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HFAC safety

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Posted on January 7, 2017 at 08:49:46
dave slagle
Manufacturer

Posts: 4754
Location: NYC
Joined: April 27, 2001
This is just a silly thought but here goes.

I just found out that the impedance of a body is not linear with frequency. I always found that I could pinch the leads of a transformer and have some interesting effects on the shape of a 10K square wave. Knowing the body has something like 50K-200K resistance when grabbing a DVM and since simply placing that value resistance across the leads on the bench did very little I came to the conclusion that we must be more complex than just a simple R so I played with adding some C and still was unable to get results that matched what my body did to load a square wave.

The other day I looked back at the subject when I was measuring something with a friend and this time I tried to measure the simple Z and saw two things that I hadn't considered. First the Z went down as frequency went up which tells me there must be some capacitance involved and the phase of the impedance is moving - even at 100hz. (-90 is capacitive, 0 is resistive and +90 is inductive). Here is a simple impedance plot of my body from hand to hand (across the chest cavity) and the pattern is consistent with what my meters say.





We have all been taught that it takes 100ma across the chest cavity to kill us and that typically refers to the 100V at 50-60hz line voltage. Applying ohms law with the 100K impedance of my body I only come up with 1ma, so I tried grabbing my line voltage and that didn't go so well. As it turns out once the voltage is high enough to "establish a conductive path" the resistance (impedance?) breaks down to a lower value causing a runaway situation and since we know that line voltage can kill you it seems clear that that 100K initial value @ 50-60hz must break down to 1K if you hang on long enough.

I realize this was a long story to get to my question but if my graph is representative and the bodies impedance at 60khz is 1/10th of what it is at 60hz, do we need to consider that the voltages in use for filament supplies might be dangerous too?

I'm guessing they aren't since these supplies are readily available without warnings but that makes me wonder what piece of the puzzle I am missing / overlooking in this.

dave

 

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And indeed it does..., posted on January 7, 2017 at 09:04:30
Ivan303
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Posts: 34969
Location: Cadiere d'azur FRANCE - San Francisco
Joined: February 26, 2001
One of the clever ways of differentiating 'cells' from 'plasma' in blood is based on the theory that plasma is basically a salt solution which conducts electricity rather easily while red cells act as small capacitors being mostly resistors at low frequency and highly conductive at high frequency.





 

RE: HFAC safety, posted on January 7, 2017 at 09:07:34
RayP
Audiophile

Posts: 558
Location: Maryland
Joined: June 30, 2005
To add a safety reminder just a little off topic.

When you turn off your halo AC unit with the attached capacitor, the cap remains charged. I now put a bleeder resistor across the cap.

ray

 

RE: HFAC safety, posted on January 7, 2017 at 09:27:20
Frank E
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Posts: 718
Joined: May 20, 2000
Contributor
  Since:
September 8, 2000
As a longtime employee of an electric utility I was taught that 40mA of 60Hz current can be lethal.
As it happened, I suffered an electric shock from an open-circuit 60Hz current transformer passing 0.7A, where I was unable to let go and lost consciousness relatively quickly. I came to lying on the floor covered with blood, and learned that a couple of coworkers had pulled me free using a "shepherd's hook".
It took 3 years for my physical and brain injuries to plateau and I never managed to get back to work.
If you make contact with an electric circuit of lethal magnitude via your bloodstream it is a much more serious scenario.

 

MUCH Less Than 100mA Will Kill, posted on January 7, 2017 at 11:08:26
Triode_Kingdom
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Posts: 5817
Location: Texas
Joined: September 24, 2006
"We have all been taught that it takes 100ma across the chest cavity to kill us"

That is NOT an established axiom, nor is it true. The amount of current needed to interfere with and stop the heart is a few mA or less. Because of this, all biomedical equipment in clinics and hospitals uses special low-capacitance transformers to avoid the possiblity of shock. The UL standard for leakage current of Class II devices in the patient care area is 150uA.




--------------------------
Buy Chinese. Bury freedom.

