How Much Current Can The Human Body Withstand?

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It is not the Voltage that can kill humans, it is the current that kills. Humans have died at as low as 42 volts. Time is also a factor. A current of 0.1 ampere for a mere 2 seconds can be fatal.  As Voltage = Current x Resistance the current depends on body resistance. The internal resistance between the ears is only 100 ohms, while it is around 500 ohms when measured from finger to toe.

Electric shocks are frequently portrayed in physical comedies. The act proceeds as usual: the comical protagonist inadvertently gets hold of a wire, ignorant of the high current flowing through it. He receives a lethal shock that results in a stereotypical shimmy, a charred face and hair standing on end like an umbrella flipped inside out by the wind.

Asking the question of why this fatal accident is perceived to be humorous is unsettling… interesting, but unsettling. A plausible answer can be found here. However, that discourse is irrelevant for now. What concerns us is why we aren’t impervious to electricity in the first place and how much of it will actually kill us.

This is, of course, crucial knowledge for safety purposes. We find cautious messages on electrical boards and generators imprinted with the universally recognized emblem of danger: a human skull floating above two crossed bones. This symbol is accompanied by the rating of this machine, highlighting the high voltage at which it operates, letting you know that contact with it would probably kill you.

The use of voltage has set a psychological trend in us. We believe that 10,000 volts would be deadlier than 100 volts. This is, however, only partially true. Electrocutions are often implemented using household voltages of 110 Volts, or in some instances, as low as 42 Volts!

Of course, more voltage draws more power, but it is not the caliber that kills us, but the bullet it shoots. Regardless of the voltage, the real cause of death is the current that is forced through the body.

Birds on Wires

This is the same reason why birds reposing on wires aren’t electrocuted. (Credits: palickam/Shutterstock)

However, we shouldn’t discard voltage entirely. Without voltage or a potential difference, there’d be no current at all. This is the reason why hanging on a wire wouldn’t electrocute you unless you touch the ground. Hanging from the wire forms an equipotential with the wire, whereas touching the ground immediately creates a potential difference, which draws a huge current through us.

So, how much current will kill us?

Electrocution: How much current will kill you?

A current of 10 mA or 0.01 A provides a severe shock, but it wouldn’t be fatal. As we approach 100 mA or 0.1 A, muscular contractions begin. It is imperative to realize that because of the heart’s low resistance, a current of magnitude as small as 10 mA through it is enough to kill us.

However, the current never reaches the heart, as the resistance of our skin is higher, thereby absorbing this current entirely. If this paltry current were to reach the heart by any means, it would almost certainly be fatal.

When the current increases beyond 1000 mA or 1 A, the muscular contractions augment to an extent that does not allow us to let go of the wire. This tenacity is ironically a consequence of muscular paralysis. At this point, the heart experiences ventricular fibrillation, an uncoordinated intermittent twitching of the heart’s ventricles that produces ineffective heartbeats, which could result in death if help is not summoned immediately.

Further increase in current towards 2000 mA or 2 A produces burns and unconsciousness. The muscular contraction induced by the shock is now so severe that the heart plunges into clamps. Exposure to such an amount of current could lead to dreadful internal burns, and the clamps may lead to cardiac arrest. Death is possible.

However, the clamping mechanism is devised in a way that it is surprisingly lucrative, as it protects the heart from ventricular fibrillation. Chances of survival are scant, but redeemable if the victim receives immediate attention. Defibrillators are medical devices that are utilized by doctors to save shock-impinged victims.

The repercussions can be summarized in a tabular form like this:

Porbable-effect-on-human-body

Why aren’t we impervious to current?

Even though it takes a certain voltage to make a current flow, the amount of current barging into our bodies depends on the measure of how permeable the body is to current, or simply, its resistance. The resistance to current varies depending on the condition of the skin – whether it is dry or wet. It is estimated to be 1000 ohms for wet skin and greater than 5,00,000 ohms for dry skin.

The resistance also varies depending on the points of contact. The internal resistance between the ears is only 100 ohms, while it is around 500 ohms when measured from finger to toe. It is due to this finite resistance that we aren’t impervious to current.

Another major factor is time. The ordeal’s extent depends on the amount of time that the body is exposed to a given current. For instance, a current of one-tenth ampere for a mere 2 seconds can be fatal.

References

  1. The Ohio State University
  2. The Physics Hypertextbook
  3. Wikipedia
The short URL of the present article is: http://sciabc.us/8iEgC
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About the Author:

Akash Peshin is an Electronic Engineer from the University of Mumbai, India and a science writer at ScienceABC. Enamored with science ever since discovering a picture book about Saturn at the age of 7, he believes that what fundamentally fuels this passion is his curiosity and appetite for wonder.

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