Volts vs. Amps 😕

Factual Questions
1

Can someone explain that old rule of thumb “it’s the amps, not the volts”…or do I have it backwards?

Let’s use a human body as a resistance. Let’s assume it’s some constant value of some resistance “R”. Then, isn’t it obvious that the greater the volts “V”, the greater the resulting current “I”? (Knowing V=IR)

So, again I ask…is it the volts or the amps that’s the worry?

I am assuming the current flows through the human evenly, if that matters, although studies show teh current sometimes will only flow across the surface of the skin if the voltage is within some range which I forget.

Working with only one hand, as a good electrician should,

  • Jinx
2

Er, actually it’s both of them together.

Most often*, the damage done by electricity to a body is proportional to the power expended, which is equal to volts times amps (V x A) and is measured in watts (W)**.

This explains how you can touch one of those electrostatic charge generators*** at the Science Centre at 250 000 V, and not be harmed: there’s negligible current, so almost no power. 250 000 V x 0.000 000 01 A = 0.025 W.
[sub]numbers pulled out of the air[/sub]

[sub]*I’m ignoring any voltage breakdown effects you might get in areas of high field strengh (where different parts of a body might be at dramatically different voltages), and also things like dissimilar metal migration in areas of high current flow.

**I’m assuming a purely resistive sircuit and ignoring reactance here.

***I’m also ingnoring frequency effects, such as the current-flowing-across-the-skin effect you mentioned.

Van de Graaf generators*.

*****Also the name of a band.

What, you want a complete recap of 2 years of electronics school?[/sub]

3

As I understand it (get someone who actually knows the details in here, since my knowledge comes from a lab I took 6 years ago and paid minimal attention to in any event), it is indeed true that the current is what matters.

You see, while a higher voltage will of course result in a higher current, where you err is in thinking that the higher the current, the worse the effects. For small enough currents, you won’t even notice a thing, then as current becomes larger, it can do some bad things to you, then you die, and at even stronger currents, the effects become so strong that things sort of lock down before they become fatal. Of course, you need to be disconnected from the current and revived before you die from heart failure (I THINK that’s what I was told), but the actual damage isn’t irreversible.

Or so I dimly recall from something I read a long time ago. The more detailed answer is anxiously awaited, but I wanted to be high up on that list of people who answered for once… :slight_smile:

Or, on preview, Sunspace said something before me. But I wanted to give my 1/2 cent’s worth anyway.

4

Hmm… on further thought, in the case of a human body, I seem to remember that relatively small amounts of current (0.25 A?) will do damage (possibly because they disrupt existing currents in, say, the heart), but the human body can withstand quite a lot of voltage (i.e. the VdG generator from my previous post).

But for things like resistors, which get damaged by heat, the power is what counts.

Maybe an EMT will happen by to clarify things.

5

“One day, I heard a hospital intern claim that 110 volts ac from the wall socket is not dangerous because they told him in medical school that it’s not the voltage that kills, it’s the current.”

“…110 volts ac from residential wall sockets is the most common cause of electrocution in the United States. In addition, medical studies reveal that the 50 to 60 Hz frequency used in ac power distribution almost worldwide is the most dangerous of frequencies.”

“Higher and lower ac frequencies are less dangerous than 60 Hz ac (but not safe!) According to medical experts who have studied electrical shock, the kiling factor is * current density * in a certain area in the right atrium of the heart called the * sinoatrial node. *”

"Current flowing through that section of the heart can induce fatal V. Fib. In general, for limb-contact electrical shocks through intact skin (macroshcok), the following approximations are accepted:

1-5 mA Level of perception
10 mA Level of pain
100 mA Severe muscular contraction
100-300mA Electrocution

Keep in mind these are approximate, and not to be taken as guidelines to approximate ‘assumed risk.’ Death can occur under certain circumstances with considerably lower levels of current. For example, when you are sweating, or standing in salt water, then risks escalate tremendously."

Is High Current, Low Voltage Safe?

“…an electronics technician I know recently injured himself severely when he cut himself on a +5 Vdc, 30-ampere computer power-supply terminal. A large amount of current flowed in his arm, and caused severe pain and some physical damage. Second, high current is extremely dangerous if you happen to be wearing jewelry… … the large current turned the watchband red hot, and gave him serious second and third degree burns.”
Carr, Joseph J.

