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Trade electricians often work on live household circuits carrying 120V AC. I've gotten shocked by accidentally touching such hot wires often enough to recognize the 60Hz AC buzz. Fortunately, human skin has pretty high resistance: even when drenched in sweat, mine measures in the hundreds of kΩ. If we assume a lower-end skin resistance of 120kΩ, then through unbroken skin 120V service will only pass 1mA through my body.

Conventional "wisdom" says that working in a main panel calls for more caution because one might make contact with 400A service, as compared to the 20A one is facing working at the end of a typical circuit. But this sounds incorrect:

If skin has significant enough resistance that at 120V only milliamps flow through it, then is a 200A service supply any different from a 20A breaker in terms of the experience of touching a hot wire (holding all else equal)?

Does ground contact matter in terms of skin contact with residential electric service? E.g., at 1mA, 120V, and half a 60Hz cycle would an insulated 20-gallon vat of saline take enough charge to slow the current significantly relative to the current it would receive if it were connected to the ground?

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  • \$\begingroup\$ @FakeMoustache - Please read the question, not the title. I am asking whether my analysis of these scenarios is correct. Whether these levels are "dangerous" is ancillary/illustrative. \$\endgroup\$ – feetwet Feb 21 '16 at 18:25
  • \$\begingroup\$ Reading the other question will give you more insight into this topic. If you only want yes/no answers you might want to go elsewhere. \$\endgroup\$ – Bimpelrekkie Feb 21 '16 at 18:31
  • \$\begingroup\$ @FakeMoustache - Yes, that other topic is informative and worth reading. Perhaps you can link it as a "Related or Informative" comment instead of marking this question as a duplicate (which it is not). Also I highly suspect my analysis may be missing things, so while a "Yep, you've nailed it, nothing else to say" answer might be gratifying I expect answerers (like you) will have useful notes, corrections, and/or additions. For example, just now it occurred to me that perhaps the contact area with the conductor might be a relevant factor? \$\endgroup\$ – feetwet Feb 21 '16 at 18:37
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    \$\begingroup\$ Odd that you noted hundreds of kiloohms (kilohms?), I had a safety training a work that said an average human from palm to palm is about 1 k resistance. Not saying you're wrong, just wondering why such a variation. \$\endgroup\$ – Swarles Barkely Feb 21 '16 at 18:42
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    \$\begingroup\$ Do not forget the effect of inductances: A low voltage but very high current source might seem harmless, but with very high currents, a even tiny inductances can generate very high voltages. Lets say you accidentally short your power supply with a metal tool, then huge currents can flow and when you try to stop the short, the inductance of the tool would induce a high voltage to keep the current constant even after the short has been removed for example by flowing through the high resistance path of the tool handler. Any high powered supply is also carries a risk of burns. \$\endgroup\$ – Jan Lucas Feb 21 '16 at 23:12
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You assume that voltage and current are independant, they are not.

It is indeed the current which kills (can stop your heart for example) but the voltage is needed to make that current flow in the first place.

Conventional "wisdom" says that working in a main panel calls for more caution because one might make contact with 400A service, as compared to the 20A one is facing working at the end of a typical circuit. That has probably more to do with the dangerous consequences of a short circuit than the danger to a human. The 20 A sise is fused so a short circuit will blow the fuse. At the 400 A side there will be no fuse or one with a much higher rating so more current has to flow until that one blows.

it doesn't matter how much current is on the supply: The body will only carry the milliamp or so admitted by its resistance. That is correct.

Indeed a mains of 20 A or 200 A, there is no difference regarding you feeling a shock as the current only needs to be a few mA. Both can deliver that current.

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Don't assume that skin resistance is constant.

The voltage-current characteristic of human skin is non-linear and depends on many factors such as intensity, duration, history, and frequency of the electrical stimulus. Sweat gland activity, temperature, and individual variation also influence the voltage-current characteristic of skin. In addition to non-linearity, skin impedance exhibits asymmetric and time varying properties. These properties can be modeled with reasonable accuracy.[17] Resistance measurements made at low voltage using a standard ohmmeter do not accurately represent the impedance of human skin over a significant range of conditions.

