# How much voltage/current is "dangerous"?

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...), but they sometimes do not use 1.5 V with the low amperage levels, some use a DC motor to vibrate and complete the trick.

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) would be considered hazardous?

No old telephone lines have always been 48vDC well at least since from 1950s, if your skin is wet you can feel it slightly, like on your forearm. Now the ring voltage is 90-110vAC with a 2 on 4 sec off cycle (USA). It will ring your bell but good, should you be touching the wires when someone calls. The ring voltage rides on top of the 48vDC, so its present on the same two conductors that the voice voltage(DC) is on. Luckily it's 4 seconds off will give you a chance to get off the conductors with a scream (of pain).

• 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. Commented Sep 5, 2011 at 19:20
• 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. Commented Sep 6, 2011 at 6:34
• @AndrejaKo: It's dangerous to touch both terminals of a 12 V car battery? Commented Sep 7, 2011 at 16:02
• @AndrejaKo: That's a burn, though, not a shock. You're not conducting a current, the metal is, and you are being burned because you're touching the hot metal. Commented Sep 7, 2011 at 20:15
• 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. Commented Sep 9, 2011 at 20:20

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:

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:

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.

• 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. Commented Sep 6, 2011 at 16:58
• Ah yes, thanks. I should have said "page" rather than "reference". Commented Sep 6, 2011 at 17:17
• Most of the painful data dates back to second world war. Thinking of which ruins my appetite for todays breakfast. Commented Mar 7, 2015 at 9:11
• Some of this data doesn't make sense. How can "Threshold of perception" be highre current than "slight sensation"? Commented May 13, 2015 at 13:30
• Hi Olin. Yes - I wondered about that too. Maybe they got them the wrong way round? Commented Apr 17, 2017 at 2:36

I add to this answer ongoingly, either as new information comes to my attention, or when the subject is raised elsewhere on SEEE, or when 'pushback' occurs to what seems to be to be a very well established and very important fact. That is

• "12V and even substantially higher DC voltages are extremely unlikely to kill you.
But, it has happened, and can happen in exceptional circumstances.
Do not be scared of such voltages, but do be aware of the unlikely but potential dangers."

FACT:

• 12 VDC CAN kill and has killed people.

• While 12 V is almost always safe, worst-case situations can and have led 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 the discussion and solid 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.

The references, especially A review of hazards associated with exposure to low voltages make it very clear that a number of reputable peer reviewed sources substantiate this fact.

I have received a substantial amount of "pushback" in the 11+ years since I posted this answer.
A summary of my answer is: "12V sources can cause death in very unusual worst case circumstances. While this is exceptionally unusual the possibility exists. The most likely situation would be application either across the chest or from chest to one limb in conditions where the chest was wet. Death could be from ventricular fibrillation or paralysis of respiratry muscles. Voltage would usually need to be applied for a prolonged period. In exceptional circumstances muscle lockup in conjunction with a secondary effect such as drowning may occur.

This paper A review of hazards associated with exposure to low voltages summarises a range of peer reviewed literature. It makes it clear that electrocution at 12V would be very very rare indeed, and that conditions to make this possible in some cases are 'easily enough' arrived at.

I suggest that people who wish to comment on plausibility first read the relevant parts of this paper.

At the end I've added EXTRACTS FROM PAPER A review of hazards associated with exposure to low voltages.

There are two main "what kills" issues.

• One is general trauma - burns, etc., and that is obviously very situation and person-dependent. 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 fibrillation.

Also possible are respiratory muscle paralysis and muscle lockup followed by death by a secondary cause.

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 20 mA and in about 20 ms = well short of a heart cycle.
See graph below.

Image from above article:

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

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

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

A 12 V "car battery" or any high current source from a few volts up MAY kill in the very worst case. Hand to hand, I have 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 causes annoying minor shocks (as experienced in e.g., 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 the 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 12 V 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 himself (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.

A Summary of Surveillance Findings and Investigative Case Reports - Part I. Electrocution-Related Fatalities.

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 dependent 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 12 V, if resistance did not change with the 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.
[email protected]
Convenor IEC TC108/WG5, IEC 60990 'Measurement of touch current and protective conductor current"

A careful but less than an 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 hearsay reports and actual cause of death may not have been electrocution.

### Table 1. Estimated Effects of 60 Hz AC Currents

Amps Effect
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.

February 2023:

This paper cited by Nick Bolton provides much useful material

A review of hazards associated with exposure to low voltages - 18 pages. Published ~= 2004.

Interestingly - this answer has 2 downvotes* - which is interesting considering the undoubted truth it tells. Maybe the downvoters 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.

• And a 3rd on August 11 2022

EXTRACTS FROM PAPER A review of hazards associated with exposure to low voltages.

The letters at the start of each extract indicate where the extract may be found within the paper. These are of the form Page number, column (l or r), n = 10ths of a way down the column.
So eg 13 l.8 = page 13, left column, 8/10th way down column.
Below some extracts I indicate the current drawn with 12V applied for the lowest resistance mentioned.
Note that I have consciously "cherry picked" these extracts, and in some cases not included conclusions from the information provider. This does not make them less valid - read and see.