  • Old time radios! : restoration and repair *
    1990
6

>>Can someone explain that old rule of thumb “it’s the amps, not the volts”…or do I have it backwards?<<

One easy way to remember this is “West Virginia”, or “WVA”; where Wattage=Voltage X Amperage. Think of electricity like water flowing through a hose. The voltage is the amount of water, while the amperage is the rate of flow. The wattage is the measure of both combined. In order to deliver a large payload wattage, the hose must be able to accomodate the volume and rate simultaneously. To address your question more directly, a large amount of voltage can be dealt with by the human body if it is delivered at a slow rate (a trickle or light spray from the nozzle of a small hose.) If you increase the rate of flow, and in turn the size of the hose, we move toward the force of a firehose. This, at the very least, will get your attention, and may become injurious to sensitive parts of the body. When we crank it up to say the rate of flow seen in a real lightning bolt, (as opposed to one of those plasma globes) the force is so tremendous that its introduction to the body can be similar to being hit by a cannon ball.

>>Let’s use a human body as a resistance. Let’s assume it’s some constant value of some resistance “R”. Then, isn’t it obvious that the greater the volts “V”, the greater the resulting current “I”? (Knowing V=IR)

So, again I ask…is it the volts or the amps that’s the worry?<<

The resistance of the body has a great deal to do with the damage that is potentially done, but the aforementioned must be considered before voltage can be thought of as a determining factor.

In an electrical circuit, a resistor is designed to allow a specific amount of current to pass, while dissipating the remainder as heat. The non-conductive materials used in resistors are designed according to the specifations required by the application. Since people aren’t “designed” for such instances, the human body can be fall anywhere within the range of “decent to poor” resistors. If you get a “zap” from touching the hot lead of a regulated power source, you will most likely live to tell the tale. On the other hand, if you are wading in a pool of water in the basement while working on the main circuit breaker of your house (unregulated 220v), and accidentally touch the hot lead, you will, most likely, find yourself a smoldering corpse in a pool of recently-warmed water; as you, as a resistor, failed to be made up of the sufficient materials required to deal with such an electrical discharge.

>>I am assuming the current flows through the human evenly, if that matters, although studies show the current sometimes will only flow across the surface of the skin if the voltage is within some range which I forget.<<

Again, it is not the voltage that dictates the characteristics of the flow. Electricity will tend to find the path of least resistance from its origin to the nearest ground. Since the surface of the skin often provides this path, many have found themselves lucky to be spared the organ disruption that heavy current can cause to vital organs.

>>Working with only one hand, as a good electrician should<<

And finally, the idea behind this notion is that if you are connected to a ground by the right hand, and come into contact with a live wire by the left, the current can make its “path of least resistance” loop though the chest–you know, the area where the heart is…

I think you get the idea!

Hope this helps,

Al

7

One important distinction between voltage and current is that voltage is merely potential. Amperage represents flowing electrical current. Exposing onesself to a voltage is essentially meaningless; you must have amperage to experience current flow (by definition).

8

Nice post, Al. I would offer, however, that voltage is more akin to pressure in the water flow analogy.

9

Since finding theremin plans and deciding in an LED-intensive costume for Halloween, I’ve been studying basic electronics. But all the sites I’ve found are either very basic (Ohm’s law gets 3 pages plus 2 more of diagrams) or very advanced (Now as we all know a selenium PNP transponder works best when combined with a flyback-quantum-mosfet). Can any Dopers recommend a good site or book?

I am NOT doing any thing involving the wiring of my house and am NOT building any weapons or other illegal projects (My place already looks like a bomb went off. Building an actual bomb , which of coures would detonate on the workbench, would thus be pointless). HMM...no smiley with charred face and wild hair ala Wile E Coyote.
10

Ok…

I have seen several amounts of current that represent the minimum amount to kill a person. Typically, it is listed around 2ma (2/1000 of an amp). To start with, the minimum amount to kill will not be a “likely” amount to kill. Anyway, we will use 2ma for this.

To kill anyone with a minimum amount of current requires that you pass that current through the chest cavity and cause the heart to misbehave. (i.e. fibrillate ) For this argument, we will say that 2ma is the least amount of current that has been demonstrated to sometimes cause fibrillation. Obviously, higher currents would increase the odds. With a high enough current, it would be a near certainty. Interestingly enough, the easiest way to save someone dying after this type of shock, is to shock them again until the heart picks its normal rhythm back up.

So, having established that passing some current through a person’s heart is the way to use the minimum current, how is that accomplished? The first rule is that the current is going to flow from the entry point to the exit point (source to drain). A person can safely handle ANY amount of voltage if there is no discharge path (witnessed by the fact that birds safely sit on high tension wires with no ill effects). The next rule is that the path the current follows between the source and drain should cross the chest cavity and through the heart. The more directly the current crosses the heart the more dangerous it is. So, ways to achieve this would be:

  1. Applying the conductors one each to each side of the chest. (best)
  2. From on hand/arm to the other
  3. From arm to leg. (Left arm to right leg is good)
  4. From head to leg… etc.