 

RE: MUCH Less Than 100mA Will Kill, posted on January 7, 2017 at 11:24:26
dave slagle
Manufacturer

Posts: 4754
Location: NYC
Joined: April 27, 2001
I remember the 1/10th of an amp from high school. Google will fight this battle and give you hundreds of different opinions.... In any case the topic I was trying to start a discussion on was if HFAC was more dangerous because of the bodies decreasing impedance with frequency.

dave

 

RE: MUCH Less Than 100mA Will Kill, posted on January 7, 2017 at 11:30:57
Triode_Kingdom
Audiophile

Posts: 5817
Location: Texas
Joined: September 24, 2006
"do we need to consider that the voltages in use for filament supplies might be dangerous too?"

The supplies I use are isolated from earth ground. There's no current path unless I touch both output leads at the same time. A 30V pk-pk square wave at 66kHz might be uncomfortable or even painful, so I try not to do that. :)

It's also worth mentioning that at some frequency below 1MHz, skin effect begins to take hold. AC current won't travel into the body, but instead flows near the surface. Depending on the frequency, this will create a burn, rather than a shock.


--------------------------
Buy Chinese. Bury freedom.

 

RE: HFAC safety, posted on January 7, 2017 at 13:02:35
megasat16
Audiophile

Posts: 196
Location: SoCal
Joined: April 15, 2015
The current of 1uA at 1000v is much more dangerous than at 10V. The normal heater voltage for the filament is around 20V or less so it is well below dangerous level even at around 200KHz or 2MHz HFAC is applied at these low voltages. The normal skin resistance varies from places to places on the human body. Skin on the hands and foot are thicker so they have higher resistance. So, a lot of variables in play here.

But it sure is dangerous to touch the low voltage HFAC or DC or regular AC filament wire when the B+ potential is applied to the tube. Normally, it would be OK but in some cases with failing tubes or God knows what else; the leakage current from HV B+ may flow through your body.

A function generator can be set to 20V at 200KHz HFAC (square / sine / triangular, rectangular) and you can hold the lead without any adverse side effects. So, it is mostly quite safe but does not mean absolute in anyway. But cracked skin, wet skin, thin skin, and a thousand other things can reduce the normal skin resistance to considerable level so never say never and better not touch any bare wire or electrical contacts when power is applied. Accidents do happen and many have lost their lives over charging a cell phone.

.
.
.Thou shall not stand where I type for I carry a bottle of Certified Audiophile Air and a Pure Silver Whip.

 

RE: HFAC safety, posted on January 7, 2017 at 14:26:28
SteveBrown
Audiophile

Posts: 2270
Location: Portland, OR
Joined: November 14, 2002
I learned the hard way about HF shock - in high school I was a Ham operator and I was outside trying to do some measurements of my "inverted V" transmitting ant. which was energized to 75w at about 3.8Mhz. I got the end of the antenna from my SWR bridge a bit too close to the transmitting antenna and zap! Left a dark pit of a hole in one finger as the signal found a nice short path to ground!

 

RE: GFIs are set to trip below 5ma, posted on January 7, 2017 at 15:31:43
deafbykhorns
Audiophile

Posts: 651
Location: Florida
Joined: October 17, 2003
If your doing some live testing, do it with a GFI circuit so you don't feel the effects for more than 40 milliseconds....

Never been shocked by more than 60cycles, I might have to ask a couple of my Navy buddies how 400hz feels. One would think the body would react like a capacitor.

Personally, I don't feel anything below 50 volts.

 

Same Here, posted on January 7, 2017 at 17:16:38
Triode_Kingdom
Audiophile

Posts: 5817
Location: Texas
Joined: September 24, 2006
I was 14 years old, touched my 80M antenna running about 50W, burned a hole into my finger. That sucker hurt for a week!


--------------------------
Buy Chinese. Bury freedom.

 

Modern electrosurgery units do realtime impedance matching, posted on January 7, 2017 at 18:37:07
Jim Doyle
Audiophile

Posts: 1146
Location: Medford, MA
Joined: July 8, 2001
Also they run about 500kHz up to about AM radio frequencies. No, the electrosurgery will not stop the heart - because the frequency is too high.

 

YEP!! (nt), posted on January 7, 2017 at 18:58:00
SteveBrown
Audiophile

Posts: 2270
Location: Portland, OR
Joined: November 14, 2002
nt

 

RE: HFAC safety, posted on January 8, 2017 at 08:09:28
tube wrangler
Manufacturer

Posts: 1636
Location: USA
Joined: January 29, 2007
Hi Dave!

