Milliamps and death

[Rick Law]

Perhaps the most important electrical signal of a human's life is the heart beat waves typically shown in the EKG.  The EKG is a complex non-sinusoidal wave but a regularly repeating pulse sequences.  The strongest pulse is probably the "QRS Complex" and the "R Wave".  The tail of the "R Wave" is the start of "S Wave" which is the negative deflection that follows the "R Wave".

The maximum delta typically would be between the R Wave's peak and S Wave's bottom, it is less than 100mV.  Typically 50-70 mV.  If an external current passing your body creates a signal of near that magnitude, it will interfere with your heart pulse.

So, when "applied" properly, your body can reach room temperature with just a small current.  I don't know exact how much it will take, but I sure am not in a hurry to reach room temperature.

[Infraviolet]

Post #21, perhaps the main reason people don't discuss DC shocks is because DC at high voltages is rather rarer than AC at high voltages. Afterall the whole point of AC is the ease with which you can step AC voltages up and down with transformers. There are some HVDC power lines thesedays in a very few places, and maybe some high voltage DC in some parts of some modern electric cars (400V ish), but otherwise it virtually doesn't exist, and until recently existed even less.

[TimFox]

Post #21, perhaps the main reason people don't discuss DC shocks is because DC at high voltages is rather rarer than AC at high voltages. Afterall the whole point of AC is the ease with which you can step AC voltages up and down with transformers. There are some HVDC power lines thesedays in a very few places, and maybe some high voltage DC in some parts of some modern electric cars (400V ish), but otherwise it virtually doesn't exist, and until recently existed even less.

A normal third-rail supply to subway trains and similar applications is 600 V DC, which can be quite lethal.
This has been true since the 19th century, and continues to the present day.
Overhead wire electric trains usually run on even higher voltages, and traditionally that was DC or low-frequency AC.
Overhead wires are now commonly at 50 or 60 Hz.

A summary (from Wikipedia) on electrification for ground-level (third-rail) and overhead wire in use now in UK:

3 Existing systems – overhead line (OHL)
    3.1 National Rail: 25 kV, 50 Hz AC overhead
        3.1.1 Existing
        3.1.2 2010s Network Rail electrification programme
    3.2 Other systems
        3.2.1 1,500 V DC, overhead
        3.2.2 750 V DC, overhead
        3.2.3 Other overhead systems

4 Existing systems - third and fourth rails
    4.1 National Rail: 650 V - 750 V DC, third rail (top contact)
    4.2 630 V DC, fourth rail (top contact)
    4.3 750 V DC, third rail (bottom contact)
    4.4 750 V DC, fourth rail (top contact)
    4.5 600 V DC, third rail (top contact)
    4.6 250 V DC, third rail (top contact)
    4.7 110 V DC, third rail (top contact)
    4.8 100 V DC, four rail

[nightfire]

The fact that DC (and frequencies significantly apart from 50 Hz) is less dangerous to humans is also reflected in appliance testing. Here the so called "50Hz filter" is employed in device/appliance testers, that essentially filters low and high frequencies in an attempt to reflect the sensitivity of a human body.
This is/was in the EN61010 Picture A1, if I remember correctly.

[geggi1]

Gottfried Biegelmeier the father of RCD

[BeBuLamar]

There is some current going from the 5V bare wire thru the human body and to the ground but it's very very small and he doesn't get shock.

Why do you say that? Most low voltage supplies are isolated, so there is no path to ground for any current to flow.

Although the 5V is derived from a 100:1 ratio transformer but I measure I do have 5VAC to ground.

[IanB]

There is some current going from the 5V bare wire thru the human body and to the ground but it's very very small and he doesn't get shock.

Why do you say that? Most low voltage supplies are isolated, so there is no path to ground for any current to flow.

Although the 5V is derived from a 100:1 ratio transformer but I measure I do have 5VAC to ground.

What do you mean by "derived from"? What what do you mean by "ground"?

There are too many things that are unknown in your statement.

I can assure you, however, that if I take a properly constructed step down transformer with 120 V AC on the primary, and 5 V AC on the secondary, there is no voltage between the secondary side and the earth beneath our feet. That is the whole point of isolation in transformers.

[james_s]

100mA flowing through a circuit doesn't make it dangerous, the current has to flow through your body to hurt you. If you insert yourself into a low voltage circuit you are not going to have anywhere near 100mA, you need a much higher voltage than 12V to push 100mA through the resistance of your body.

[vk6zgo]

Catchy title huh!

Okay, I’m old enough not to be embarrassed by this question. Let’s not get off topic and talk about AC and its issues, I’m sticking with DC for this question.

I need some clarity… as a beginner… If 100 milliamps can kill, how come in some videos you can see the host twisting wires together in a live circuit at 100 milliamps with no issue?
Now the voltage was 12 volts. I just need clarity. The circuit was complete, no faults. At what point does the danger become an issue? Does it depend on voltage? I’m aware that’s the force etc.
Can this be answered simply without anecdotal stories? Thanks.

Ohms Law! I=V/R, where I is current in Amps, V is voltage (obviously enough, in Volts), & R is resistance in Ohms.
For a current of 100mA (0.1A), R would need to be 120 ohms.

The device, which is being connected does offer that resistance, but a human appears as a very much higher resistance, so the 12 V cannot cause a 100mA flow through the human.

