs at 20 mA . These limits are very much user and situation dependant but give an order of magnitude indication.

With very informal equipment I measured 1500 ohms resistance across two areas on my abdomen. I decided not to measure across my chest in the vicinity of the heart. I used flat contacts with no skin penetration. At 12V, if resistance did not change with current flow (and I'd expect it to probably drop) a current of 8 mA would be produced. Measurement with skin penetrating electrodes may reasonably be expected to increase this significantly.

A superb discussion of electrical safety, current levels in various situations and consequences can be found here. The writer's competence and bona fides are above reproach*. The discussion relates to the provisions of standard IEC60990 'Measurement of touch current and protective conductor current'. This is a "for money" standard that I do not have access to, but excerpts from it are provided in the above reference and elsewhere.

  • '*' P E Perkins PE.
    p.perkins@ieee.org
    Convenor IEC TC108/WG5, IEC 60990 'Measurement of touch current and protective conductor current"

A careful but less than exhaustive examination of the above document and other related web material makes it very clear that

  • "Electrocution" from a 12 Volt DC source would be extremely unlikely

  • In worst case situations it could happen.

Related:

Full copy of standard ECMA287 - Safety of electronic equipment

Touch current comparison data paper - P Perkins

NIOSH - worker deaths by electrocution

Accounts of two deaths by electrocution. One at 12V. One at 24V. Note that BOTH these are unsupported heresay reports and actual cause of death may not have been electrocution.

Table 1. Estimated Effects of 60 Hz AC Currents
1 mA Barely perceptible
16 mA Maximum current an average man can grasp and "let go"
20 mA Paralysis of respiratory muscles
100 mA Ventricular fibrillation threshold
2 Amps Cardiac standstill and internal organ damage
15/20 Amps Common fuse or breaker opens circuit*
* Contact with 20 milliamps of current can be fatal.
As a frame of reference, a common household circuit breaker may be rated at 15, 20, or 30 amps.


Interestingly - this answer has 1 downvote, and surprisingly few upvotes considering the undoubted truth it tells. Maybe the downvoter and anyone who doesn't think it is a good answer would like to tell me why? The aim is to be balanced and objective and as factual as possible. If it falls short please advise.

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I have also touched 50VDC(48V) and it just annoyed me and gave me a tickle. I know a guy whom had a 100kW power supply turned on while he was working on it, he lived without long term consequence. I do have some lead acid 9V batteries though. –  Kortuk Sep 6 '11 at 10:00
    
Please provide a reference for the claim that 12 V has killed someone –  endolith Sep 7 '11 at 16:42
    
@endolith - My certain memory that I read about it long ago BUT you could find it and other references and do us all a service BUT I'll do so if I get time if you don't do so first. While there you could add references for all the other did and didn't claims. eg 1200 VDC didn't kill me, 230 VAC often doesn't / often does etc. Edison's linemen dying (something he tended to hide) ... . –  Russell McMahon Sep 7 '11 at 16:47
    
@RussellMcMahon: You could have impeccable memory and the original source could still be wrong. I'm skeptical is all. Minimum human internal resistance is still 300 ohms. –  endolith Sep 7 '11 at 18:34
    
"Peng and Shikui (1995) presented 7 cases of electrocution by AC or DC voltages ranging from 25-85 Volts. In all cases, the contact site was on or near the chest, the contact time was “long”, and skin burns were observed. In addition the authors note that all victims were working in an enclosed, high humidity and high temperature environment" That's the lowest I've ever heard of. –  endolith Sep 7 '11 at 19:00

It's not the voltage but the current that kills, is a popular yet still incorrect incomplete answer. It is the ENERGY that kills. With static electricity you will will be exposed to voltages much, much, much higher than 110/230V and that is not dangerous. So obviously high voltages are not that dangerous in some cases. Why? Because the time is so short that the total energy you are exposed to is so low. Please see the video It's not the volts that kill you, it's the amps at youtube that explains this topic in more detail.