Now the interesting part…… How much voltage is required to drive 2ma of current through a human body? As mentioned before, the amount of current flowing through a circuit depends upon the voltage and the total resistance, following the law of I (current in amps) = E (electromotive force in Volts) / R (resistance in ohms). In this case, essentially all of the resistance is from the contact between the person’s skin and the conductor. For dry clean skin, the resistance value of the human body is around 100000 ohms. This would indicate that it takes about 200 volts to be dangerous with only dry skin contact. And this again assumes that current is flowing across the heart. But, there are a number of ways to reduce the resistance of the junction between the skin and a conductor.

  1. Make the conductor contact a very large section of skin.
  2. Use a conductive jelly or fluid at the contact point (i.e. saltwater)
  3. Break the skin, or insert the conductor into the skin.

Any of these would reduce the amount of voltage necessary. Of course, there are ways of accidentally doing the same thing. Having sweaty skin, or standing in the rain leaps to mind. Being very sweaty could lower your resistance to 25000 ohms making it conceivable to cause electrocution with 50 volts. Of course standing in the shower or tub can make house voltage (110) quite dangerous. (Don’t use a hair dryer while still in the shower kids…… The warning labels on hair dryers REALLY say that. Of course, if you were dumb enough to want to use a hair dryer while still in the shower, you probably aren’t bright enough to read the warning label, eh?)

So, house current is typically quite safe, even 208/220 is unlikely to kill a person. However, under wet conditions, people have been killed by as little as 48 volts. People have also been exposed to voltages well into the 10’s of thousands of volts and survived. (I’ve witnessed this personally working on a RADAR) For example, if the current enters and exits on the same limb, it is very unlikely to be fatal, but can do extensive local damage (burns to the skin or worse) and be truly painful. I’ve personally been in contact with 220 a number of times, and power supplies of 1500 volts a few times. (1500 volts hurts enough to quickly condition a person not to repeat it.)

In summary:
So, while it is true that the current is what kills you…. It takes enough voltage to drive that current through your body’s resistance. There are ways of reducing a body’s typical resistance that can vastly lower the amount of voltage required to drive that much current also. A typical car batter can deliver 400+ amps through a low resistance load. But, touching the terminals of the battery with your dry hands won’t even push enough current through you (0.12 ma) for you to even feel.

11

I’ve always disliked the “it’s the amps that kill you” line because it’s fairly meaningless. It’s kind of like saying - “when you burn yourself, it’s not the temperature that hurts you, it’s the rate heat flows into you.” True, but big deal.

We’re used to encountering fixed voltage sources such as mains voltage and batteries. The current they will pass through you is indeed governed by Ohm’s law, V=IR. scotth’s post covered this very nicely.

Confusion arises when we consider static electricity with its potentials of thousands of volts. The explantion frequently given for the harmlessness of static is that “the currents are so small”. Why are they so small? Why doesn’t ohm’s law apply? It DOES, and the currents from static discharge are large, but they only last for a tiny instant of time. It’s the amount of available energy that’s small - the capacitance involved in most static discharges is miniscule.

It’s analogous to the sparks coming off a grinder which don’t burn you - the sparks are orange-hot but they cool down instantly on touching your skin because they are so small. Same with 10000V static electricity - the voltage drops to zero instantly on contact, because the charge “reservoir” is tiny. Try charging a big capacitor to 10000V and you still have “static” electricity, but discharge it through you and it could kill you.

Worry about the voltage. If you have a low voltage supply which can supply very high currents such as scotth’s car battery, it can’t push enough current through you to hurt you. If you have a high voltage supply which can only supply 0.1 amp, that’s enough to be trouble.

12

I see a lot of talk about “what the source voltage is” and “what the source current is” etc. This type of talk can be very misleading to a lot of people.

So I’ll start at the beginning…

Let’s say there is an electrical power source. This can be a wall outlet, output of a power supply, battery – anything.

You walk up and touch one of the terminals. Is it going to hurt you? Kill you?

First of all, we must determine what components make up the “circuit.” They are as follows:

  1. Electrical power source
  2. Your body
  3. Resistor

Electrical power source
As mentioned above, the electrical power source can be pretty much anything. For the sake of discussion, let’s assume it’s a DC supply w/ two terminals ( “+” and “-” ). Let’s also assume it’s a non-isolated supply, i.e. the “-” output is tied to earth ground. Furthermore, the electrical power source can be modeled as an ideal voltage source in series with a known resistance. (This is a 1st-order model, but good enough for this analysis.) The voltage source is really called the Thevenin equivalent voltage source, and the series resistance is called the equivalent source resistance or, more accurately, *Thevenin resistance. *

Your body
During an electrocution incident, your body can be 1st-order modeled as a 2-terminal resistor. Of course, the actual value of resistance depends on a lot of things, and can vary wildly from person-to-person. Very dry non-broken skin can be as high as 500,000 ohms from hand-to-hand; wet skin can result in a hand-to-hand resistance less than 1,000 ohms. Sweaty hands typically register between 20,000 and 40,000 ohms. For broken skin, an arm or leg is typically 500 ohms, while the trunk is 100 ohms. Again, this depends on a lot of factors, and actual resistance may be very different from the values I stated. Also, I believe your body resistance is somewhat non-linear (i.e. the resistance is not constant with current), but we’ll ignore this factor.

