Does current or voltage kill?

  1. As an electrician I hear it far too often that its not the voltage that kills, its the current. I find it fustrating because ive worked with people who fear car batterys because their high amperage output or got electrocuted by 600v and turns around to tell me good thing voltage dosent kill.

    Now I try to explain that the current results because of voltage and that the current magnitude could even be a measure of the damage cause by the voltage and to fear voltage but I always get a question that I cant respond to well.

    What about high voltage coil packs or spark plugs that can apply hundreds of thousands of volts and do not do severe damage.

    Now I know they are current limiting. So if they are current limiting there is no longer a high voltage applied across you. Because low current also means low voltage when resistance is the same(resistance of the body). So I always felt the voltage actually applied to the body by a spark plug is always much smaller than what is popularly beleived.

    Now is this the case and is this the view that other electrical engineers view this?
  2. jcsd
  3. Matterwave

    Matterwave 3,865
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    Gold Member

    From a physicist's point of view, you need both. You need a certain voltage to overcome the resistance of your body, and then you need a certain current to actually do damage.

    But the necessary voltage and currents are not very high. A current of a few milli-amps across the heart will be enough to kill. A voltage of 100V or something like that is potentially enough to get a few milli-amps across your heart. So, a standard electrical socket is pretty dangerous because it can provide both the voltage and current necessary to kill you.

    But voltage itself is not enough to kill. There are sources of very high voltage that don't provide very much current at all as would be the case with static electricity.

    So really it's "a combination of high voltage and high current kills".
  4. How can u have a high applied voltage without current though? Since your voltage is just of your current times resistance?

    So if you have a device that outputs a high voltage and low current you must have a set resistance for such a value.

    So I always look at it as your voltage divided by your body resistance gives you your current. So if your trying to asses the danger of equipment it should always be the applied voltage that matters.

    I guess my question is how can you have a peice of equipment with a higher applied voltage(spark plug) but have a lower current compared to something with a lower voltage(say a wall plug) causing a higher current with equal resistances?does this not defy ohms law.

    When I think of static electricity I dont think of a high voltage applied to the body but instead to the air gap. Is this the case?
  5. the power source could be design limited to a very small current, like a Taser. Also, real world sources have limitations to them. If you took a battery, and boosted its voltage to very high levels, you would only be able to deliver a small current because of the power limitations of the battery. No matter how much you want to draw from it, it cant drain any faster than the chemical reactions allow.

    Essentially you are right, in that it takes a high level of voltage to create a lethal current, but there are many factors that can adjust the level voltage necessary. If your skin is wet, it resists less. If the electric shock occurs over a period of time, it breaks down an easier path for the voltage to travel. And like I said, high voltage can be current-limited. At the end of the day, its a matter of how much current is passed through your heart. so technically its the current that kills, but since lethal current is pretty low, it is high voltage that is dangerous.

    And static electricity is essentially just high voltage with 0 or nearly 0 current.
  6. Matterwave

    Matterwave 3,865
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    The voltage source may be unable to maintain a large current if the resistance is too low. For example, I might have a capacitor plate charged via a source to 5000V. If I touch the two capacitor plates together with a wire, there's no way any source can maintain the 5000 volts over the new configuration since you basically created a short circuit. The amperage has some maximum value it can read, and the voltage between the two plates will decrease dramatically.

    Every realistic current source or voltage source has an internal resistance. If you short circuit it, current will be limited by the internal resistance.

    So, say I have static electricity built up on by body. I might reach very high voltages compared to the ground, but I will only have a set number of extra electrons stored in my body. When I discharge this electricity, the current is quite low because there's a finite amount of current I could possibly drive.
  7. nsaspook

    nsaspook 1,390
    Science Advisor

    I work daily with high voltage, some bias (1-5 kv) supplies would only deliver a small current in the mill-amp range into a low value resistance but they are connected to large panels or plates with substantial capacitance for storing charge so the energy available to discharge into a human is greater than you would think from a pure DC voltage into resistance calculation. One of worst shocks I ever had was from a current limited source like that and it wasn't the shock that really hurt, the muscle spasm from it caused me to fall back and knock myself out from hitting the floor with my head.
  8. Edit :I didnt read all posts before comment ting
    Last edited: Jun 9, 2014
  9. I think I know where this may go... a taser is a non ideal current source with a peak voltage of 20kv so when your add a load... say your skin... the voltage would drop
  10. But a capacitor would still push full current momentarily. .. thinking back to circuit analysis (its been a couple months so I may be wrong) I=C dv/dt or I (current)=I(initial)e^-at... so once you close the circuit with your finger should you not momentarily feel a full current associated with the capacitor... and as your voltage decreases your current matches?

    Btw thanks for the replys this is something that has troubled me so far through my first couple years of EE
  11. nsaspook

    nsaspook 1,390
    Science Advisor

    The RC time might be a milliseconds at peak voltage but that's plenty long to burn a channel in your skin directly to the nerves and blood vessels for a really good shock while it decays to some low current value limited by the power supply.
  12. Note that the voltage read by a voltmeter is by no means the same voltage that a source would apply to a person. If the internal resistance is very large, then the applied voltage will be small.

    Saying that current kills never made much sense to me. Of course current implies voltage, and vice versa. A sustained lethal current is associated with a sustained lethal voltage.

    (I know I'm rehashing other posts, but I sometimes appreciate seeing the same answers in different words.)
  13. HUH ??? I don't think Ohm would agree w/ you.
  14. Take a 10V source with 1 MOhm of internal resistance. An ideal voltmeter would read 10V across the open terminals, would it not? But, a 1 Ohm resistor connected to the terminals would have a negligible voltage applied to it, wouldn't it? Isn't that the whole idea of Thevinen analysis?

    Edit: What I meant was that the measured open-circuit voltage will not be the same as the closed-circuit voltage.
  15. D H

    Staff: Mentor

    Last edited by a moderator: Sep 25, 2014
    1 person likes this.
  16. Thanks for sharing that video, D H. That's fantastic :D
  17. Matterwave

    Matterwave 3,865
    Science Advisor
    Gold Member

    I feel very nervous watching that video...
  18. IMO Current kills, period. Regardless of the voltage, X ma kills (pick an X). If you have to force that current through skin resistance, then higher voltage is required. If you use needles to break though the skin, then less voltage is required. If the skin is dry, resistance is higher, so more voltage is needed to get that current.

    Now, keep in mind X is not a fixed value, since it depends on the path of the current. But, for a given path, the current must reach X to kill.

    You can feel a 9V battery across your tongue, but not across your fingers.
  19. Ah. I thought you meant the internal resistance of the PERSON and were talking about an ideal source in which case your statement made no sense. Given your description, yes, you are correct.
    Last edited: Jun 10, 2014
  20. In my opinion:
    Current thru the heart kills. Or, so much power went thru the body that the person was charred, very gruesome in that case.

    High voltage may kill, BUT: will that high voltage maintain its high value? static charge certainly can not.
    will that high voltage cause a current to flow thru heart and screw up human heart's internal pace maker? Lightning does not always kill because huge amount of current went thru the body but not enough to disturb the heart.

    we had a guy somehow walked between a 13.8kv breaker while the unit was on. high current went thru him, but we had a big belly, he lived.
  21. On this forum, it's generally better to stick to the facts rather than your opinion.

    I agree that lightening does not always kill. That is well documented. But how can a huge amount of current go through the body without being enough to disturb the heart?

    I assume you mean "big belly laugh". Would it have been funny if he had died?
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