Basic factors resulting in electric injury

Current is the most effective factor. The more is current the higher is risk of injury. Threshold (minimum) currents (at the 50Hz frequency) are classified below:

- sensitivity threshold current is 0.5 - 1.5mA of alternating current (AC) and 5 - 7mA of direct current (DC);

- cramp threshold current (current that results in irresistible muscle contraction of the arm holding the conductor during electric current passing through body) is 10 - 15mA, AC and 50 - 80mA, DC;

- fibrillation threshold current (current that induces heart fibrillation during electric current passing through body) is 100mA, AC and 300mA, DC.

Acceptable level of current in normal (non-emergency) operation of electric facilities should not exceed such values: 0.3mA, AC, 50Hz; 1mA, DC. In work zone with high air temperature (over 25°C) and high air humidity (over 75%) acceptable level should be 3 times decreased.

Electric resistance of human body is resistance to the current passing through part of the body between two electrodes contacting body. It consists from surface skin layer resistance and inner organs resistance.

For practical calculation the value of human body resistance is considered equal to 1000 ohm, however several inner organs are differently sensitive to current.

The most dangerous for human being is current at the frequency range from 20 to 200 Hz. At the frequencies lower and upper that range danger of electric injury reduces and completely disappears at 450 - 500 Hz, meaning that current at such frequencies can’t bring to fatal injury of heart, lungs or another vital organs. However that current retains the danger of burns possible from electric arc or in places of contact with the wire under voltage. Fibrillation current value at 50 - 100 Hz frequencies doesn’t practically change; it’s 2 times higher at 200 Hz and 3 times higher at 400 Hz.

Direct current is 4 - 5 times more safe than alternative current at 50 Hz. Passing through body it causes muscle contraction of smaller intensity and is not so painful as alternative current of the same value. A person feels pain only in moment when power is switched on or off as muscles contact convulsive. Comparative analysis of alternative and direct current is true only for voltages under 500 V. There’s assumption that if voltage is higher 500 V DC becomes even more dangerous than AC at 50 Hz.

Duration of exposure to current is critical to determine its consequence: probability of major or even fatal injury grows with time person is under voltage. Long time exposure to current makes it possible to coincide with vulnerable heart phase (cardiocycle). Moreover current grows with time that’s explained by decreasing of body resistance because of local heating of skin and exciting tissues.

Negative consequences studied in p.3.2. accumulate and reinforce with time. It’s in fact that heart is differently sensitive to current at its different work phases. It’s most vulnerable at T-phase that lasts for 0.2 sec. If current, of the certain value, coincides with T-phase it results in fibrillation.

Consequence of electric shock depends on the way current passes through the body. If it crosses vital organs such as heart, lungs or brain that will be very dangerous. If it goes around vital organs they will be effected just by reflex reaction. It’s still dangerous but unlikely. Human body has about 15 ways all together current may pass through. These ways are called loops, the most frequent are: hand-hand, right hand-feet, left hand-feet, foot-foot, head-foot, head-hands. The most dangerous loops pass through heart and brain.

It’s in fact that healthy and physically strong people sustain electric impact easier than weak or sick one. People who’s got a problem with skin, cardio-vascular system, organs of inner secretion, lungs, nervous system are most vulnerable to electric shock.

Psychological training for risk of electric shock is practicable. Majority of cases shows that unexpected electric shock even of a small voltage may have heavy consequence. On contrary when a person is ready for possible shock the heaviness of consequence is significantly decreased. In this context attention, focusing in work and fatigue are considered. Experience and skills are important to evaluate situation and find effective measures to break free from under the voltage and allow avoiding accident. In accordance to all said safety rules require medical examination of personality who works with power facilities at the beginning of work and periodically.

Electric danger work area classification. Consequence of electric impact depends on environmental conditions. All occupational areas are divided by electric danger into three categories.

1. High danger areas are ones having one of the following conditions: high humidity exceeding 75%; floor (metallic, earth, ferroconcrete floor) or dust (metallic or carbon dust) conducting electricity; high air temperature exceeding 30 °C; likely simultaneous touching of unguarded metallic cases of electric equipment and grounded constructions.

2. Extreme danger areas are those having one of the following conditions: overall humidity about 100%; presence of chemical or organic active substances (vapors or condensates, which act destructive on insulation and conductors in the work area constantly or during long time); simultaneous action of two conditions referred to high danger areas.

3. Areas without high danger don’t have any condition to make high or extreme danger.