Dangers of AC

Thread Starter

Wolframore

Joined Jan 21, 2019
2,342
Hi @DickCappels we are discussing this in shop right now, could you share some information about the dangers of AC? We were taught 40V is the danger threshold, wish we had a little more data.
 

DickCappels

Joined Aug 21, 2008
7,740
60 Hz is an excellent frequency with which to cause atrail fibrillation Here is Google's explanation:

An electric shock may directly cause death in three ways: paralysis of the breathing centre in the brain, paralysis of the heart, or ventricular fibrillation (uncontrolled, extremely rapid twitching of the heart muscle).


and,

Also, the AC current can have a direct impact on our heart. ... Therefore, AC current is more dangerous than DC current because it has a greater magnitude than its RMS value; it directly affects our heart as the frequency of AC current interferes with the frequency of the electric pulses of the heart.



The threshold of damage depends on the kind of contact. When I was starting out in electronics I worked at a tiny power supply company. My boss told me of a guy who took a bet and connect a 12V car battery, one pole to each arm and climbed into a bathtub full of salt water and was electrocuted. True story? I don't know but I don't take chances.
 

crutschow

Joined Mar 14, 2008
27,743
We were taught 40V is the danger threshold, wish we had a little more data.
The danger is the amount of current through the body (especially that across the chest and through the heart).
The limit is normally considered to be about 30mA, (a GFCI will trip at 4-6mA) so the maximum AC voltage is determined by your body and skin resistance (for wet but unbroken skin).
The lowest value for these will likely keep the current below the lethal (but not painful) level at about 40Vac.
 

ThePanMan

Joined Mar 13, 2020
86
I've been hit with a 65KV charge from a CRT. Maybe that's why I stopped growing up. Geez, can't begin to guess how many times I've been shocked with some sort of AC, inductive or capacitive discharge. Once took 120VAC 400 cycle from left hand right wrist (aircraft power). Knocked me on my butt and left me dazed for five minutes. And it was a hot day and I was sweaty. So conductivity was really good that day.
 
I caught a frayed extension cord (120 VAC) between my toes while electric sanding my car. I was shirtless and bare footted laying on the ground on my side. Intentionally screaming allowed me to let go. I figured it would work. It did. I was like 115 or 16. I did play with HV at a young age and during work.

Work involved fixing a 15kV DC regulated 1.5A power supply (e-beam evaporator) and a 0-100 kV at 0.1A (X-ray diffraction power supply). We can also thow in a 14.56 MHz 1000 W RF power supply that used tubes. The scanning Electron Microscope often had high voltage troubles too.
 

dcbingaman

Joined Jun 30, 2021
344
If you must work around live conductors. To verify that the hot wire is off never touch it with the palm of your hand but always the back of your hand. The hand will form a fist and the arm will pull away from the conductor. If you grasp a conductor with the palm of your hand, the muscles will contract the hand causing you to grasp the conductor and not be able to let go. Not to mention the palm of your hand tends to be more moist than the back of your hand. The dryer your skin the better.
 

Reloadron

Joined Jan 15, 2015
6,064
You work with electricity long enough you are going to get bit a few times. Years ago on old B&W TV sets the CRT high voltage was driven by the horizontal output driving a flyback transformer. The common HV rectifier was a 1B3 Tube. Using a well insulated screwdriver one could touch the 1B3 tube cap and draw an small arc to test for HV. Using a pencil with a graphite point is not a good idea. I remember that one to this day. Latched onto a lawnmower spark plug as a kid, also a bad idea. I still remember it was a REO engine. Then there was the butter knife in the toaster. I have no clue over a long career how many times I got bit but many were shocking memorable experiences. :)

Surprised nobody has mentioned Topsy The Elephant during the AC verse DC wars of long ago. I never did care for Thomas Edison. :)

Ron
 

MikeA

Joined Jan 20, 2013
259
Are these thresholds for putting a finger on something? Or grabbing metal with wet hands?

I once got a real good shock from a 12V car battery. While standing on a layer of autumn wet leaves. While trying to wrestle with a battery post clamp. Grabbing the post and clamp real hard. My hands were dry as far as I can remember. I didn't think 12V could do that, until I got zapped.

 
To become an old salt in electronics, one must pass the triple challenge:
1- burn your fingers with a hot soldering iron. Even better if you are working with short pants, and molten solder falls on your leg.
2- have a component explode close to your face.
3- receive a shock of at least 100 volts.
Bonus points: step barefooted on an upside down DIP IC.
 

MisterBill2

Joined Jan 23, 2018
9,294
Of course electricity is dangerous! That is why it is important to focus on what you are doing while working with it. And it is also important to understand what you are doing. For those chained to fear I recommend an entirely alternate profession that does not involve electrical anything. Certainly the hazards exist and so they must be respected.
Unfortunately the current thinking seems to be that stupid people who open electrical control enclosures with the power on must be protected from grabbing the live terminals.
 
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Thread Starter

Wolframore

Joined Jan 21, 2019
2,342
Here’s another good source I found:
Makes me want to measure my resistance... keep in mind he’s using RMS but interesting nonetheless.

Resistance of Humans
Humans are conductors of electricity and have electrical resistance similar to any other material. The human body's resistance to current flow varies depending on:
  • internal and external moisture;
  • exposed sub-epidermal tissue;
  • and skin thickness.
Human resistance is about 10,000 ohms on the high side and as little as 1,000 ohms if the person is wet. Remember, ohms is the unit of measure of a material's resistance or impedance to current flow. Current flow is obviously higher as the resistance goes down.
As an example, let's see how much current flows through a person if he or she contacts a typical 120 volt household circuit. On the high side, with human resistance around 10,000 ohms, we can compute the current flow by dividing the voltage, 120, by the resistance, 10,000. This yields .012 amps, or 12 milliamps.
This is well above the perception level of 1 milliamp, and slightly below the 15 milliamp "let go" threshold. We feel it, but we can let go and have no lasting physical damage.
If we are wet or standing in water, we become a much better conductor, thereby offering less resistance. The current flow is again found by dividing the voltage, 120, by the lowered resistance of 1,000 ohms, which yields 0.12 amps, or 120 milliamps, of current flow. This is easily enough current to send the heart into fibrillation and cause electrocution.

source:
https://c03.apogee.net/mvc/home/hes/land/el?utilityname=harrisonrea&spc=foe&id=4657
 
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