Which is more dangerous AC or DC power?

     Alternating current (AC) and Direct current (DC) have slightly different effects on the human body, but both are dangerous above a certain voltage. The effect on a particular person is very difficult to predict as it depends upon a large number of factors - amount of current, duration of flow, pathway of current, voltage applied and impedance of the human body. 

Having said that, I would rate AC as more dangerous owing to the following reasons,

1. To produce the same excitatory effects, the magnitude of DC flow of constant strength shall be two to four times greater than that of the AC. i.e more DC current is required to induce the same harmful effects as AC current.

Why? The main difference between the effects of AC and DC on the human body result from the fact that excitatory actions of the current are linked to the changes of the current magnitude especially when making and breaking the current. Excitatory actions of the current include stimulation of nerves and muscles, induction of cardiac atrial or ventricular fibrillation. To produce the same excitatory effects, the magnitude of DC flow of constant strength shall be two to four times greater than that of the AC.

2. Accidents with DC are much less frequent than would be expected from the number of DC applications, and fatal accidents occur only under very unfavorable conditions, for example in mines. This fact is highlighted in the IEC publication 60479 - Effects of current on human beings and livestock. This reveals that DC is only an 'occasional culprit' compared to the 'serial killer' AC.

3. Ventricular Fibrillation is considered to be the main cause of death by electric shock.The probability of a human suffering from Ventricular Fibrillation is much higher in the case of AC than DC.

Why? For shock durations longer than the cardiac cycle, the threshold of ventricular fibrillation for DC is several times higher than for AC. For shock  durations shorter than 200 milli seconds, the threshold of fibrillation is approximately the same as for AC measured in RMS values.

4. The total impedance of the human body is higher for DC and decreases when the frequency increases. Since the impedance for DC is higher, the severity of electric shock would be comparatively lesser than AC.

Why? The impedance of the human body is one of the factors influencing the effect of electrical current on humans. The total impedance of the human body depends upon a number of factors (including the frequency of electrical supply). Therefore, the impedance of the human body is higher for DC and decreases when the frequency increases.

5. It's comparatively easier to let go of the gripped 'live' parts in the case of DC than AC. This is in contrary to popular belief.

To quote the popular belief, one such argument is shown below,

"AC would allow your muscles enough time to pull your limb away from the 'live' part because of the alternating cycles (AC frequency) which pass through zero. DC current, on the other hand, has continuous flow due to the absence of frequency oscillations and therefore you can't pull your limb away from the 'live' part.''

I could see many such 'flawed' arguments prevalent on the internet - especially the science forums debating on this topic. But, this is simply a myth.

Why? 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. As mentioned in the IEC publication 60479 - Effects of current on human beings and livestock, the let-go of parts gripped is less difficult in the case of DC. This is based on experimental evidence.

Given the above reasons, we can safely conclude that AC is more dangerous than DC. Nevertheless, you should always avoid contact with high-voltage electrical conductors, regardless of the type of electrical current.

            No less important engineers than Thomas Edison and George Westinghouse argued that point ad inifinitum without reaching any conclusions that stuck. while debating the merits of their different power systems (Edison championed DC and Westinghouse championed AC which, as we all know own for its technical merits, safety not being one of them).  A great deal was made about electrocution as a form for death sentence and each one advocated the power of his opponent for this.

For example, are we talking peak voltages or RMS voltages? Different ways of measuring things. For example the AC Mains is 120V RMS. The equivalent DC for the same power is 120VDC. But the AC has much higher instantaneous peaks of 170V.  Its not the power that kills so its very hard to compare AC to DC voltages in terms of killing ability and absolute voltage.