High frequency current

[Qacer]

I just got into a discussion with a friend. We were talking about electrical safety, and I told him that it is also the alternating nature of 60hz current that disrupts our heart. I then went on to say that due to skin effect a higher frequency current is "less likely" to pierce our heart because it will be mostly travelling on the top of our skin. Theoretically, I think I'm right, but is that really the case? Are there other factors besides the amount of current? After all, enough DC current can still kill us.

 

[TuxDave]

umm... I figured that if you get enough current running through your heart, regardless of frequency, you'd die within a couple of seconds.

As for the skin effect, I never heard of it applied in combination with the human body and high frequencies. I know in semiconductors, high frequency currents will tend to move towards the outer edges of the wire due to its self-inductance properties. As for humans, I figure that current tends to move along the skin only because it offers a lower resisistance path towards ground (for the most part).

 

[Geniere]

Well there is only some validity to your post. The prime factors are voltage, current, current density, frequency, the physiological response of various tissues, and the entry/exit points of the current. Skin effect can be ignored at 50-60hz, being significant only at much higher frequencies. Muscle response to DC is moderately less than to AC at 50-120hz; the ?let go? threshold is a little higher and you might say DC is a tiny bit safer. Ignoring the heart and respiratory muscles, electrocution is simply ?cooking? the body. How quickly the body is cooked is a function of current and voltage (power). With properly placed electrodes it is possible to fibrillate the heart or tetanize the reparatory muscles with much less power (increase current density).

Human skin is a very poor conductor of electricity at any frequency.

 

[DrPizza]

The frequency is the most significant safety issue, as far as someone being killed by household electricity. The frequency is very close to the natural frequency of the signals sent to the heart. Thus, even a mild shock, far less than that necessary to "cook" the vital organs, can throw the heart into electro-mechanical dissociation... the heart starts beating very irregularly and is unable to pump blood to the rest of the body. To try to give you an idea of why it isn't the actual "burn" that's doing it, keep in mind that you can be killed by household currents and household voltages... Sure, touch a 20,000 V line and you're going to absorb quite a bit of energy. But, consider how long it takes to heat a cup of water with a 15 amp current at 110-120 volts.

Some other info that may or may not be interesting or of help...

CPR no longer teaches a precordial thump (smacking your fist on someone's chest at the start of CPR), but if I recall from helping my mother-in-law study for some sort of advanced emergency room certification (she was an RN - the certification allowed her to be in charge during codes) for electro-mechanical dissociation, the thump is often quite effective (2nd to the paddles they use, and more effective than regular CPR) Why this isn't part of a regular CPR course, I don't know... I suppose it has to do with KISS (keep it simple, stupid)

Also, a demonstration I do every year while my physics class is working on parallel vs series circuits is to wire hotdogs into each type of circuit - the hotdog itself serves as a resistor. I can assure you that your body has more resistance than 6 inches of hotdog, thus the current through the hotdog is higher than the current through the human body at 110-120 volts. Nonetheless, it takes about 1 1/2 minutes for the hotdogs to cook (in the parallel circuit... much longer in the series - challenge: figure out how much longer ) Ballpark franks work great for this demonstration (because they "plump" - we use stopwatches to time them on that basis.) The students enjoy the hotdogs for breakfast, they see yet another working example of the different types of circuits, and I make them write a paragraph or two, with calculations, showing why the series circuit takes longer, and how much longer.
I always lead it into a discussion of why electricity is dangerous.

 

[AnnoyedGrunt]

Mmmmm, I like that hot dog experiment. I can microwave 2 Ball Parks in 2 minutes @ 50% power, and it sounds like I could do a very large number using the "wall socket" method in less time, as long as I don't blow a fuse. Hmmm, I know what I'm doing at the next BBQ.....BTW, how evenly do the Hot Dogs cook?

Anyhow, it's been while since I looked @ circuit problems, but I'm feeling like reliving my past so lets see:

For the initial case, we have resistors in parallel, so that means:
R=Hot Dog Resistance
V=Voltage potential (120V if in a wall socket)
I=Current in the dog
N=number of dogs

So, the overall Parallel Dog resistance will depend on the # of dogs, and the total resistance is:
1/R(total) = 1/R + 1/R + ...... for the number of dogs = N/R
1/R(total) = N/R
R(total) = R/N

The Series dog resistance is:
R(total) = R + R + R..... = N*R
R(total) = N*R

So, if V=IR(total) and Power = VI = (I^2)R(total)
Then I = V/R(total) and P = (V/R(total))^2(R(total)) = (V^2)/R(total)

So:
P(parallel) = N(V^2)/R
P(series) = (V^2)/RN

So:
P(parallel) = (N^2)P(series)

So, this means that the time it takes will grow exponentially (or is it geometrically) with the number of dogs. Since I don't know how many dogs were cooking in the original case, I can't say for sure how long it will take, but if you cooked 2 dogs in 1.5 minutes in parallel, it would take 4X as long to cook 2 dogs in series (and 16X as long if you cooked 4 dogs). Also, as you add more dogs to the parallel circuit, the overall resistance goes down and you can actually cook them faster (until you blow a fuse, anyway). Seems kinda conuter-intuitive.

