1. 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.
DC current does elecrolysis of body fluids and products of it are toxic. So it may happen that few hours after shock one dies because of poisoning.
2. 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.
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 can get through your skin a bit as well.
Also, 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.
3. Nikola Tesla proved that there is one thing called the "skin effect".
You see, if you have an AC current running through a conductor, such as a wire, then the current is not uniformly distributed in it (if you look at the cross-sectional area). On the surface of the wire, current density is highest. The more you go into the center of the wire, current density is weaker. For example, current density near surface is 1 A/m^2, and near the center of the wire it is 0,01 A/m^2.
The intensity of current decrement depends on one thing: the frequency of applied voltage. If the frequency of voltage is very, very high, then almost all of the currrent runs on very surface of the wire. As you decrease the frequency, more and more current is going to the center of the wire. At zero frequency, you have DC current and it is all uniformly distributed across the wire.
Actually, the current density falls exponentially with the frequency of applied voltage - in the direction of the radius of the cross-sectional circle of the wire. That explains all.
Well, since human body is also a conductor, then the same thing applies to it. If you have a current of very high frequency running through your body (e.g. 100 MHz), then nothing can happen to you no matter what the voltage is. The current is "only skin deep", it goes a milimeter of your skin in depth, and therefore it is not harmful to any vital functions, heart, muscles, and everythig else.
Tesla had several performances on various fairs proving this effect. He used to run million volts AC through his body, with very high frequency. And he always remained unharmed. Cute, isn't it? 
Well, that should be the answer for you. Less frequency - more dangerous. But don't try it at home
4. Voltage at high frequency WILL cause RF burns! even at low power, 4 watts at 27 MHz (typical CB radio freq) will cause tiny blisters when localised, but no electrical shock, the higher the frequency the more surface the burn tends to be at the same power levels. This is happening because at higher frequencies all the current is "concentrated" in the skin surface.
Example: If 1 mA is running mostly through upper 1mm of your skin - at, let's say, 10 MHz, and no current runs beneath (this approximation is OK because of exponential nature of decreasing), than at 100 MHz, the same 1 mA will be running through upper 0,01 mm. And that tiny surface of the skin is receiving a great amount of current.
Speaking purely of danger, AC is more likely to deliver a shock than DC because the alternating current causes dielectric breakdown of insulating material to occur at lower voltages than DC.
5. At a given frequency of AC, a pearticular voltage would be enough to overcome skin resistance. If I recollect correctly, the normal household supply @ 50-60 Hz in most countries becomes hazardous at voltages beyond 120V, hence in several countries they prefer a 110V 60Hz supply.DC voltages are not used primarily because of transmission problems, but they are exteremely harmful. Thats why one of its commercial uses is in electric chairs. But given that as already mentioned welding uses low voltages high current electric arcs, and DC currents of low voltages are often used for welding without any danger.
6. Physical differences between shocks by AC and DC current? Both are dangerous and can be lethal in high amounts.
Effects of an AC or DC Currents on the Human Body
The three basic factors that determine what kind of shock you experience are the amplitude of the current, the duration of the current passing through the body, and the frequency.Direct Currents actually have zero frequency, as the current is constant.
However, there are physiological effects during electrocution no matter what type of current.
The factor deciding the effects of the AC and DC current is the path the current takes through the body. If it is from the hand to the foot, it does not pass through the heart, and then the effects are not so lethal.
However DC current will make a single continuous contraction of the muscles compared to AC current, which will make a series of contractions depending on the frequency it is supplied at. In terms of fatalities, both kill but more milliamps are required of DC current than AC current at the same voltage.
If the current takes the path from hand to hand thus passing through the heart it can result in fibrillation of the heart. Fibrillation is a condition when all the heart muscles start moving independently in a disorganized manner rather than in a state of coordination. It affects the ability of the heart to pump blood, resulting in brain damage and eventual cardiac arrest.
Either AC or DC currents can cause fibrillation of the heart at high enough levels. This typically takes place at 30 mA of AC (rms, 60 Hz) or 300 – 500 mA of DC.
Though both AC and DC currents and shock are lethal, more DC current is required to have the same effect as AC current. For example, if you are being electrocuted or shocked 0.5 to 1.5 milliamps of AC 60 Hz current is required and up to 4 mA of DC current is required. For the let-go threshold in AC a current of 3 to 22 mA is required against 15 to 88 of DC current.
Facts about Electric Shock
- It is the magnitude of current and the time duration that produces effect. That means a low value current for a long duration can also be fatal. The safe current/time limit for a victim to survive at 500mA is 0.2 seconds and at 50 mA is 2 seconds.
- The voltage of the electric supply is only important as it ascertains the magnitude of the current. As Voltage = Current x Resistance, the bodily resistance is an important factor. Sweaty or wet persons have a lower body resistance and so they can be fatally electrocuted at lower voltages.