For sinusoidal electrical stimulation less than 10 volts, the skin voltage-current characteristic is quasilinear. Over time, electrical characteristics can become non-linear. The time required varies from seconds to minutes, depending on stimulus, electrode placement, and individual characteristics.

Between 10 volts and about 30 volts, skin exhibits non-linear but symmetric electrical characteristics. Above 20 volts, electrical characteristics are both non-linear and symmetric. Skin conductance can increase by several orders of magnitude in milliseconds. This should not be confused with dielect breakdown, which occurs at hundreds of volts. For these reasons, current flow cannot be accurately calculated by simply applying Ohm's law using a fixed resistance model.

Source Wikipedia.

Grounding

My body is actually the ground when I touch a hot wire.

No. If fully isolated your body is only capacitively coupled to ground. The electrostatic Human body model uses 100 pF for static discharge models. Assuming this is all capacitance to ground its impedance will be 33 MΩ at 50 Hz.

Touch something earthed and current now has a much lower resistive path. Fatal electrocution would be much more certain if "standing in a puddle" of water.

High currents

If working on equipment capable of delivering high currents arc-flash protection should be worn. This includes arc-flash rated visor, balaclava, shirt / jacket, gloves, proper shoes and trousers.

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  • \$\begingroup\$ Can you elaborate the grounding aspect for AC current? I assume that on contact the supply has a RMS potential w.r.t. my body of 120V -- i.e., same as w.r.t. perfect ground. Unless it can pass enough current into my body during a half cycle to change my relative potential then how does a conductive ground beyond my body increase the flow through my body? \$\endgroup\$ – feetwet Feb 21 '16 at 19:01
  • \$\begingroup\$ "Between 10 volts and about 30 volts, skin exhibits non-linear but symmetric electrical characteristics. Above 20 volts, electrical characteristics are both non-linear and symmetric." There must be a mistake in the Wikipedia article: I expected one of those symmetrics to be asymmetric, based on the sentence construction. \$\endgroup\$ – Wayne Conrad Feb 21 '16 at 19:52
  • \$\begingroup\$ @feetwet: The supply has potential w.r.t. ground due to neutral - earth bonding at the local transformer. The ground completes the circuit from the live wire, through you, through the ground and back to the transformer. The current flowing through you depends on the sum of all the series resistances / impedences. Decreasing the resistance to ground increases the current through the body. \$\endgroup\$ – Transistor Feb 21 '16 at 20:30
  • \$\begingroup\$ @WayneConrad: I didn't notice that and don't know the correct answer. The main point I was trying to get across is that skin resistance is not linear and shouldn't be assumed to be a particular value. \$\endgroup\$ – Transistor Feb 21 '16 at 20:33
  • \$\begingroup\$ My understanding is that isn't strictly true for household AC service. E.g., the "ground" can literally be a conductor driven into the earth a short distance from the main panel. If you don't get shocked when you touch the earth, or anything touching it, then you have the same potential with respect to the service as its "ground." To see what I'm getting at please reference this comment. \$\endgroup\$ – feetwet Feb 22 '16 at 0:14
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How much current a power supply is capable of delivering is only part of the equation (Ohm's Law) - the other half is how much current the load wants to draw, determined by its resistance (DC) and impedance (AC).

In short, you are correct that in this situation it doesn't matter whether you touch a downstream mains circuit protected by a 20A CB and lighter wiring, or a 400A main feed with a hulking big CB and half-inch-thick bus bars; both are capable of delivering enough current to kill any animal.

When you touch the Live of an AC-mains, a circuit may form between that point and Neutral (if your other hand happens to be in contact with that, for example), or that point and Earth, the danger is largely the same, because it only takes tens of mA current to put the heart into fibrillation (as distinct from actually cooking someone with several amps running through them).

