Why is AC current harmful to the body, but DC current is not?

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First of all, I would tell you that your point of view is wrong. Both are equally lethal, it is like asking what causes the worst death, a truck hitting you or a car, either way you die!

When to choose between D.C. or A.C for which is harmful, I would rate A.C more dangerous to D.C. Below are some of the reasons for A.C. to be considered more dangerous. I am mentioning these reasons not based on accidents or deaths but at the same voltage level.

  1. One of the reasons for which A.C. is considered more dangerous is arguably it has more ways of getting into human body. As the voltage is alternating in nature, it can cause current to enter and exit your body even without a closed loop since there exists some capacitance between the body and the ground (For those who don’t know, capacitor allows A.C. and stops D.C.). But, D.C. cannot do that.
  2. A.C. might be more deadly than D.C. at same voltage. Generally, the rating of alternating current is expressed in terms of RMS values which are lower than peak values. So by 120 volts A.C. it actually means 170 volts (120*√2=170) peak-to-ground voltage. So if you have an AC Supply of 120 volts, it reaches a maximum of 170 volts and a minimum of -170 volts, 50 times each second. But, if it was DC the voltage would have remained constant at 120 volts.
  1. is considered as one of the main reasons of death by electric shock. Human body is easily prone to fibrillation when subjected to A.C. compared with D.C. Either AC or DC currents can cause fibrillation of the heart at high enough levels. This typically takes place at 30 mA of A.C. (rms, 60 Hz) or 300 – 500 mA of D.C. Fibrillation is a condition when all the heart muscles start moving independently in a disorganised manner rather than in a state of coordination.AC’s alternating nature has a greater tendency to make the cardiac muscles go 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 defibrillators 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.
  2. The total impedance of the human body is higher for D.C. and decreases when the frequency increases. As the impedance for D.C. is higher, the severity of electric shock would be comparatively lesser than A.C. You see, human bodies have a fair amount of impedance for electrical current, which is why we can endure extra-low voltage current. But if the electrical current constantly changes its direction, our body’s impedance for it would decrease. And the more frequent the changes, the less our impedance. While the frequency of household AC electricity is 60 Hz in the US and 50 Hz elsewhere, the frequency of DC electricity is 0 Hz anywhere, because it never changes direction. Therefore, we would generally have lower impedance for AC, thus increasing the risk of danger.
  3. The let-go threshold of D.C. current is greater than A.C. current. 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.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 mA 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. Therefore, the belief of getting enough time to escape from A.C. is a misconception.

Alternating currents and direct currents both have different effects on human body and actually they both are dangerous above a certain voltage. But remember it is the magnitude of current that damages human body rather than voltage. According to OSHA (Occupational Safety and Health Administration), some of the probable effects of various current levels are mentioned below.

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