12 r.5 The 1988 European Organization for Nuclear Research, (CERN) “Dangers due to electricity” safety Instruction (IS-28) is “essentially based on IEC publication 479-1” (see above) including statistical resistance values (Table 1). In addition the IS-28 report specifies a total body resistance of >650 Ohms under moist/wet conditions and 325 Ohms for ‘immersed skin’. Presumably based on minimal ‘let-go’ thresholds, the IS-28 report specifies “as a rough guide to complete safety, the current limit should be considered as 10 mA” for <20 ms.

Noting that 12v and 325 Ohms gives 37 mA

5 l.4: DiMaio and DiMaio (2001) concluded that for 120 V, dry skin may have a resistance of 100 kOhm; dry and calloused skin up to 1000 kOhm; moist skin 1 k or less; and moist, thin skin as low as 100 Ohms

100 Ohm = 120 mA at 12V

5 r.8: Hart (1985) reported an internal resistance of 400-500 (hand-to-hand) and 450-500 (handto-foot). He found internal body resistance from hand-to-forearm was 140 Ohm and from finger-to forearm 700-800 Ohm

140 Ohm = 85 mA at 12V

5 r.9 - 6l.1: Statistical impedance data was developed by Underwriters Laboratories (12 VDC, relatively large electrodes, ‘wet’ conditions; Whitaker 1939) Low Voltage Electrocution - 6 - M Bikson for children (3-15 years; 14-58 kg) and for adults (18-58 years; 45-95 kg).

• For children they found a resistance variation from the 5% to 95% rank of 1.7 to 4.47 k (hand-to-hand) and 0.9 to 2.04 k (two hand-to-two feet).
• For adults they found a resistance variation from the 5% to 95% rank of 1.28 to 2.45 k (hand-to-hand) and 0.63 to 1.16 k (two hand-to-two feet).
• 60 Hz AC resistance values can be 60-90% of these DC resistance values (Reilly 1998).

6 l.2: Thus the majority of research reports consider worst-case (large contact area, wet conditions) total body resistance (limb-to-limb) to be slightly greater than 500 Ohm. Worst-case resistance across the chest can be less than 100 Ohm. Under nonworst-case conditions (e.g. small contact size, dry skin) total body resistance values quickly increase to greater than 2 kOhm.

100 Ohm = 120 mA at 12V

"Let-go" limits are well below those usually needed for electrocution. However, an inability to release grip can and has lead to injury and death. My flounder-fishing friend could have drowned worst case.

9 r.8: Let-go comments: Gilbert (1939) used the release-grip endpoint in determining an average let go current of 21 mA. Whitaker (1939) using the release-grip endpoint current found a range of 8.4 mA to 14 mA, average 11 mA. Thompson using a rotate-handle endpoint reported an average ‘let-go’ current of 11.7 mA, maximum 28 mA. In the commonly referenced initial report of Dalziel (1938), using the release-grip criteria, the average endpoint was 17.7 mA, maximum 25 mA (male subjects). These reports were reviewed by Reilly (1998; Table 2). Dalziel and Lee (1969, 1972) summarized results from 124 males and 28 females; the average ‘let-go’ currents were 22.3 mAPEAK male and 14.8 mAPEAK female Low Voltage Electrocution - 10 - M Bikson while the lower 0.5 percentile values were 12.7 mAPEAK male and 8.5 mAPEAK female. Thus across studies 8.5 mAPEAK appears a safe ‘let-go’ threshold

13 l.4: Based on the reports summarized above, respiratory paralysis can lead to death but requires several minutes of contact; currents as low as 30 mA PEAK across the chest may induce paralysis.

13 l.8: Based on the reports reviewed above, for less than 1 min duration electrical contact, currents 40 mA may be necessary to cause ventricular fibrillation,

THIS MACHINE (ONLY VERY VERY OCCASIONALLY) KILLS. BE SENSIBLE - DON'T LET IT
to SEEE Jan 2020 question Can a switching power supply kill you? adds usefully (imho) to this discussion.

This image from that answer (and another) provide information on IEC claimed limits. From here with my annotations. Th AC3 area is probably of most interest in the current (unintended pun noted) context. While the B boundary suggests 5 MA at 10 seconds, it is not clear why, if 5 mA will cause ANY result, why there is a 10 second period during which it is not relevant.
My above recorded account of a friend's muscle lockup experience at 12 volts (while standing in salt water & holding a metal fish spear) is very much an extreme case - and could easily prove fatal for others in similar circumstances if experienced when assistance was not available.

• Please provide a reference for the claim that 12 V has killed someone Commented Sep 7, 2011 at 16:42
• @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. Commented Sep 7, 2011 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. Commented Sep 7, 2011 at 19:00
• Prolonged exposure to even very low DC currents can kill body tissues by electro-chemical effects. Medical devices with electrodes that attach to the body (e.g., heart monitors) are rigorously tested to insure that DC current can not flow in the leads. Commented May 13, 2015 at 19:06
• 24VDC is considered Safety Extra Low Voltage or SELV in hospital environment and that should tell you enough about how deadly 12VDC is. Driving voltage inside the body with needles makes things much worse but still, 24VDC doesn't hack it beyond possibly cooking other electronics. Commented Feb 3, 2023 at 12:22

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.