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Your statement about the energy being the issue is incorrect from everything I've heard and reasonable logic. It is the current that kills. The volts only matter in how much current they can cause, which depends on how well the potential is coupled to your body. That's why wet skin is a lot worse, because you get more current for the same volts. Energy can kill in some situations by cooking your tissue, but that's way more current than would kill you for other reasons in most cases. A few extra Watts is no big deal for the body to dissipage. –  Olin Lathrop Sep 5 '11 at 21:54
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Maybe calling it incorrect is wrong, but my point is that only considering current is incomplete without also considering the time. With a static discharge you might be exposed to 8A at the very start. 8000mA is a magnitude above the dangerous levels already mentioned, and yet still only annoying. –  hlovdal Sep 5 '11 at 22:17
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@hlovda: Yes there is a time component to causing harm, but thats current and time, still not energy. Energy is simply the wrong metric unless you're doing damage by cooking. –  Olin Lathrop Sep 5 '11 at 22:32
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@Olin, I disagree, for there to be high current you need the voltage. I agree that 480V with 1mA rated current will not be dangerous, but is .1V with 1000000A rating? Only if you can get it to conduct. You need to know both conditions to have a complete picture. I hate that people act as though you only need to know current and hlovdal is making the same point here. You are not at danger with a 1000A source and 1V unless you touch it with something that will conduct alot of current at 1V. But a 40kV source with 100mA is actually pretty dangerous. –  Kortuk Sep 5 '11 at 22:56
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I think it would be fairest to say that an electrical event is not dangerous, regardless of peak voltage or current, if the total energy is below a certain level. Likewise, if the current and voltage are below a certain level, a person can--given enough time--safely absorb an arbitrarily large amount of electrical energy. Further, if voltage is sufficiently low, the amount of current that can flow as a consequence of such voltage will be too low to cause harm. –  supercat Sep 6 '11 at 2:17

All the answers given are correct to an extent :

  1. Electrical current will cause muscles to contract and can lead to respitory and cardiovascular seizures.
  2. The electrical energy imparted on the body will burn and cause serious internal injury.

But this only holds true for a given voltage, a certain voltage is needed to traverse the skin and this of course is a function of the impedance. For example the 9V battery on the tongue gives a slight shock but you wont feel anything if you hold the battery in your hand.

The rule of thumb is 50 VAC or 120 VDC is considered the danger limit, take these as guidelines as the limits will change with humidity and other environmental factors.

Whether or not these voltages are lethal really depends on the situation. For example, if you are working inside and power cabinet and you touch 1000 VAC with your elbow resting on the earthed shell, you will most likely BBQ your forearm and need an amputation. Do the same thing with 1000 VAC in your left hand and earth in your right hand and its game over.

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I agree with the other answers about that is the current that kills, but most off the other answers forget that the internal resistance of a body is not constant.

  1. How big is the body, a child, a small woman and a big man do not have the same mass.
  2. Contact area, i.e. how moist is the skin and how thick is it.
  3. How far shall the current travel in the body, longer distance means higher resistance (just like any other cable out there). So there is a big difference if you have 2 wires connected directly to your chest or if one cable is attached to your hand and the other to your foot.

Then with this input you can calculate how big the current will be at the different voltages.

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Yes these are certainly factors but once current reaches the nerves resistance becomes incredibly small. However the initial voltage required to cross the skin into the nerve system of our bodies remains relatively constant despite age, size and contact area with the conductor. –  klonq Sep 6 '11 at 7:56
    
@Johan - I'm being picky, but I'm not sure I agree with your opening comment, the fact that body resistance is not constant is the main theme of my answer? also Russell mentions varying risks dependent on internal resistance. –  Oli Glaser Sep 6 '11 at 13:06
    
@Oli Glaser How about the change in phrase from "all" to "most" ;) –  Johan Sep 6 '11 at 13:24
    
@Johan - thanks, that's fine :-) –  Oli Glaser Sep 6 '11 at 13:35
    
The resistance of the human body depends on voltage, too. :) Larger voltages reduce the resistance of the body and increase the current more than if the body had a fixed resistance. –  endolith Sep 7 '11 at 19:31

From my experience;
Once, I connected output of a transformer to a voltage doubler to obtain 65V DC voltage. When I touched it with my two hands, it didn't shock me, it didn't even made me feel it. If I hold my breath and stay really calm like a training Buddhist monk, I barely felt a very tiny vibration at my fingers.
I didn't measure current then. I am a male with an average body, and my hands were not dirty at that time.

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I know some will probably frown at this, but +1 for the vision of a buddhist monk trying to measure electricity with mindpower, a la Shaolin Temple kung fu flick training scene.. :-) –  Oli Glaser Sep 6 '11 at 19:55
    
@Oli Glaser; Thank you :) –  hkBattousai Sep 6 '11 at 21:34
    
And, on some other occasion, you'd be unfortunate and die. Dry hands at 65 VDC is most often non fatal. Wet hands and bad luck and you could have a bad day. –  Russell McMahon Sep 7 '11 at 16:49
    
Was it still 65 V when you were touching it? –  endolith Sep 7 '11 at 18:35
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@endolith Yes it was. Since output impedance of the voltage source was low enough, it didn't change its voltage value after I touching it. –  hkBattousai Sep 11 '11 at 4:39

From my experience.

I have built a single-pulse high voltage source that charged a 6 uF capacitor to 600 Volts and discharges it through a transformer's primary winding so that it's about 30 kV at the secondary. I got a shock from it through a 1 cm air gap, and it caused me to lose hearing and vision for a few seconds. Fortunately both recovered completely, but it was scaring even to switch this circuit on. I was lucky not to have bought a 400 uF capacitor battery for that voltage.

I don't think the voltage means much above a certain threshold, but the energy does definitely.

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