Resistor
This is the resistance between your body and the “-” terminal of the power supply. I’ll call this the “ground resistance.” It can be very low, e.g. if you’re standing barefoot in a puddle. In can even be zero, e.g. if your left hand touches the “+” side of the power source, and your right hand touches the “-” side. Of course, it can also be very high, e.g. if you’re wearing rubber-soled shoes.

So here’s what the circuit model looks like:

Positive terminal of Thevenin voltage source connected to Thevenin resistor.
Other end of Thevenin resistor connected to your body.
Other end of your body connected to ground resistor.
Other end of ground resistor connected to negative terminal of power supply.

With me so far?

Now the risk factor to your body depends on the current through your body. This is because body resistance can be just about anything (see above), thus voltage value alone is doesn’t tell you a whole lot. (I = V/R). Currents above 10 mA are considered “bad” for an adult, though it is possible to receive a lethal shock from currents below 10 mA. The GFCI outlet in your bathroom is designed to break the circuit when it detects a leakage of current of 4 to 6 mA.

So given the model I described above, what determines the current through your body?

  1. The value of the Thevenin voltage source = Vs (volts)
  2. The value of the Thevenin resistance = Rth (ohms)
  3. Your body resistance = Rb (ohms)
  4. The value of the ground resistance = Rg (ohms)

The current through your body (Ib, in mA) is thus:

Ib = 1000 * Vs / (Rth + Rb + Rg)

Therefore, for a given voltage source, the way to decrease the current through your body is to increase Rb and/or Rg. (You usually don’t have much control over Rth.)

Consequently, the voltage across your body (Vb, in volts) is:

Vb = (Vs * Rb) / (Rth + Rb + Rg)

So what does all of this mean? Let’s look at some examples.

Let’s say you put one hand on the “+” terminal of a car battery, and the other hand on the “-” terminal. Vs = 12.6 V, Rth = 0.01 ohms, Rb = 100,000 ohms, Rg = 0 ohms. The current through your body is 0.126 mA. No problem here. Notice that the maximum current capability of the car battery (about 500 A) makes no difference, except for the fact that more current capability = lower Thevenin resistance.

Let’s say you have a skin break in each hand, and do the same as the previous example. Vs = 12.6 V, Rth = 0.01 ohms, Rb = 1000 ohms, Rg = 0 ohms. The current through your body is 12.6 mA. Not good.

Let’s say you’re very well grounded and you touch a Van de Graaff generator. (This type of device generates a “static” voltage using a motor-driven belt and combs. As a result, it can generate fairly high voltages, but the Thevenin resistance is extremely high. It’s also very non-linear, but we’ll ignore that fact for this analysis.) Let’s say it can generate up to 10,000 volts… Vs = 10,000 V, Rth = 1,000,000,000 ohms, Rb = 100,000 ohms, Rg = 0 ohms. (I’m guessing at these values.) The current through your body is 0.01 mA. Definitely no problem here. Also notice that the voltage across your body is only 1 V. Because the Thevenin source resistance is so high, 10,000 V is never actually across your body. In other words, before you touch the Van de Graff generator, the voltage is indeed 10,000 V. But it drops to 1 V as soon as you touch it. Of course, this is with Rg = 0. To get the notorious “hair raising effect,” Rg must be very high. (Wear rubber-soled shoes!) This means your entire body will “float” at a fairly high voltage above ground, but the current through your body is (still) extremely small.

Now let’s talk about AC. Assume you have rubber-soled shoes on and you touch the hot side of a 120 VAC outlet. Vs = 173 V (max), Rth = 0.001 ohms, Rb = 100,000 ohms, Rg = 10,000,000 ohms (due to the shoes). The maximum instantaneous current through your body is 0.017 mA; you probably won’t feel a thing. (But because you’re a capacitor, you might feel a charging current. But that’s another topic.) Now let’s say you’re standing barefoot in a puddle and you touch the hot side of a 120 VAC outlet. Vs = 173 V (max), Rth = 0.001 ohms, Rb = 1000 ohms, Rg = 100 ohms. The maximum instantaneous current through your body is 157 mA. You’re dead. (I have also read that 60 Hz is more lethal than DC.)

So there you have it. To re-cap:

  1. Too much current through your body is bad.
  2. The value of the current is determined by 4 things: Thevenin voltage source, Thevenin resistance, body resistance, and ground resistance. Thus, voltage is a factor, but not the only one.