And you often hear about people being killed by testing batteries with their tongue,
The chances of such reports being true are close to zero.
I have never heard of such a report that is reputable or traceable to source*.
I cannot think of a mechanism that would make doing this likely to be fatal.
I do not recommend that others do this without advice from their attorney, GP, surgeon, insurer and spouse., BUT, I have 'tested batteries with my tongue' probably hundreds of times in my lifetime at voltages up to 9V / PP3 batteries. The sensation is 'interesting but nothing like the sort of effects I have experienced from more substantial and life threatening electric shocks.
Electrocution by AC mains:
See references at end.
Short: You usually need to pass a certain amount of current through your heart muscle long enough to cause fibrillation of the heart. The trigger point is inexact and will vary widely with circumstance and person. Many people receive severe mains electric shocks and survive with little or no apparent damage. Others receive what may appear lesser shocks and die. Luck and more is involved and not doing it is the best protection.
I have read accident reports of people who died due to a finger to finger shock while unplugging a cord from a mains socket. There was probably a secondary path to earth through the body as well.
Death from electric shock from mains AC at residential voltage levels (say 100 VAC - 240 VAC range) is usually caused by fibrillation of the heart. Current passing through the heart interferes with the natural pace making cycle and the heart goes into an oscillatory mode that does not support pumping. It usually takes a current of look_it_up mA for a period around a certain criticaL heart timing parameter. The details are not as important as is knowing that you want to keep current away from your heart or, if possible, away from your body.
I have had probably dozens of AC mains shocks over a long lifetime - some severe, most less so. I've also managed a hand to hand 1200 VDC shock (not at all recommended) and one RF kick from a 100W + voice triggered transmitter ("hey - look, the aerial is disconn ... AGH!!!")
I've made an increasing habit of trying REALLY hard to avoid mains shocks and I can't now remember the last time I had one. The odd very high impedance tickle is known to happen from time to time - but as these are a step on the path to too much current they too are avoided.
Ground fault interrupters / GFI = Earth leak circuit breakers / ELCB are designed to detect leakage of current from a circuit to ground and to then terminate the power supply in less time than the critical period. Say around 10 mS from memory. I purposefully tripped an ELCB with my body long ago to see what it felt like. Back of hand contact as below. The shock was substantial and painful even though it lasted only for a few mS.
A major problem with electric shock is that the body's muscles tend to contract under current frow and the victim may grip the live item and not be able to release their grip. This effect can occur at as low s 12V DC under "ideal" conditions. (Such as eg standing in salt water holding a metal fish spear and a LED lantern powered by 12V and mounted on hand held metal pole. A friend did this and a fault caused a return path via the fish spear and water and their legs and they report having their hands clamped on the spear and being unable o release it. ) DC is worse than AC for this but AC can be every bit as bad as is requried to be fatal.
If you ever MUST touch something which may be "live" then I've seen it recommended that the back of the hand or fingers be used so that if the muscles contract the limb will contract away from the conductor and will; not instead grasp it. This is in fact the precaution that I have taken personally for many years but see your attorney surgeon etc as above and take this as comment and not advice.
Electric shock - OSU
Hyperphysics page - Electric Shock
Wikipedia Ventricular fibrillation
Wikipedia - Electric shock - magnitude says:
The minimum current a human can feel depends on the current type (AC or DC) and frequency.
A person can feel at least 1 mA (rms) of AC at 60 Hz, while at least 5 mA for DC.
At around 10 milliamperes, AC current passing through the arm of a 68 kg (150 lb) human can cause powerful muscle contractions; the victim is unable to voluntarily control muscles and cannot release an electrified object. This is known as the "let go threshold" and is a criterion for shock hazard in electrical regulations.
The current may, if it is high enough, cause tissue damage or fibrillation which leads to cardiac arrest;
more than 30 mA of AC (rms, 60 Hz) or 300 – 500 mA of DC can cause fibrillation.
A sustained electric shock from AC at 120 V, 60 Hz is an especially dangerous source of ventricular fibrillation because it usually exceeds the let-go threshold, while not delivering enough initial energy to propel the person away from the source.
However, the potential seriousness of the shock depends on paths through the body that the currents take. If the voltage is less than 200 V, then the human skin, more precisely the stratum corneum, is the main contributor to the impedance of the body in the case of a macroshock—the passing of current between two contact points on the skin. The characteristics of the skin are non-linear however.
If the voltage is above 450–600 V, then dielectric breakdown of the skin occurs.
The protection offered by the skin is lowered by perspiration, and this is accelerated if electricity causes muscles to contract above the let-go threshold for a sustained period of time.
If an electrical circuit is established by electrodes introduced in the body, bypassing the skin, then the potential for lethality is much higher if a circuit through the heart is established.
This is known as a microshock. Currents of only 10 µA can be sufficient to cause fibrillation in this case.
Added - March 2016.
- A reader mentioned a report of a man being killed using a multimeter on an Ohms range and pushing the probe tips through his skin into his body so the current flowed through his core resistance of (it says) around 100 Ohms.
It is essentially certain that this account is fabricated
(and most "Darwin award" stories are. I'd be genuinely pleased to know of a reputable link to the original claimed report of ant reputable report where this happened.
Note that it is substantially different to connect to a 9V powered Ohm meter than to directly contact a 9V battery's terminals.
Battery across the tongue - my assessment above stands.
Battery hand to hand with probes/contacts pushed through skin into body core !!! - Ouch! - I'd not recommend it ir do it purposefully. Death may be possible, but seems unlikely. 12V across chest near heart using sharp tipped probes HAS caused death. (Reference not to hand). However ....
An Ohm meter even on a low Ohms range is designed to provide a voltage such that at zero Ohms external load the meter is just delivered full current.
An example of a Simpson 260 meter as claimed to be used by the cited report.
There are a range of model 260 Simpson meters but all seem to be 20,000 Ohms per volt meters meaning it needs I = V /R = 1V/ 20,000 Ohm = 50 uA of current to achieve full scale deflection. The original 1930's model had a 50 uA movement and all others since also seem to have had. This is typical of this class and age of meter.
While the designers MAY arrange for 100 uA or 1 mA of full scale current to flow on low Ohms with probes shorted, the odds of it being designed to deliver 100 MA or even 10 mA in such conditions is minimal.
Questionable 1999 Darwin Award report
Simpson model 260 multimeter as reported to have been used:
Original 1930's model 260 - even this had a 50 uA movement.
260 series 2
This chart (source shown on right hand margin of image) shows combinations of time-current which may cause various effects, ranging from perception through death.
I'd err on the low side when deciding what might be harmful.
Note that (Ventricular) fibrillation usually kills if not reverted promptly.
The orange AC4-1 zone suggests a 5% to 50% chance of fibrillation.
The suggested boundary conditions for potentially lethal AC4-1 include
50 mA for 1 second, or
100 mA for 500 ms, or
200 mA for 250 ms, or
500 mA for 50 mS.
The lowest current suggested to possibly cause death is about 30 mA (for > 5s).
Aim for lower or none of any of these examples.