Which shock is dangerous: 1.High V &Low I. 2.Low V & high I.

[waqarrashid33]

Which shock will be most dangerous for human
1.High Voltage and low current
or
2.High current and low voltage

 

[schip666!]

Current Kills. Voltage overcomes resistance to produce current.

Getting a dangerous current flow in a body, without skin penetration, is more difficult with a low voltage source, but still possible.

 

[BruceW]

I never really understood this. V=IR, and the resistance is defined by the material the person is made out of, so surely the ratio of current to voltage is fixed?

 

[Quinzio]

Current kills.
Of course Ohm law is valid for the human body as well. Resistance varies with the path of the current, with humidity of the hands (or the exposed part), or if the parts are wet indeed.
Human body resistance can be measured roughly with an ordinary ohmmeter, usually is 10-100 k ohm.
The more the current passes near the heart, the more is lethal. E.g. if current flows from left to right hand, it passes near the heart.
80mA can kill.

 

[Philip Wood]

1. and 2. are not valid alternatives. I'll explain. What matters is the current that flows through you (especially through your thorax) when a voltage is applied, say between your two hands. The larger the voltage the larger the current, even if your body doesn't obey Ohm's law exactly. So you can't choose current and voltage independently of each other.

Now test your understanding with this question. Suppose you put your hands on the terminals of a 12V battery when it is supplying 200A (a large current) to the starter-motor in a car (automobile). Will you feel a shock?

The first thing to be clear about: the 200A is through the starter-motor and the battery - not through YOU, so it's irrelevant. Second point: unless your hands are wet (so lowering contact resistance) your resistance will be too high for a low voltage like 12V to drive a dangerous current through you. You probably won't feel a thing. [But don't try it in case there is a fault in the car involving the high-voltage electrics for the spark-plugs.]

 

[256bits]

For the V=IR rule you have to take into account the voltage source and its internal resistance. The assumptions being made above make 2 assumptions:
1. internal resistance of the source is 0
2. the source can supply unlimited current.

An electric fence to keep cattle enclosed will not kill a cow or you or even cause debilitating muscle contraction, even though its voltage is in the upper thousands. The same thing for the shock from the car spark plug high voltage. Both will give you a good scare but nothing like that from a high voltage power line which will stop you dead in your tracks.

 

[boneh3ad]

So in light of all this, if you had an AC power system running at, say, 5 kVo-p and 5kHz with roughly 100 mA running through it under normal operation, how bad would that be to get your finger too close to one of the leads? I ask because I did that and was wondering how much I lucked out.

 

[BruceW]

So if I include the internal resistance of the voltage source, then:
V - I R_{internal} = IR
So the voltage (through me) is not as great as it should be at very high currents.
And this is just for D.C. right?

 

[256bits]

So if I include the internal resistance of the voltage source, then:
V - I R_{internal} = IR
So the voltage (through me) is not as great as it should be at very high currents.
And this is just for D.C. right?

I would think it should work for AC also. I am thinking of a step-up transformer ( say 100 to 1 for sake of argument ) with very fine secondary wire and thus a high resistance which would then limit the output short circuit( you ) current. The output power rating of the transformer would have to be taken into account, to see if that output current is enough to stop your heart and or breathing, in which case the internal resistance would not help you.

bY THE WAY, The car and fence charger by the way supply a transient jolt and not continious - ms for the car and around half second for the fence charger.

As an aside, anyone ever get jolted by one of the bug zapper contraptions. They aren't too kind on bugs, though.

 

[Philip Wood]

Hope 256 bits won't mind me emphasising: The voltage between the electric fence wire and the ground is probably over a thousand volts but only if nothing and nobody is bridging the gap. Suppose you do so accidentally. Current will flow through you - between the part of you touching the fence and your feet. BUT as soon as this happens the voltage between these bits of you will drop, probably to less than 50V, because of the internal resistance of the electric fence unit. So it's still a case of lowish current through you because of lowish voltage across you. What the ORIGINAL voltage was between the wire and the ground is hardly relevant.

 

[256bits]

So in light of all this, if you had an AC power system running at, say, 5 kVo-p and 5kHz with roughly 100 mA running through it under normal operation, how bad would that be to get your finger too close to one of the leads? I ask because I did that and was wondering how much I lucked out.

Interesting.
Would anyone know how much of the voltage is "consumed" by the spark gap versus what is left over for the zap to the finger?

 

[256bits]

Philip Wood - exactly - you said it better than I could have.