Thank you!!!!! You're certainly onto something here for sure.

A physical, alive body is a dynamic (always moving, always changing)
SET of moving targets.

All bodily systems are always changing-- both electrically and
certainly chemically, and so are all of our senses, including hearing.

Hearing (part of feeling, which is part of realizing) could be described as mechanical, chemical, and electrical-- and, much more!

When man tries to study any ONE of these things, he makes a fundamental
mistake every time: he assumes that about 1,000,000 other things-- which affect what he is studying-- are not simultaneously changing as he analyzes!

We're stuck with this: results count above all. COMPLETE results where
EVERYTHING is considered (admittedly an almost impossible task) while
everything else is changing as well, is the only useful SET of considerations.

Results are what counts. YOU count! You're a good thinker... widen your
horizons-- check out what happens to all parameters when one swims, surfs
and almost lives in Ocean water. Everything assumed here on DIY does-- and MUST change.

--Dennis--

 

RE: HFAC safety, posted on January 8, 2017 at 09:35:38
Alpha Al
Industry Professional

Posts: 2431
Location: N. Carolina
Joined: February 16, 2004
Contributor
  Since:
December 3, 2015
A transmitting dipole or inverted V can build up very high voltage, even at low power levels. It's highest at the ends if it's center-fed.

 

Yeah but it made a cool spark! (nt), posted on January 8, 2017 at 09:49:54
SteveBrown
Audiophile

Posts: 2270
Location: Portland, OR
Joined: November 14, 2002
nt

 

RE: Modern electrosurgery units do realtime impedance matching, posted on January 9, 2017 at 04:33:05
PakProtector
Audiophile

Posts: 11273
Joined: May 14, 2002
hey Jim, long ago you built a 211 amp and posted a few pic. Can you send me the scary one with the connections made with clip leads?
cheers,
Douglas

Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

 

RE: I offer this as regards to causing death, posted on January 9, 2017 at 18:25:56
Russ57
Audiophile

Posts: 3752
Location: South Florida
Joined: November 16, 2001
I was always taught that what causes death is AC hertz close to natural heart beat.

AC goes back and forth from positive to zero to negative so one assumes it gives you a chance to get free (if low enough hertz) but in reality it cause contractions. DC (IMHO) tends to hurt/burn more.

IMHO electrocution is secondary to "blowing yourself up". Think about dropping a crowbar across a 2,000 amp, 3 phase, 480 bus bar. I know from actual testing 2,000 amp breakers will pass upwards of 10 times that current rating for a brief period in a direct short.

 

yah..., posted on January 11, 2017 at 03:38:58
PakProtector
Audiophile

Posts: 11273
Joined: May 14, 2002
there is a sensitive interval across the heartbeat. Apply a shock during this time, and Bad Things can happen. DC has to hit this interval, and it has one chance. AC has multiple chances.
cheers,
Douglas

Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

 

RE: yah..., posted on January 11, 2017 at 09:17:35
Triode_Kingdom
Audiophile

Posts: 5817
Location: Texas
Joined: September 24, 2006
"DC has to hit this interval, and it has one chance."

That's sorta true. The heart is most sensitive to external current during that time. However, it can be stopped at any time if enough current is applied. DC causes muscles to clench, so the victim is often subjected to the charge for a much longer interval. This tends to even the chances of DC electrocution (vs AC), assuming the applied voltage is large enough. The bottom line is, don't try this at home. :)





--------------------------
Buy Chinese. Bury freedom.

 

RE: yah..., posted on January 12, 2017 at 07:27:01
Lew
Audiophile

Posts: 8778
Location: Bethesda, Maryland
Joined: December 11, 2000
This reminds me of the "War of Currents" that took place in the late 19th century over the question of which form of electrical power was "safe"-est. Edison vs Westinghouse (who bought the patents on AC from Tesla).

 

RE: HFAC safety, posted on January 17, 2017 at 09:08:15
Tom Bavis
Audiophile

Posts: 864
Location: Upstate NY
Joined: May 25, 2007
At frequencies in the megahertz range, the skin effect becomes significant, and current will not go very deep. I have read that if you're in a strong radio frequency field the first thing you'll notice is warmth in the ankles (so get away from that transmitting antenna!). Contact with RF voltage will result in deep, painful burns; really little danger of electrocution.

 

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