# Why is AC more “dangerous” than DC?

After going through several forums, I became more confused whether it is DC or AC that is more dangerous. In my text book, it is written that the peak value of AC is greater than that of DC, which is why it tends to be dangerous. Some people in other forums were saying that DC will hold you, since it doesn't have zero crossing like that of AC. Many others also say that our heart tries to beat with the frequency of ac which the heart cannot support leading to people's death. What is the actual thing that matters most?

After all, which is more dangerous? AC or DC?

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I think he means that, since he clearly describes it in his question –  markovchain Mar 29 '13 at 3:04
No, it's not! Depends on lots of factors. People have given good answers. But here's a fun fact. A US Navy guy died out of a 9V DC battery. The spark which you see off metal door knobs and sweaters in winter are a minimum of 10,000 V. I didn't die of it. It's the current that matters –  Cheeku Mar 29 '13 at 3:57
well zero crossing is the term which i found by reading the answers from other forum.and i too used it here because i thought that in ac when moving from positive peak voltage to negative peak voltage it crosses zero. –  idiosincrasia23 Mar 29 '13 at 7:42

The RMS (root-mean square) value of an AC voltage, which is what is represented as "110 V" or "120 V" or "240 V" is lower than the electricity's peak voltage. Alternating current has a sinusoidal voltage, that's how it alternates. So yes, it's more than it appears, but not by a terrific amount. 120 V RMS turns out to be about 170 V peak-to-ground.

I remember hearing once that it is current, not voltage, that is dangerous to the human body. This page describes it well. According to them, if more than 100 mA makes it through your body, AC or DC, you're probably dead.

One of the reasons that AC might be considered more dangerous is that it arguably has more ways of getting into your body. Since the voltage alternates, it can cause current to enter and exit your body even without a closed loop, since your body (and what ground it's attached to) has capacitance. DC cannot do that. Also, AC is quite easily stepped up to higher voltages using transformers, while with DC that requires some relatively elaborate electronics. Finally, while your skin has a fairly high resistance to protect you, and the air is also a terrific insulator as long as you're not touching any wires, sometimes the inductance of AC transformers can cause high-voltage sparks that break down the air and I imagine can get through your skin a bit as well.

Also, like you mentioned, the heart is controlled by electric pulses and repeated pulses of electricity can throw this off quite a bit and cause a heart attack. However, I don't think that this is unique to alternating current. I read once about an unfortunate young man that was learning about electricity and wanted to measure the resistance of his own body. He took a multimeter and set a lead to each thumb. By accident or by stupidity, he punctured both thumbs with the leads, and the small (I imagine it to be 9 V) battery in the multimeter caused a current in his bloodstream, and he died on the spot. So maybe ignorance is more dangerous than either AC or DC.

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Do you have a source for that "death by 9V battery through blood stream" story? –  us2012 Mar 29 '13 at 13:56
+1 for 'ignorance is more dangerous than either AC or DC' (although I would have said "stupidity") –  sds Mar 29 '13 at 14:19
@us2012 I read the story in a book or something by the infamous Darwin Awards. –  krs013 Mar 31 '13 at 0:19
I doubt that 9V death story is true. –  Fermi paradox Apr 27 at 16:24

Direct current (DC), because it moves with continuous motion through a conductor, has the tendency to induce muscular tetanus quite readily. Alternating current (AC), because it alternately reverses direction of motion, provides brief moments of opportunity for an afflicted muscle to relax between alternations. Thus, from the concern of becoming "froze on the circuit," DC is more dangerous than AC.

However, AC's alternating nature has a greater tendency to throw the heart's pacemaker neurons into a condition of fibrillation, whereas DC tends to just make the heart stand still. Once the shock current is halted, a "frozen" heart has a better chance of regaining a normal beat pattern than a fibrillating heart. This is why "defibrillating" equipment used by emergency medics works: the jolt of current supplied by the defibrillator unit is DC, which halts fibrillation and gives the heart a chance to recover.

There is a table with bodily effects at http://www.allaboutcircuits.com/vol_1/chpt_3/4.html

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If you have a dc current of $x$ volts, this is the maximum voltage you can get from it.

If you have an ac current of $x$ volts, the maximum voltage is more than $x$ (I forgot how to calculate it, maybe it $x\sqrt2$, someone correct me if I'm wrong). This is because the voltage rating is the average of the oscillating voltage (after taking all positive).

And, a higher voltage means more dangerous, right?

EDIT: Check this wikipedia example out: http://en.wikipedia.org/wiki/Alternating_current#Example

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i think its depends upon the relative value of rms value of AC and absolute value of DC. sir if we touch a AC carrying wire with one hand ,current will not flow in our body if we not connected to ground(potential difference zero= no current). but as we touch wire our body potential reach the value of wire potential, can our body support such a high potential? that may be cause of shock or death. pls correct me if m wrong.

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Dc current is more dangerous than ac beacause the voltage is Dc is same I.e 240volt but in ac the average voltage is 110volt by the cause of rms

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This doesn't make sense. If the rms voltage is lower than the peak-to-peak voltage, why would that make AC more dangerous? –  Ben Crowell Jul 27 '13 at 22:59
This is incorrect. Human body conductivity is dominated by reactive resistence. –  Vorac Sep 13 '13 at 13:38

## protected by Qmechanic♦Jul 27 '13 at 17:19

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