Short theoretical information

Electric current while passing through human body influences on it in thermal, electrolytic and biological manner. Thermal action is characterised by heating of living tissue, up to their burns; electrolytic - by decomposition of organic liquids, in that number blood, changing their chemical content; biological - damaging bio-electrical processes and, so, the functions of nervous system and tissue.

Varieties of outcomes of electric current influence on living organism are divided conditionally into two basic types: local electric traumas, at which the local strike may arise - electric burn, electric signs (marks), skin metallization, and general striking, when whole man organism may be damaged because of violation of normal activity of vitally important organs and systems, - it is an electric blow.

Electric burn is the most widespread electric trauma. Current burn is contra-distinguished in networks with voltage not above 2 kWt, which caused by passing through the body tissue current with strength not more than 1 A, and arc burn, which arises from electric arc temperature, that may reach more than 3500 °С. Arc arises at accidental short current in plants with voltage not more than 6 kV, and also in networks higher than 10 kV at man approach to current conducting parts, which are found them under voltage.

The Electric signs are the blots of grey or daffodil colour on skin surface in place of contact with current-conducting parts. They, as a rule, are painless and in process of time must ascend.

Skin metallization is worming of metal steams into skin depth, when body part is found itself in immediate contact from place of electric arc formation. Struck part of the skin has a rough surface, colour of which is determined by colour of metal compounds that wormed into skin. A sick feeling of burn and presence of foreign body disappear on dying off degree of damaged skin. A big hazard is eye metallization.

Electric blow is excitation of living organism tissue by electric current that is accompanied by spontaneous convulsive muscles shortening, which frequently may cause a mortal outcome.

Electric current operates on organism reflexly through nervous system. At the beginning an excitation phase takes place: a reaction on pain appears, arterial pressure rises and other. Then a braking phase treads: nervous system becomes weak, arterial pressure falls down, breathing weakens, a pulse falls and becomes more frequent, a depression state arises. By Result of excitation and further braking of work of nervous system can be bringing of cardiac activity and breathing - an electric shock.

Cardiac activity may cease at straight action of electric current, that passes over domain of the heart (primary fibrillation), and by reason of reflex arteries spasm (second fibrillation). The heart fibrillation causes violation of blood circulation and, if not to take measures, which proceed cardiac activity, then death may happen.

Suppression of breathing, its paralysis is outcome of electric current influence on muscles of thorax that provide a breathing process. If such influence is long, then asphyxia may happens - asphyxiation from lack of oxygen and carbonic acid surplus in human body.

Electric shock is a difficult nervously reflex reaction on irritation by electric current. With shock may arise deep discords of breathing, circulation of the blood, nervous and other systems of organism. Shock may continue from several minutes till ten days and if not to take measures on guiding out of such state, then biological death may happen.

Hazard degree from influence of electric current depends on row of factors: strength of current, that passes through man; type and frequency of electric current; human body electric resistance; current paths in the body; duration of current influence; human individual peculiarities; conditions of external environment; factor of attention.

Strength of current is a basic factor determining a human striking outcome. Voltage in electric chain, in which man found himself, and also resistance of his body, influence on striking outcome by electric current only in that degree, in which they determine the current strength, that passes through human body.

Current with different strength acts differently on man. Following reference values of current are picked out: perceptible current (passing through organism current bringing perceptible irritation) is equal to 0,6... 1,5 mA for variable current with frequency 50 Hz and 5... 7 mA for constant current; a limit for unletting current (passing through human body current bringing to insuperable convulsive muscles shortening of the hand, in which conductor is clutched) is equal to 10... 15 mA for variable current with frequency 50 Hz and 50... 80 mA for constant current;

A limit for fibrillating current (current causing heart fibrillation at passing through human organism) is equal to 100 mA for variable current with frequency 50 Hz and 300 mA for constant current.

Type and frequency of electric current also may influence on outcome of man striking. Constant current is less dangerous and its limits 3-4 times higher than for variable current with frequency 50 Hz. However this may happens only at voltage 400 V. In diapason 400... 600 V hazard of constant current and variable current with frequency 50 Hz are practically identical, and with further voltage increasing a relative danger of constant current increases too. Physiological processes on living cells cause this.

With increasing frequency of electric current from 0 to 50 Hz thresholds of irritation reaction by current fall down. With further frequency growth thresholds of current begin rise. Noticeable lowering of danger of human striking is observed at frequencies higher than 1000 Hz. Experiments over animals have proved that impressive voltage with frequency 50 Hz causes death in 100 % of the cases and at frequency 200 Hz animals perish. Fibrillation current under this effect increases twice, and at frequency 400 Hz - more than three times. At frequencies near to 10 kHz danger of electric blow fully disappears and only burn danger omits at touch to current-conducting parts.

Electric resistance of human body is different not only for different peoples, but for different body tissue also, that conditioned by physical, biochemical and biophysical phenomena. That's why it is non-linear and unstable value. Volumetric resistivity for current with frequency 50 Hz composes [Ohm-m]:

horny layer of dry skin 1-10⁵.. 1-10⁶;

bone 1-10⁴.. 2-10⁶;

fatty tissue 30... 60;

muscle tissue 1,5... 3,0;

blood 1,0... 2,0.

So-and-so, skin, specially its horny top layer is, which consists of died horny cells, has large resistance.