I'm sure I screwed up somewhere, so hopefully I don't get banned from this forum, hehe.

Man, I can't believe I just did that, and now I'm nervous about looking too dorky if I post it all.

I need to go rest now....

-D'oh!

 

[Qacer]

This guy tried experimenting on himself:

Shock at high frequencies

He said that the shock factor was more pronounced at the lower frequencies compared to the higher ones.

 

[Mark R]

At very high frequecies AC, there is little effect on the body - I'm not quite sure why this is, but presume it's due to the slow response time of biological systems.

In surgery, high voltage current is routinely used to seal blood vessels and destroy diseased tissue. This is called diathermy. The voltages used are around 1000-2000 volts with a frequency of about 50,000-100,000 Hz.

The lower the frequency, the more significant the effect on the body. Below about 100 Hz, down to DC there is relatively little difference - although DC is slightly more damaging because it can cause electrolysis of the tissues.

The most dangerous effect of electric shock is abnormal cardiac rhythms (specifically ventricular fibrillation or ventricular tachycardia). The treatment for these is a high voltage DC shock delivered by a defibrillator (while waiting for a defibrillartor, a single precordial thump is currently recommended, as long as the collapse was witnessed on a cardiac monitor). In other abnormal heart rhythms (electro-mechanical dissociation - normal electrical activity, but no blood pumping e.g. due to severe blood loss; or asystole - no electrical activity) a DC shock is not appropriate and dangerous - the only treatment is CPR.

 

[Geniere]

DrPizza
The frequency is the most significant safety issue, as far as someone being killed by household electricity. The frequency is very close to the natural frequency of the signals sent to the heart.

Not to my knowledge. The heart is not directly controlled by the brain, but indirectly through the Vagas and Parasympathetic nerves that release respectively Noradrenaline (speeds heartbeat) and Acetylcholine (slows heart beat). Internal to the heart is a small comma shaped cluster of cells (sinus node) that responds to those agents. The sinus node has an intrinsic rhythm of about 70 beats per minute. Cardiac muscle contraction is complex but has to do with the inversion of potassium and sodium concentrations external and internal to the cell. Upon inversion a voltage change occurs and is propagated through the heart. This is known as polarization and depolarization and produces the squiggles on an ECG. The heart has its own ?nervous system? but it is composed of specialized heart tissue and is of much different physiology than a nerve cell. The heart is contained in myocardial tissue that insulates the heart from external electrical influences, but can be bridged by high potentials. The atria and ventricles are insulated from one another so the wave cannot pass directly from atria to ventricle. The sinus node depolarizes, the voltage field propagates from cell to cell directly and also through specialized tissue to the AV Node. There the wave is delayed, as obviously the ventricles should not contract while the atria are pumping blood into them. After being delayed, the wave propagates throughout the ventricles via the specialized tissue (bundle branches) and cell to cell. All individual cardiac cells have intrinsic (escape) rhythms so if the cardiac conduction system fails the atrial cells will contract at about 60 beats per minute while the ventricular cells will contract at about 40 beats per minute. Life sustaining but that?s about it. Diseased cardiac tissue may under go spontaneous depolarization and produce a voltage wave that propagates through the heart causing either ventricular or atrial fibrillation.

When interpreting an ECG, the heart is considered a radiating dipole antenna. Vector analysis can be used to pin point diseased tissue.

I am of the OPINION (a little hedging here) that most electrocutions are due to ?cooking? rather than induced cardiac fibrillation. With typical household voltages it would be difficult to achieve current densities through the heart sufficient to cause fibrillation. Intentionally placed electrodes may do it but I don?t think so. Defibrillators deliver up to 40 or so amperes to provide a few mille-amperes of current flow through the heart.

But, consider how long it takes to heat a cup of water with a 15 amp current at 110-120 volts.

You probably want to re-think that, as 120vac would not cause a 15 amp current flow through tap water.

I can assure you that your body has more resistance than 6 inches of hotdog?

I did human resistivity (DC) measurements cc.1969 to select the best performing brand of ECG electrodes. To do this it was necessary to abrade the skin (remove dead cells). I found readings from about 10 volunteers to be (going by memory) 200-1000 ohms. Interestingly the readings were almost the same when measuring from finger to finger of the same hand compared to finger to toe. Never did a hotdog!

