The effect of electric current on the human body. Scheme, description

ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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The effect of electric current on the human body. Encyclopedia of radio electronics and electrical engineering

Free technical library

Encyclopedia of radio electronics and electrical engineering / Electrical safety, fire safety

Electrical energy is too serious a thing to be trifled with. In educational institutions where electricians are trained, safety is singled out as a separate subject, in which students must pass an exam. And after graduation, any professional electrician must undergo regular training on the rules for operating electrical installations and safe methods of working with them. Those who have not undergone such instruction are simply not allowed to work.

It is known from a school physics course that water conducts electric current quite well, but worse than aluminum and copper conductors. The human body is also a good conductor. But what processes occur in the body when an electric current passes through it?

There are two types of electric shock:

- an electric shock (shock) that affects the entire body, which does not cause burns, but leads to paralysis of the heart or breathing, and often to both at the same time;

- electrical injury - electric shock to external parts of the body: burns, metallization of the skin, electrical signs.

The nature of the occurrence of electrical burns is as follows: when an electric current passes through the body, heat is released in its tissues, sufficient to heat the places of contact with voltage up to 60-70 ° C. The protein that is part of the tissues at this temperature coagulates, from which burns or electrical signs appear on the body (round or oval swellings of hardened, deeply affected skin of a yellowish color, outlined by a white or gray border). If the metal (conductors, other parts) under the influence of current during the burning of the arc splashes and evaporates, electrometallization of the skin occurs - the affected areas are impregnated with metal particles.

In addition to electric shock caused by direct contact with a current-carrying element, the so-called touch injury is possible, which occurs when a person comes into contact with a grounded installation in which voltage has appeared on the case. In this case, a person is not affected by an electric current as such, but by a voltage arising from the potential difference between the reference points under his feet and his hands, which have contact with the body of the electrical installation. If the ground is broken or has too much resistance, then the touch voltage can be dangerous. The degree of electric shock depends on the following indicators and parameters: on the magnitude and type of current flowing through the human body; on the duration of its impact; from the mental and physical state of a person at the time of the defeat; from the path through which the current passes in the body. It is most dangerous when the path of most of the current passes through the heart (from one hand to another or from hands to feet), and the current is alternating.

It has already been said that electric current is characterized by three parameters: voltage, strength and frequency. It is not voltage that is dangerous for the human body, but current strength: if at a strength of 0,6 mA a person is just beginning to feel an alternating current, then at a strength of 12-15 mA he can withstand it for no more than 10 seconds and, although he will experience severe pain in the fingers and hands, able to get rid of contact with current-carrying elements on their own; when struck by a current of 20-25 mA, a person experiences very severe pain and paralysis of the hands, therefore, he cannot get rid of its effects on his own; current strength of 50-80 mA causes respiratory paralysis; and already at 90-100 mA leads to heart paralysis and death.

If we consider the degree of danger of alternating current depending on its frequency, then the following pattern is observed: the higher the frequency of the current, the more dangerous it is for a person (the most dangerous frequency of alternating current is 50-60 Hz). The fact is that with an increase in the frequency of the current, surface currents begin to spread over the surface of the body, strongly heating the skin, so a high-frequency current injury usually leads to burns, but not to an electric shock. A person has less sensitivity to the action of direct current: he begins to feel it at 12-15 mA, and respiratory paralysis occurs only at 90-110 mA. As we have already noted, the human body has resistance, which is measured in ohms (Ohms). At the same voltage, the current strength will be the greater, the lower the resistance of the current-carrying conductor. The resistance of dry and intact skin (its stratum corneum) can reach 40-000 ohms.

Physical, physiological and psychological processes occurring in the body (excessive sweating, intoxication, nervous excitement, overwork) sharply - up to 800-1000 Ohm - reduce the resistance of the human body. In addition, the stratum corneum of the skin has a very small thickness - 0,05-0,2 mm - and is easily penetrated by a voltage of 250 V. In this case, the resistance decreases by 100 times and falls the sooner the longer the current is applied to the human body. This explains the fact that even a small voltage can cause electric shock: with a human body resistance of 700 ohms, a voltage of only 35 V will be dangerous, and when working with such a voltage, it is necessary to use insulating protective equipment - rubber gloves or tools with insulated handles.

Author: Korshevr N.G.

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