For a Mains supply of 230v, or even 120v, a current of 100mA could flow through a person, as 2300ohms or perhaps 1200 ohms are feasible resistances for skin contact resistances.

There is a lot of literature on this stuff, but suffice to say, you can pretty much fart around with 12v & under till you are blue in the face, & your chance of injury is up there with being hit by a meteorite.

[vk6zgo]

Perhaps the most important electrical signal of a human's life is the heart beat waves typically shown in the EKG.  The EKG is a complex non-sinusoidal wave but a regularly repeating pulse sequences.  The strongest pulse is probably the "QRS Complex" and the "R Wave".  The tail of the "R Wave" is the start of "S Wave" which is the negative deflection that follows the "R Wave".

The maximum delta typically would be between the R Wave's peak and S Wave's bottom, it is less than 100mV.  Typically 50-70 mV.  If an external current passing your body creates a signal of near that magnitude, it will interfere with your heart pulse.

So, when "applied" properly, your body can reach room temperature with just a small current.  I don't know exact how much it will take, but I sure am not in a hurry to reach room temperature.

You are already "above room temperature", unless you have just come in from a blizzard.

[IanB]

You are already "above room temperature", unless you have just come in from a blizzard.

I think you missed the point. If you are "at room temperature" you are dead. The point being, it doesn't necessarily take a large current through your body for you to end up cooling down to room temperature and being taken to the mortuary 

[BeBuLamar]

There is some current going from the 5V bare wire thru the human body and to the ground but it's very very small and he doesn't get shock.

Why do you say that? Most low voltage supplies are isolated, so there is no path to ground for any current to flow.

Although the 5V is derived from a 100:1 ratio transformer but I measure I do have 5VAC to ground.

What do you mean by "derived from"? What what do you mean by "ground"?

There are too many things that are unknown in your statement.

I can assure you, however, that if I take a properly constructed step down transformer with 120 V AC on the primary, and 5 V AC on the secondary, there is no voltage between the secondary side and the earth beneath our feet. That is the whole point of isolation in transformers.

It's a transformer stepping down from 480VAC.The voltage we have is actually slightly over 490VAC and the output is quite close to 5V. One end of the output is connected to the ground wire which is connected to the ground wire all the way to the panel and also all the metal frame of the machine. Measuring from one of the wire that supplying 5VAC and the ground wire show about 5VAC.One end show 0V.

[jonpaul]

AC frequency....mains vs RF vs HF all different issues.

Tesla experienced RF burns

Jon

[james_s]

RF burns suck. I got one once while playing with a Tesla coil. I tried to turn on the light and when my finger touched the grounded screw holding the cover plate on the light switch I got a burn.

[Zero999]

There is some current going from the 5V bare wire thru the human body and to the ground but it's very very small and he doesn't get shock.

Why do you say that? Most low voltage supplies are isolated, so there is no path to ground for any current to flow.

Although the 5V is derived from a 100:1 ratio transformer but I measure I do have 5VAC to ground.

What do you mean by "derived from"? What what do you mean by "ground"?

There are too many things that are unknown in your statement.

I can assure you, however, that if I take a properly constructed step down transformer with 120 V AC on the primary, and 5 V AC on the secondary, there is no voltage between the secondary side and the earth beneath our feet. That is the whole point of isolation in transformers.

It's a transformer stepping down from 480VAC.The voltage we have is actually slightly over 490VAC and the output is quite close to 5V. One end of the output is connected to the ground wire which is connected to the ground wire all the way to the panel and also all the metal frame of the machine. Measuring from one of the wire that supplying 5VAC and the ground wire show about 5VAC.One end show 0V.

There will be a tiny interwinding capacitance between the primary and secondary. 5VAC is probably even too good to be true. If the meter has an input impedance of 10M, that would be just 500nA of leakage. The transformer probably has a screen, which is coupling the secondary to earth.

[8753ES]

Typically, a person can sense currents of 1mA or more, with currents over 40mA being possibly lethal as they pass through the heart. Therefore, it is how much current flows through the body and where it flows that is of concern. When a person becomes part of a circuit, there are four parameters that determine the current:
1) source voltage
2) source resistance
3) contact resistance, and
4) internal body resistance for the current path.
The second two parameters are a function of body physiology. The contact resistance is mainly caused by the dead skin layer where contact is made. Contact resistance is typically on the order of 100 ohms for sweaty skin to 100 kohms for very dry skin. Beware that if the skin is cut, the contact resistance becomes negligible. The internal body resistance is fairly low due to the fact that nerves and blood vessels make good conductors. Any limb-to-limb internal resistance can be approximated as about 500ohms.

The effects of electricity are felt differently, depending on frequency. The most dangerous frequency range is from about 5Hz to about 500Hz and peaks in danger right at about 50/60Hz, the frequency of power lines. The frequency sensitivity has to do with the physiology of the human nervous system, which typically communicates via pulse trains in this range of frequencies. A 60Hz current has approximately two to three times the danger as the same current at DC. As opposed to DC signals, these AC signals can cause muscles to lock up, leaving a person unable to let go of the voltage source.

[Zero999]

A common myth is the skin effect protects against electric shock, or harm from AC. The skin effect depth even at 2.45 GHz is around 10cm or so in human flesh. If this wasn't the case, you wouldn't be able to cook meat in the middle in a microwave oven. It's probably more true of the higher microwave bands above 100GHz which behave more like infrared, causing surface, rather than deep burns, but will still damage the eyes and testicles