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Can someone please explain to me in simple terms how is it possible to have high voltage and low current and low voltage and high current and what actually does harm to human body.

Here is what I don't understand:

12V / 1000 Ohm = 0.012A (12mA)

300V / 1000 Ohm = 0.3A (300mA)

So if I go and touch 12V battery terminals, my body is 1000 Ohms 12mA of current flow through me and it is safe.

Then if I go and touch 300V battery terminals and 300mA of current flow through me it is fatal.

Yet 12V battery vehicles have fuses rated for 30A and more. Well, that means if there is a short etc. 30A will flow through a wire and break the fuse.

30A in a short circuit wire with a 12V is safe and 0.3A is a 300V is not? How is this possible?

MrGerber
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Adam Cimler
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    A short circuit is very close to 0 ohms so if you add that in your equation you will see high current with both voltages. – R.Joshi Mar 06 '18 at 08:44
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    _"12mA of current flow through me and it is safe."_ - 12mA of current through you **is not safe**. If you manage to get all of that current flowing through the wrong places (e.g., your heart), you're at serious risk. – marcelm Mar 06 '18 at 09:54

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First of all, 30A in a short circuit wire with 12V is not safe. Chances are the thing will go bang or get extremely hot before you even have the chance to touch it.

As for your actual question 'how is it possible to have high voltage and low current and low voltage and high current?'. That question you have sort of answered yourself. It's Ohm's Law. you have used the equations to work out the current in those batteries. Now, change the resistance in that circuit and you will see the voltage changes, hence that is how it is possible to have high voltage and low current, and also low voltage and high current.

As for what damage can these currents do to a human.... Well, this link HERE will tell you all you need to know about fatal currents and electrical safety. There is also a discussion on what damage can be done in a question asked on this site HERE if you wanted a bit more of a read!

MCG
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In terms of the Human body, what causes harm is the current, or number of electrons flowing per second. The voltage is the "pressure" needed to overcome the resistance to the flow.

Having said that, if a high voltage is needed to drive a moderate or otherwise "safe" current though the body it can cause burns from the dissipated energy.

In short, what will kill you is fairly complex given the variability of skin resistance, the current paths involved etc.

Rule of thumb - anything over 30mA is seriously bad no matter what the voltage.

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Dirk Bruere
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  • Ok, so if anything over 30mA is seriously bad, let's look at standard DC bench top power supply that delivers 0-30V 0-10A. You wanna tell me that such DC power supply selling of the shelf for hobbyist and even kids building circuit at home for example, is completely safe? – Adam Cimler Mar 06 '18 at 09:14
  • I do not disagree with what you say, I just can't get my head around it. – Adam Cimler Mar 06 '18 at 09:14
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    @AdamCimler Depends. Can that 30V drive 10A though your body? Generally, the answer is "no" because skin resistance is so high. So high that with dry hands touching the outputs you would probably feel nothing. OTOH, if you used needle electrodes to pierce the skin the resistance would fall to the point where enough current might flow through your arms to stop your heart. – Dirk Bruere Mar 06 '18 at 09:24
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    Now we talking, so it is the low voltage that simply won't allow enough current to flow though me. So even I am exposed to 30A in a car 12V circuit I will simply act as giant resistor. Thanks mate, appreciate it. – Adam Cimler Mar 06 '18 at 17:04
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I love electricity! The problem with your question is that you're looking at voltage and amperage, and not thinking about the end result... wattage. Wattage is the combination of voltage and amperage, so 120V@10A is the same as 240V@5A. Resistance limits flow, but voltage stays constant. Dry human skin has a relatively high resistance to current flow, which is why that 12V battery won't hurt you. So, if your car battery specs were swapped to be 1000V@12A, it would kill you, even though it's wattage is the same. And you can use smaller wires for high voltage/low amperage to get the power (watts) you need.

Kevin James Yancey
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If you have a short circuit on a 12V battery you have a short circuit and these 30 amps will take the path with the lowest resistance (namely the short circuit itself), this has nothing to do with you touching it.

po.pe
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As far as I understood its not the voltage nor the current (to a specif limit of course) which harms the body. Its the energy or lets say power over time which can effect serious issues to the body. I think it is a more medical question. Maybe a PhD can answer this question in a serious scientific way. I just believe (how should someone be tested?) the physical body is a varistor thus a non constant resistor.

Daniel Gschösser
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    It definitely is the current that harms the body. Your body is controlling your muscles with small electric signals. An external induced current flowing through your body would irritate or "overrule" these signals. This leads to the effect that e.g. a wire that has been touched cannot be released due to muscle contraction as long as current is flowing (from a certain degree on). Now the big danger is if the same thing happens to your heart muscle. – po.pe Mar 06 '18 at 08:59
  • Very interesting I never though of it this way, Thanks – Adam Cimler Mar 06 '18 at 17:09