Mark R: In surgery, high voltage current is routinely used to seal blood vessels and destroy diseased tissue. This is called diathermy?.

Actually it?s called Electro-surgery. Diathermy is used in physical therapy to warm muscle tissue.

In my day the Electro-surgical device operated at about 300,000hz. A sharp tipped ??knife? provided a entry point of very high current density. The exit point (usually buttocks) was a large area plate to diffuse the current so as to minimize skin heating and tissue damage other than at the surgical site.

 

[DrPizza]

Wow, thanks for the detailed information about the heart.... But, you haven't changed my opinion that it isn't that the heart is getting "cooked" during most electrocutions.

The typical electrocution does not last long enough to "cook" the vital organs. You've calculated the natural resistance of the human body.... now use Ohm's law to calculate the amount of current, then calculate the amount of energy. It would take considerable time to "cook" the organs.

I did a little googling:
It only takes as little as 74 mA to cause cardiac fibrillation... at 250 mA, there's a 99.5% probability of fibrillation..
That's not a lot of current, particularly in terms of cooking anything.e of 5 seconds)

The frequency of the electrical current is as important as magnitude when evaluating electrocution and electrical shock injuries. Humans and animals are most susceptible to frequencies at 50 to 60 hertz. The internal frequency of the nerve signals controlling the heart is approximately 60 hertz. Ventricular fibrillation occurs when 60 hertz current from the electric shock interferes with the natural rhythm of the heart. The heart loses its ability to pump and death quickly follows. Ventricular fibrillation can occur at current levels as low as 30 milliamps for a two year old child and 60 milliamps for adults. Most adults will go into ventricular fibrillation at hand to hand currents below 100 milliamps (0.1 Amp).
Humans are able to withstand 10 times more current at DC and at 1000 hertz than at 50 or 60 Hz..

Note: 10 times more current for DC.... "cooking" cannot be the answer for this. Plus, it wouldn't explain the nearly harmless nature of higher frequency currents... the skin effect is true - it doesn't travel near the heart.... but, the skin effect would not eliminate "cooking." There would still be the same amount of resistive heating = "cooking" albeit not in the vital organs.

I tried googling for the solution: I'm not going to post all the links, but try these search terms:
"most dangerous frequency"
+electrocution +frequency
electrocution

I also tried
+electrocution +cook but found no useful information, except for stories about death row, with executioners commenting about "you've gotta give it time to cook the blood in the heart" - but I hardly find death row executioners to be qualified sources of knowledge.

 

[Geniere]

Originally posted by: DrPizza

The frequency of the electrical current is as important as magnitude when evaluating electrocution and electrical shock injuries. Humans and animals are most susceptible to frequencies at 50 to 60 hertz. The internal frequency of the nerve signals controlling the heart is approximately 60 hertz. Ventricular fibrillation occurs when 60 hertz current from the electric shock interferes with the natural rhythm of the heart. The heart loses its ability to pump and death quickly follows. Ventricular fibrillation can occur at current levels as low as 30 milliamps for a two year old child and 60 milliamps for adults. Most adults will go into ventricular fibrillation at hand to hand currents below 100 milliamps (0.1 Amp).
Humans are able to withstand 10 times more current at DC and at 1000 hertz than at 50 or 60 Hz..

My failing memory tells me there are several errors in the quote you've provided. If you have a link, I'd appreciate it.

Thanks

 

[DrPizza]

My computer is crapping out at the moment (thanks Azureus...) Once these downloads finish, I'll be able to search. However, The information in my previous post as very similar to information I learned in a graduate physics class last summer, and similar to information on numerous sites. Just do a few google searches. Search for "most dangerous frequency."

 

[DonPMitchell]

High frequency skin effect is why tesla coils don't really give deep shocks. But radio frequency power is very inefficient for lots of reasons, energy gets absorbed more in transformers, is lost by radiation and reactance in power lines, etc.

The increase lethality of AC power was used in the early 1900s by proponents of DC power systems. Edison, who sold DC power systems, convinced the state of New York to use AC electricity to execute prisoners, the electric chair was a publicity stunt to help discredit Westinghouse's AC power company.

Transformers are smaller and more efficiet for modestly higher frequencies, not radio frequency but like 133 Hz or 400 Hz. The Navy used 400 Hz power, and the old Cray I super computer required 400 Hz. At the U of Minn, we had a Cray 1, and in the basement was a big motor generator to feed it 400 Hz power.

The most efficient power lines are high voltage DC, for example. Because then all you have is ohmic resistance, no inductive resistances (reactance). The Russians did that first, to send power from the Volga to Moscow, and Canada and the US have also build some big DC powerlines.