 

[chrisbaird]

I never really understood this. V=IR, and the resistance is defined by the material the person is made out of, so surely the ratio of current to voltage is fixed?

Ohm's law assumes that the voltage source can provide an infinite current if it is drawn. In reality, sources can only provide so much current. For instance, a Van de Graaff generator produces megavolts. The typical human has a hand-to-hand resistance of a few kiloOhms, so a naive application of Ohm's law says that someone touching the Van de Graaff will get a shock of kiloAmps. And yet children do it all the time in science classes without harm. The reason it that a Van de Graff generator is a high voltage/low current source. It just can't supply very much current.

 

[boneh3ad]

Interesting.
Would anyone know how much of the voltage is "consumed" by the spark gap versus what is left over for the zap to the finger?

I suppose I should say I got it close enough that it did arc to my hand and luckily only give me a few second-degree burns. haha

 

[BruceW]

So am I right in thinking that the danger to a human is due to the true current which flows through their heart/brain/muscles? And that this true current isn't necessarily given by Ohm's law?

 

[chrisbaird]

So am I right in thinking that the danger to a human is due to the true current which flows through their heart/brain/muscles? And that this true current isn't necessarily given by Ohm's law?

Yes.

1. low voltage & low current: safe (think small batteries)
2. high voltage & low current: safe (think Van de Graaff)
3. low voltage & high current: unsafe (think arc welders)
4. high voltage & high current: very unsafe (think power lines, lightning)

 

[Altrepair]

Bruce, the three conditions needed for a lethal shock are a high enough voltage source, the voltage source must be able to supply enough current, and your body's resistant must be low. The body resistant will likely be low enough, but the other two conditions will have to met in order to receive a lethal shock.

An example is that your typical 120 volt outlet is deadly, but if I wired a 100 Mega ohm resistor in series with the hot side and touched it with one hand and completed the circuit by touching the neutral side with my other hand then I will not receive a lethal shock, but I would definitely feel it. A $5000+ lab digital multimeter would read 120 volts regardless of whether the 100 mega ohm resistor was there or not because they have a very high internal resistance that barely puts any load on the circuit. Our bodies would be a large load on such a circuit that has 100 mega ohms of internal resistance wired in series to the voltage source that the vast majority of the voltage drop would be across the resistor.


NOTE: For legal reasons do not actually try this experiment! Resistors can be faulty!

1. low voltage & low current: safe (think small batteries)
2. high voltage & low current: safe (think Van de Graaff)
3. low voltage & high current: unsafe (think arc welders)
4. high voltage & high current: very unsafe (think power lines, lightning)

Number three is not correct. A one volt, 10 million amp hour battery would not kill you as an example. You must not forget to factor in the body's electrical resistance. I believe an arc welder is more of a current source than a voltage source and current sources vary their voltage to try to maintain the same current. So a current source can be lethal. We are talking about voltage sources here.

 

[Philip Wood]

Chris Baird: I think your 2. and 3. could be misleading. In 2, the voltage falls almost as soon as you complete the circuit. That's the only reason why the current is low enough not to damage you. It might be that in the first microsecond or so after touching it the voltage is still high - in which brief time the current will also be high. The point is that we don't get high voltage (across you) and low current through you AT THE SAME TIME.

In 3. the high current doesn't flow through YOU, even if you touch the wires to the arc. The voltage used is, I believe, fairly safe - not enough to drive an appreciable current through you (as your resistance is in the order of kilohms) [see my post of 6.34]. There are, though, plenty of other dangers which aren't specifically electrical!

 

[pervect]

Low voltages (below 50 volts), aren't regulated by, for instance, the NFPA 70E, because they really aren't very hazardous as far as shock goes.

You can still injure yourself if you try hard enough - for instance, you can melt wires and possibly even tools if you short them across an automobile battery. This could burn you, and/or start a fire. But it's pretty unlikely you'll be able to even feel a shock, much less electrocute yourself, from a 12 v battery, even if it's capable of generating high currents.

 

[Drakkith]

How is a battery able to generate high current but only 12 volts?

 

[Quinzio]

Talking in this way about what is safe/dangerous is very misguiding.
Almost all human made electric sources are able to source those 100mA that would kill (depending on many ifs, they they could kill).
So Ohm law DO APPLY TO THE DAMNED HUMAN BODY.

What's the matter here about talking of Van Der Graaf generator ?
Who has ever seen in reality such generator in the last year of life ?
Who cares if it's not dangerous ?
Instead how many people daily handle household appliances like washing machines, irons, and so on ?