• 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 Commented Sep 5, 2011 at 19:45
• see my answer to this question: electronics.stackexchange.com/questions/9222/… which is a pretty much a duplicate of this one. Commented Sep 5, 2011 at 20:43
• @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. Commented Sep 5, 2011 at 22:17
• @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
Commented Sep 6, 2011 at 6:29
• 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. Commented Sep 7, 2011 at 16:05

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.

• 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. Commented Sep 5, 2011 at 21:54
• 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. Commented Sep 5, 2011 at 22:17
• @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. Commented Sep 5, 2011 at 22:32
• @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. Commented Sep 5, 2011 at 22:56
• 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. Commented Sep 6, 2011 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.

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.

• 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. Commented Sep 6, 2011 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. Commented Sep 6, 2011 at 13:06
• @Oli Glaser How about the change in phrase from "all" to "most" ;) Commented Sep 6, 2011 at 13:24
• 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. Commented Sep 7, 2011 at 19:31
• What I need is something like an electric dog bark trainer, but with a much higher shock energy. I can barely sense the output of the dog collars, so canines must be much more sensitive to electric shock than humans. (The purpose of this training is to break a bad vocalization habit I have acquired when playing the flute.) Commented Aug 9, 2019 at 2:56

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.

• 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.. :-) Commented Sep 6, 2011 at 19:55
• 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. Commented Sep 7, 2011 at 16:49
• Was it still 65 V when you were touching it? Commented Sep 7, 2011 at 18:35
• @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. Commented Sep 11, 2011 at 4:39

Department of Defense MIL-HDBK-454B "General guidelines for electronic equipment" says that current is what kills. From Guideline 1, point 5.2.1:

Shock hazards: Current, rather than voltage, is the most important variable in establishing the criterion for shock intensity. Three factors that determine the severity of electrical shock are: (1) quantity of current flowing through the body; (2) path of current through the body; and (3) duration of time that the current flows through the body. The voltage necessary to produce the fatal current is dependent upon the resistance of the body, contact conditions, and the path through the body. (See table 1-I). Sufficient current passing through any part of the body will cause severe burns and hemorrhages. However, relatively small currents can be lethal if the path includes a vital part of the body, such as the heart or lungs. Electrical burns are usually of two types, those produced by heat of the arc which occurs when the body touches a high-voltage circuit, and those caused by passage of electrical current through the skin and tissue. While current is the primary factor which determines shock severity, protection guidelines are based upon the voltage involved to simplify their application. In cases where the maximum current which can flow from a point is less than the values shown in table 1-I for reflex action, protection guidelines may be relaxed.

MIL-HDBK-454B Table 1-I agrees with Table XXXIII in Department of Defense MIL-STD-1472G "Human Engineering".

Voltage levels that require guarding are discussed in Guideline 1 of the MIL-HDBK-454B.

Guideline 1, point 4.5.3.1:

Guards and barriers. All contacts, terminals, and like devices having voltages greater than 30 volts rms or dc with respect to ground should be guarded from accidental contact by personnel if such points are exposed to contact during direct support or operator maintenance. Guards or barriers may be provided with test probe holes where maintenance testing is required.

Guideline 1, point 4.5.3.2:

High voltage guarding. Assemblies operating at potentials in excess of 500 volts should be completely enclosed from the remainder of the assembly and equipped with non-bypassable interlocks.

MIL-STD-1472G adds the following in Section 5.7.9.1.1:

General background [...] All electrical systems of 30 volts or more are potential shock hazards. Research indicates that most shock deaths result from contacts with electrical systems ranging from 70 to 500 volts.

In MIL-STD-1472G Section 5.7.9.1.5:

Electrical currents. The crew shall be protected from exposure to electrical currents in accordance with table XXXIV."

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.

The worst shock of my life was 700 VDC for a moment. It was only a moment because the involuntary jerk quickly broke the connection. But I had a nifty little burn blister punched through my skin and into my meat that took a long time to heal. I was in high school at the time, and my dad never found out (or my electrical engineering career would have been redirected into something productive like law, accounting, or dentistry).

From the above answers, it is not just the voltage and not just the current. For every voltage and current there is a time of exposure that is required to give an effect. However, I was taught in middle school electronics that 100 mA is lethal to half of the population and that 60 Hz is about the worst possible AC frequency. (In those days, the unit of frequency was CPS, named after Charles Proteus Steinmetz.)

So what we need is some function of voltage, current, frequency, and time for each effect given in McMahon's response above, as well as additional effects of incineration and explosive destruction.

The good thing about such shocks is that they provide an accelerated learning curve. Once you get a bite like my big one, you will take extreme care to avoid a repeat! I guess that is why electric shock is such an efficient training device. However, I do not recommend that others repeat this as an experiment, especially with both feet in grounded buckets of salt water. Then you will for sure never repeat the experience. Clearly Edison's great invention incorporates measures to increase the contact area and maximize the current flow.

Does anybody here remember the Caltech lightning lab?