- Let-go current is the highest current at which subject can release a conductor. Above this limit, involuntary clasping of the conductor is present. It is 22 mA in AC and 88 mA in DC.
- Apart from electric shock the other equally dangerous hazards of playing (or working) with electricity are electrical arc flash and electrical arc blast.
- Placing your hand in your pocket may protect you by preventing a current from traveling through the heart making a shock non-lethal.
- The severity of the electric shock depends on the following factors: body resistance, circuit voltage, amplitude of current, path of the current, area of contact, and duration of contact.
- Death may also occur from falling in case of electric shock.
- Burn injury may occur at both the entrance and exit of the current.
- Low frequency AC is more dangerous than high frequency AC.
- AC and DC both kill so treat them with respect.
7. It depends on the frequency of the AC. All AC is not equal. It's also hard to directly answer your question, because we can't experiment with electrocuting human subjects. The "let-go" current is the best experimental measure we have of the effect of electricity on humans. The "let-go" current is the lowest level of current passing through a human subject through an electrode held in the hand that makes the subject unable to open his hand and drop the electrode. (From what I understand, the experiment hurts, but it isn't permanently damaging to the subject.)
It turns out that the "let-go" current starts at some finite value at a frequency of zero (DC), passes through a minimum, and then increases as the frequency gets higher. (At high frequencies, the only known harm from AC is the resistive heating of the subject's tissues.) This means, that, using "let-go" current as the measure of hazard, there is a frequency of AC that is more dangerous than others. Ironically, this most dangerous frequency is around 60 Hertz, which is the frequency of the AC supplied by electric utilities in the US.
by
Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois
8. Edison used and promoted the usage of DC and not AC. Even, the war against AC led Edison to development and promotion of the electric chair as a demonstration of lethal AC versus his "safer" DC.
As part of this promotion, Edison publicly electrocuted dogs, cats, and is considered the intellectual killer of Topsy, the Luna Park's elephant (have you ever seen that "ugly" movie?).
Edison and Westinghouse had a common friend called Telsa who worked for Edison first and later for Westinghouse when Edison did not understand/approve his improvements and investigations to improve electric power distribution (stuff that Westinghouse accepted with his arms wide open).
Also, Edison did not pay him the offer of $50,000 if Tesla was able to redesign of Edison's DC generators. Tesla did the task very well and Edison replied to him with the famous statement: "Tesla, you don't understand our American humor!"
Edison used that as a campaign against Westinghouse to show people the killing power of AC.
The rest is history!
AC won out because of the ease of transmission(TRANSFORMERS) whereas DC would have to be produced locally(high cu losses).
Edison was a monumental jerk and more businessman than a real scientist. Even today, his shotgun method of experimentation is called 'Edisonian' as a derogatory term by modern scientists.
Tesla was only one person whose ideas Edison robbed. Frankly it's appalling Edison has such a reputation as a 'great inventor'. It was the mobs of people working for Edison that gave him his success which he never shared.
But at least Tesla got the last laugh. And we're reminded of that every time we turn on a light.
"Tesla, a man out of time" is a must read for anyone working with electricity.
Pasted from a website about Tesla:
"Arriving in New York City with four cents in his pocket, Tesla found employment with Thomas Edison in New Jersey. Differences in style between the two men soon led to their separation. In 1885, George Westinghouse, founder of the Westinghouse Electric Company, bought patent rights to Tesla's system of alternating-current. The advantages of alternating-current over Edison's system of direct-current became apparent when Westinghouse successfully used Tesla's system to light the World Columbian Exposition at Chicago in 1893."
Westinghouse was also a prolific inventor, to this day most railways still use the Westinghouse brake system.
9. AC is used in power production and transmission in that it is much more efficient.
In DC you would actually push the electron down the line from the power plant to your house and through your computer.In AC you merely jiggle the existing electrons in the wires. They move back and forth and it actually takes an electron hours to move from your wall outlet to your computer due to some intrinsic losses, etc.
Voltage is not dangerous but current kills. AC or DC current are dangerous. You can actually insulate yourself and hold onto a Van De Graff DC generator that charges your body to a million volts (I've personally done it to 250,000 Volts).
Your hair stands on end and your clothes move around as though you were in the wind but it is all static electricity. The trick is to discharge yourself "Slowly" and not create damaging current.
Or better yet don't grab hold of the generator
With DC you get stuck to the line. Where as AC will throw you off. So with DC you need someone to seperate you with a non conducting material or he too will get stuck.
What I know is that 50Hz current is more dangerous than DC current.However when the frequency increases the effect of the current decreases below that of DC current.You can test it easily with 60V AC and DC.That is,if you are not "superconductive".
I personally almost do not feel 60V DC but 60V AC tickles a little.
You can try the effect by your self If you catch the telephone line wire tip&ring ; although it's almost 50VDC you will not feel any thing. Then Try to touch the same wire while some one is calling ; The voltage is 75V AC with 25Hz ; You will FEEL the effect !
ofcourse don't try it
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