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Which high voltage is more dangerous: AC, or DC, and why?

by On Radioactive Waves
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NascentOxygen
#19
Mar21-12, 09:50 PM
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Quote Quote by jim hardy View Post
I've felt both 130VDC and 120 VAC with my fingertips many times.

They both "tingle". I cannot tell one from the other.
But was that smooth DC, or just rectified AC?
Do not try it.
I don't intend to!
Regarding "unable to let go", either will cause that effect.
Old timey electricians routinely brush a circuit with the back of their hand before touching it.
The same way as those on the land test an electric stock fence before climbing through, and not just once, either, especially in the face of reassurance that it really truly honestly is "OFF".
Integral
#20
Mar21-12, 11:57 PM
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Quote Quote by NascentOxygen View Post
I speak from experience on the AC description. I haven't experienced a shock from DC (touch wood) so cannot vouch for that.
Not sure what kind of experiance you are talking about. All you learn from getting shocked is, not to do that again.

I would expect damage/pain from an AC source would be very similar to its RMS equivelent in DC.

BTW, just for the record, I once took 500V DC through about a cm of my little finger. Quite painfull and left a burn. I have also taken a few 120VAC shocks, with nothing other then pain as a reminder.
Kurinn
#21
Mar21-12, 11:59 PM
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Artificially induced fluctuations in nerve potentials can cause them to fire at the wrong time. Both alternating and direct current sources can cause arrhythmia and subsequent cardiac arrest. I wouldn't say either one is safer in that regard. Just in general, high voltage is high voltage. Once you get to the point where you're putting a certain amount of energy into tissue, you get damage.

The safety of AC or DC depends more on ensuring that shocks don't happen in the first place, and there are various ways to accomplish that in either case. In the end, it's all about ensuring that you do not experience a potential gradient over your body.

Suggesting that either AC or DC provides an innate advantage in safety is a naive argument at best. We have Thomas Edison to blame for this. Don't trust anyone who would electrocute an elephant to make a buck.
Borek
#22
Mar22-12, 03:18 AM
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I remember reading that while in terms of energy delivered they are not different (burns are basically the same), several times smaller AC current can kill someone for the reason already mentioned - it induces ventricular fibrillation.

Edit: ventricular fibrillation can be induced by 60 mA AC and 200-300 mA DC. See http://hypertextbook.com/facts/2000/JackHsu.shtml. No idea how reliable this source is.
psparky
#23
Mar22-12, 07:45 AM
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I just listened to a 4 hour seminar yesterday on arc flash.....and how when it happens it essentially just like a bomb.

Just like a bomb.....an arc flash has a sonic boom, massive heat and flying metal parts. Any one of them can kill you in a second even if you are in full protective gear.

The boom alone can shatter ribs and destroy organs, or blast you back into a lethal sharp object........the heat can melt your skin and give your 3rd degrees burns over more than half your body in a split second.....which pretty much means your dead soon.....and flying metal parts can give lethal puncture wounds.

Ac or Dc it will not matter. And I didn't even mention being shocked.

Protective suits do a pretty good job with the fire aspect of protecting your skin....but the force of the boom and lethal flying objects it can not protect you from.

Which brings me to the left hand rule. Most disconnect switches and so forth have their handle on the right hand side of the box. Instead of standing right in front of it when you open it up....use your left hand instead and put your body out of harms way. If something does go wrong.....you will likely be much safer. If you open it with your right hand....it's like someone pointing a gun directly at you. If you open with left hand.....not so much.

And obviously, you need to be trained and certified before opening any higher voltage box....but even in your residential boxes and disconnects....better safe than sorry.

And incidentally, it is against the law to get shocked. There is no reason you should be working on any energized parts. The reason I say that...is that if you destroy someone's equipment.....a lawyer will be asking you why a particular device was energized....and you won't have a leg to stand on. Possibly literally. Strangely, there is one exception. If you can prove that a device is more dangerous when de-energized....this is the only case. Funny, I know.
jim hardy
#24
Mar22-12, 01:02 PM
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I just listened to a 4 hour seminar yesterday on arc flash.....and how when it happens it essentially just like a bomb.
If you've never seen the result of a switchgear explosion, well it's like a bomb went off. Everything is blown apart, steel is deformed and covered with black soot. I remember vividly an incandescent light bulb that had been heated to point the glass softened and it deformed into something worthy of Salvador Dali.

I always told new employees "Copper vapor follows the same gas laws as dynamite vapor. It expands according to the available energy. Don't ever roll those dice. "
Windadct
#25
Mar22-12, 01:55 PM
P: 564
First Question - Why are you asking? OSHA basically regulates all power systems with voltages over 50 V as hazardous. There are too many other variables in human safety ( Connection Points, Skin Resistance, total energy of the source, etc) to really make it worth worrying about AC vs DC.
Did you know that falls from ladders and elevated points are the most common injury to electricians - the shock makes them fall. DC / AC doesn't matter.
Stanley514
#26
Mar22-12, 03:17 PM
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How electronic DC current could be dangerous to humans?Human body suppose to exhibit ionic conductivity only?Salts do not conduct electrons...
cabraham
#27
Mar22-12, 04:27 PM
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Back in 1979, I was a summer intern at a division of GM while I was a graduate student. I worked in advanced engineering (R&D). I was working on a lab bench. The edge of the bench top nearest me had a metal strip. I did not realize at first that this strip was grounded (earthed). I was placing a probe on a 115V 60 Hz live wire when my hand slipped. My finger contacted the live 115V rms ac mains, with my elbow contacting the grounded metal strip at the edge of the bench top.

I felt the shock from my finger to my elbow. That section of my arm gave me a pulsating sensation. I could literally feel the 60 Hz frequency. I was able to let go. Maybe the muscles in the forearm were still functional, or maybe the other muscles beyond the elbow gave me the ability to let go.

It was frightening. For a couple of hours I felt an adrenaline rush, and could not think of anything else. I've never been shocked by dc so I cannot make a comparison. But I definitely felt the 60 Hz line frequency.

Frankly I don't mess with either. They both scare me.

Claude
nsaspook
#28
Mar22-12, 10:49 PM
P: 648
High voltage is dangerous and costly.



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