Another factor of how deadly coming into contact with any high voltage source (with enough current capability to put the heart into fibrillation) is what that path is through your body. Electricians are taught to, where ever possible when in a high risk situation of contact with said high voltages, to try to do so with only 1 hand active, the other in their pocket. Because they're also trained to wear rubber-soled shoes/boots when on the job. The result is that there's much less likely to be a path for current to flow from the point of contact (finger) along the arm, down or across the chest (heart) to where ever the current could otherwise go (to the other hand, down through their feet). So your body isn't necessarily a path to ground, depending on the details.

Strictly speaking, your capacitance to ground and/or surroundings is there too, but in most circumstances it's a far less significant contributor to current flow (birds land on power lines all the time, even ones at thousands of volts, and are completely unperturbed, because their capacitance to anything/everything else around them via air is utterly negligible). (BTW your phrasing of the AC issues in your question suggests you think capacitance is some inherent property of a body (human, metalic, or whatever) - it's not, it's simply the surface area of an object in close proximity to some other body/lump-of-metal/etc - whether that's the track on the PCB to the ground-plane underneath, or the area of your feet to the ground beneath them, with some insulator (your shoes, pre-preg on the PCB, etc) between them.

The thing with skin resistance is that it varies dramatically, for the individual, on their level of hydration & sweating, and also on how much surface area of their skin comes into contact with the Live conductor. Here, measurements of/by one individual aren't really helpful. And even a few tens of mA, if it crosses the heart, can be enough.

So it's a combination of factors that determines how severe an electric shock from 50/60Hz mains is.

Another aspect of these scenarios is moderate and high voltage DC (> 50 Volts DC), which can be much more dangerous. With 50/60Hz mains, that reversal of voltage results in a reversal of the current, 50/60 times per second, which can be an opportunity for a human to pull away from it (and for a switch to open). When it's DC, and you touch it, your muscles are much more likely to 'lock on', and even grab the high-V conductor even more which increases skin contact surface area, and you stay connected, and if the current path is through the heart, say goodbye.

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  • \$\begingroup\$ The grounding question is obviously a big factor here. I've been taught the same thing: When working in the service panel, don't give the current a path across your chest! But at 120V if the total current really is on the order of mA and the service is 60Hz how much can the potential of my body vary from an unlimited ground? I.e., it starts at 120V difference, and for that to decrease it has to "charge up" before the service reverses polarity, right? Substitute a vat of 20 gallons of saline solution for my body: Wouldn't that serve as an effective ground for 1 mA 60Hz AC? \$\endgroup\$ – feetwet Feb 21 '16 at 19:08
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    \$\begingroup\$ No, 20 gallons of saline solution isolated from ground still presents a capacitance to ground of a low value. You can think of the body internally as a good conductor and the insulation of your shoes as the dielectric of a capacitor the size of your feed. Since the distance between the soles of your feet and the ground makes the capacitance very small. \$\endgroup\$ – Transistor Feb 21 '16 at 20:39
  • \$\begingroup\$ OK, but in this case I'm interested in how much electricity would flow into/through my body, so capacitance is probably the wrong term and concept. Maybe it's coulombs, or coulomb volts? The insulated vat of saline starts out accepting the same charge rate as the ground, right? As it charges up the potential difference decreases and therefore the flow rate also decreases. So: At 1mA, 120V, and half a 60Hz cycle would an insulated 20-gallon vat of saline take enough charge to slow the current significantly relative to the current it would receive if it were connected to the ground? \$\endgroup\$ – feetwet Feb 22 '16 at 0:03
  • \$\begingroup\$ As @transistor has described, and as I tried to explain with the birds on a wire, hypothetically if your bag-of-salty-water body had zero capacitance to anything else, then no current would flow in/out of your body due to contacting the live conductor (with, say, you finger). But you aren't zero capacitance, there's a bit there, enough to feel some fraction of the universe vibrating at 50/60Hz. But I think I know what you're asking - that simply becoming charged up/down by 120V AC will in itself result in a current flow through your one point of contact? \$\endgroup\$ – Techydude Feb 22 '16 at 0:31
  • \$\begingroup\$ @Techydude - Exactly, that's my remaining question. \$\endgroup\$ – feetwet Feb 22 '16 at 4:44

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