 

[RedX]

What about high electric fields? Can we look at electric fields instead of voltages?

The electric field from the sun's light is around 600 V/m using Poynting flux (1 kw/m² sunlight intensity). Why isn't this deadly?

Also, I'm a little bit confused. For high frequencies, objects become poor conductors, so the electric fields should penetrate the skin. For low frequencies, such as AC mains power, objects become good conductors, so there shouldn't be penetration of the skin.

Can't we view the electric field of a power line as pointing downwards from the hot wire to the ground, and traveling in the direction of the power lines with 60 hz frequency (i.e., a transmission line)? Are the free electrons in humans standing under a power line rearranging to create zero field in the person, or should a person be treated as a dielectric instead and worry about polarization current? I imagine at high frequencies the polarization current can get really large.

 

[Drakkith]

What about high electric fields? Can we look at electric fields instead of voltages?

A "high electric field" would produce a high voltage. Voltage is simply a measure of the difference in electric potential between two points.

The electric field from the sun's light is around 600 V/m using Poynting flux (1 kw/m2 sunlight intensity). Why isn't this deadly?

Sunlight is composed of photons. Each photon can only excite an individual particle to a certain amount depending on the frequency. Most of them simply cause heat to be generated, nothing else.

Also, I'm a little bit confused. For high frequencies, objects become poor conductors, so the electric fields should penetrate the skin. For low frequencies, such as AC mains power, objects become good conductors, so there shouldn't be penetration of the skin.

The resistance of the skin is finite and whether in a DC current or AC current it usually only matters what the voltage is. Extremely high frequencies may act slightly different, but I wouldn't go putting my hand on something exposed at that frequency.

Can't we view the electric field of a power line as pointing downwards from the hot wire to the ground, and traveling in the direction of the power lines with 60 hz frequency (i.e., a transmission line)? Are the free electrons in humans standing under a power line rearranging to create zero field in the person, or should a person be treated as a dielectric instead and worry about polarization current? I imagine at high frequencies the polarization current can get really large.

A power line isn't charged like you might put a charge on an electrode. The voltage goes into causing a current flow. Unless you touch the exposed wire there won't be a potential difference because of the huge resistance of the dielectric surrounding the cable and because of the air.

 

[BruceW]

The danger to the human heart is that under normal conditions, the heart produces an electrical pulse in a steady rhythm so that it contracts in the right way. So then when an electrical pulse comes from outside the body and travels through the heart, it is like a perturbation to the system. And this perturbation can cause the system to fall out of its limit cycle. (And stop beating)

 

[RedX]

A "high electric field" would produce a high voltage. Voltage is simply a measure of the difference in electric potential between two points.

It seems to me that conceptually, voltage is less important than electric field:

E=\rho j
Ed=\rho jd=\rho jd \frac{A}{A}= \frac{\rho d}{A}(jA)=RI=V

So while voltage is proportional to distance, so is resistance, so distance shouldn't matter in determining the current.

Sunlight is composed of photons. Each photon can only excite an individual particle to a certain amount depending on the frequency. Most of them simply cause heat to be generated, nothing else.

Assuming humans are drinking water, the conductivity of drinking water is 10^-2 according to Wikipedia, so the current density due to the sun would be 6.5 amps per square meter since the E-field is 650 V/m. You are correct that we experience only Ohmnic heating due to the sun (our bodies don't store electromagnetic energy, so if energy from the sun disappears into us, it has to be Ohmnic heating), so does this mean 6.5 amps per square meter is not deadly? According to this thread, 100 mA is deadly, so if the heart were (1/65) square meters in size, the sun would be deadly?

The resistance of the skin is finite and whether in a DC current or AC current it usually only matters what the voltage is. Extremely high frequencies may act slightly different, but I wouldn't go putting my hand on something exposed at that frequency.

For a dielectric, an electric field creates a polarization. A rapidly oscillating electric field causes the polarization to oscillate, so that the electrons are now oscillating. The oscillations of these electrons constitutes a current, and the strength of the oscillation should be proportional to the frequency of the incoming wave.

A power line isn't charged like you might put a charge on an electrode. The voltage goes into causing a current flow. Unless you touch the exposed wire there won't be a potential difference because of the huge resistance of the dielectric surrounding the cable and because of the air.

I think this is the reason I've got it wrong. You need to have charge flow through you, or your body will neutralize the fields in the same way a conductor neutralizes fields. So the high resistance between the hot wire, air, you, and the ground will prevent continuous current flow.