1

First of all the actual amounts needed to kill and the body resistance is not important for this scenario. Let's say we are working with dc. Let's say that 100ma is enough to kill across the chest give or take. Let's say we have a generator with a constant voltage and maximum current of 10kv and 10ma. In the right conditions, this could put 100 watts of power across the chest. How can this not kill?

My question is based on watts = volts times amps. Exsample 10v 1a motor should give the same wattage as a 1000v 10ma motor.

Is it more likely there is not enough current to stop the heart but you would die eventually by being cooked! I have asked this question many times but nobody can give a suitable answer. Or perhaps more like the reason for the answer.

Alan Wolstenholme
  • 11
  • 2
  • 3
    It is specifically current that is fatal in most electrical accidents. Power can be too (electrical burns) but that is less common. – user1850479 Feb 23 '21 at 04:55
  • It is unlikely you’d get 1A to flow across the body at 10V. You need a bit more voltage. Once you get past 50V is where things are getting dangerous. I’m pretty sure there must be extensive research done on this and the mechanisms involved. – Kartman Feb 23 '21 at 04:59
  • 2
    What is missing from your scenario is that the generator is not able to set both voltage and current. The person is the electrical load. Depending on details, the person has some resistance. So the voltage and current put out by the generator must satisfy Ohm's law. V = I * R. If the person is a 1K resistor, the 10mA limit of the generator will make sure that the applied voltage will only be 10V. So the power will be 100mW. I do agree with you that a person will die from heating if they are receiving enough watts. Not sure 100 Watts is enough to do it quickly, though. – user57037 Feb 23 '21 at 06:23
  • Oh what about if electronics played the part of supplying that same voltage minimum after the body resistance and the current was electronically limited to 10ma? – Alan Wolstenholme Feb 26 '21 at 05:53
  • What needs to be appreciated is that lethality ratings for current are based on typical voltages and environments. / While you may need 100s of mA at the chest surface to kill you,you only need microamp level heart currents. If your very high current source can cause heart surface currents of say 50 uA you may die. – Russell McMahon Feb 26 '21 at 09:13
  • Hi mkeith. Ok, let's hypothetically say that for one reason or another the body resistance is not 1k but 1000k. That would drive the 10 ma and keep the voltage of its highest limit producing 100 watts right? – Alan Wolstenholme Feb 28 '21 at 05:43

2 Answers2

7

There is more than one reason that electricity can kill you. Probably the most popular one to talk about is cardiac fibrillation. This is caused mostly by AC circuits disturbing the regular rhythms of the heart and can be cause by as little as 10mA across the chest cavity as you suggest. Other issues can arise from less spoken about effects like electroporation of cell membranes and explosive vaporization of tissues. I'm sure you can imagine why people don't like talking about those. It's a bit disturbing. Electroporation causes your cells to become "leaky", so parts like sodium and potassium pumps can't do their jobs anymore and the cell either enters apoptosis or necrosis if it doesn't just fall apart first. This can happen at the gap between the two sides of a button battery lodged in your esophagus (some excruciatingly sad stories can be told of that). Sometimes you can survive these incidents, sometimes you can't. I'm not going to discuss explosive vaporization of tissue... for obvious reasons, but there is a good reason that you should NEVER put your hand into a Jacobs Ladder, no mater how thick your rubber boots are. Let's suffice to say, there are simply too many reasons that electricity can kill you to blame it on one phenomenon. If anybody really NEEDS the gory details, I'd be happy to discuss it further, but really? Electricity can be very dangerous when not well understood. It deserves and commands a great deal of respect.

ScienceGeyser
  • 1,526
  • 3
  • 9
  • Yes please tell us more about this. Also the jacobs ladder. How much amperage falls across your hand?electroporation of cell membranes and explosive vaporization of tissues. I'm sure you can imagine why people don't like talking about those. It's a bit disturbing. Electroporation causes your cells to become "leaky", so parts like sodium and potassium pumps can't do their jobs anymore and the cell either enters apoptosis or necrosis if it doesn't just fall apart first – Alan Wolstenholme Feb 28 '21 at 05:23
  • Depending on the specific implementation of the Jacob's ladder and the atmospheric conditions at the time, the arc temperature could be anywhere between 3,000 and 20,000 degrees Celsius. I don't think the current flowing through your hand will be your biggest concern with temperatures like that. As far as the electroporation goes, there are a lot of details that can be discussed about this and entire books have been written on the subject. Do you have any specific questions about how it works? – ScienceGeyser Feb 28 '21 at 05:52
2

The two lowest power/current ways to kill a human are defibrillation and brain damage, and 100mA AC is roughly the amount of current that is likely to stop a heart. This amount is specced in current rather than wattage because the lethal current would have different wattages and voltages with different paths of travel across the body. The wattage itself is still a danger in that when you lose muscular control you can't separate yourself from the circuit, so there are cases where people die by cooking. The wattage necessary to do that is rarely/never mentioned because it would depend on how long you were locked on and wouldn't really be a useful figure. Another example of lethal danger from wattage would be flash or explosion. An electrician, for example, doesn't need to know exactly how much wattage cooks him how fast. He needs to know he should shut off the systems he's working on when he can and use all available precautions when he can't. He uses insulated boots and a fibreglass ladder and work procedures that will minimise the chances of getting shocked at all.

Any shock that can kill you is a shock worth preventing. So if you take some safety courses, you may end up watching videos of electricity hurting or killing people in a variety of ways, but it would be rare to discuss these events by wattage in basic electrical safety when anything above the 50-100mA that can kill you is a no-no anyway.

On the other hand you may be interested in looking at arc flash suits. Because the suits are rated by how large of an arc flash (conducting fireball) you can survive, they are rated in categories and have calorie ratings required for each level of protection. The evaluation of how much danger is present in a system would involve wattage among other things, but is complex enough that it's handled by professionals.

Note also that the output of whatever is driving a load can fluctuate when the load is changed. Your 10V 1A motor circuit won't be able to pass enough current through you to more than make you tingle. Your 1000V 10mA drive circuit on the other hand could produce an initial high current surge even if it was current limited, and if it isn't current limited, its actual output will be determined by your body's impedance and the source impedance. If it drops to 50mA AC at 200VAC or 100mA at 100VAC, that could still kill you even though it represents the same wattage and is intended for 10mA output.

K H
  • 4,023
  • 1
  • 11
  • 24