In fig. 1, в, the human body resistance to constant current is higher than to variable one with any frequency. Moistened skin has in 1,5...2 times lesser resistance, than dry skin because moisture dissolves on skin surface the salts and acids, that exude from organism with sweat, and that adds to its conductivity.

б

Fig. 1. Electric resistance of human body:

a - schematic inclusion of human body into electric chain; б, в - equivalent schemes of electric human body: Ze is a full electric resistance of epidermis; Rвн is a resistance of internal tissue; Rе- is an active resistance of epidermis; Се - electric capacitance of epidermis of healthy man; 1- electrodes; 2 - horny layer of the skin; 3 - an external layer of the skin - is a epidermis; 4 - internal skin layers and hypodermic tissue; 5 - an internal skin layer is a dermis; 6 – underskin tissue; 7 - skin layer.

Resistance of internal organs in general does not depend on enclosed voltage and composes 300... 600 Ohm.

General human body resistance depends on value of enclosed voltage. With its augmentation skin resistance decreases because of clamp of its top layer, that may sometimes happen even at voltage near to 50 V. Further voltage increase still decreases general resistance, and at 300 V it comes around resistance of internal organs.

Long passing of current contributes to lowering of total human body resistance as a result of raised blood circulation in under-skin tissue and, so, to rise of sweat. At voltage 20... 30 V during 1 - 2 minutes resistance can fall down on a par on 25%.

Because human body resistance to electric current is non-linear and unstable and it is difficult to conduct the computations with it, with sufficient accuracy for practice, man body resistance conditionally is taken to be active, linear and equal to 1000 Ohm.

A current path in human body influences on striking outcomes. Danger is especially large, if current, passing through vitals organs (heart, lungs, cerebrum), acts immediately on them. If current does not pass over these organs, then its action on them only reflex and striking probability is lesser.

Frequently met current path through human body is called by "current loop". In majority of cases (55%) the electric blows take place for two basic loops: а) from one palm or both palms to legs; б) from one palm to another palm. This collision is most probable. But for these two most probable paths only 28 % of all of mortal striking occurs, and other 72% occur for outstanding loops, for example, from rear part of hand to legs (25 % of cases), from palm to rear part of hand (28 %). This brings us over to our point of view that rear part of the hand is a most vulnerable place for current on human body. Refereeing from this, a danger is determined not only by current paths through vital organs, but also how body touches to the current-conducting parts of equipment, what is a closeness of nervous outflows on this part of the body and is there a acupuncture zone.

Duration of current action on man influences on striking outcome, because with increasing of time of current action protective function of human organism falls down, and striking danger enhances. Next acceptable norms of currents are adopted in dependence to time of passing:

Passing time, с 1 0,7 0,5 0,2

Current, mA 65 75 100 250

Also likelihood of heart fibrillation beginnings is fixed, in dependence on current passing time. It is known fact that cardiac muscle in different phases of its activity is not identically sensible to electric current. It may be believed that most sensitive to current, sufficient for heart fibrillation, is a phase Т, with duration 0,2 s, period, when expires shortening of ventricles and a cardiac muscle gets across weakened state. What lesser the duration of current passing through organism, that lesser likelihood of its influence on heart in vulnerable phase, and so for striking danger.

Individual human organism peculiarities considerably influence on striking result at electric trauma. For example, effects of current of the same strength depend on man mass, its physical development. An unletting current for one person may be only perceptible for other. For women thresholds of current are approximately 1,5 times lower than for men.

Degree of current effects depends also on state of nervous system and whole organism. Consequently, in excitation state of nervous system, depression, illness (specially illness of skin, cardiac-vascular system, nervous system and other), at intoxication man is more sensible to passing current.

Conditions of external environment determine the striking outcomes in that degree, in which they are capable to change value of current, that passes through man, changing electric organism resistance. For example, with temperature rise of external environment number of difficult striking increases, and with pressure rise - diminishes. In connection with that all production workplaces, in accordance to maintenance regulations of electric equipment, are subdivided on three categories in dependence with conditions like external phenomena.

Factor of human attention also largely influences on striking outcomes. It is known fact, that for protection from dangerous irritant the internal man resources may mobilise. If man is prepared to electric blow, then danger degree sharply falls down, and unexpected blow gives rise to more difficult outcomes. Eloquent assertion: "By chance to be killed from electric current" is a surplus. To put man to death by current is extremely heavily.

At first help assignment for victim from influence of electric current it is necessary to execute such operations: to absolve from source of current; immediately to bring on ambulance; quickly to define a victim state and to provide rest to him, flow of crisp air, heat; in lack of breathing and pulse - to do artificial breathing and external heart massage.

When a number of persons, who gives help, is equal to two or more, then one of them must make a heart massage to victim (to press by open palm on thorax), second must make artificial breathing (it is necessary to make 10-12 inhales in a minute). Pressing must be taken once in a second with interruptions on 2 s after each four - six pressings. If help is lend by one man, then after two - three inhales it must be taken four - six pressings for heart massage.

CONTROL QUESTIONS

1. What is a local electric trauma?

2. What is a general organism striking?

3. What is an electric burn?

4. What is an electric sign?

5. How does skin metallization happen?

6. What is electric blow?

7. What is heart fibrillation?

8. What are the causes of breathing paralysis?

9. What is electric shock?

10. From which factors depends a man striking degree by electric current?

11. What is an attention factor?

Дата добавления: 2015-11-14