Step voltage - the potential difference between two points of the ground, located at a distance of a step. Sources interpret the distance for calculation differently. As a rule, 0.7 - 1 meter( some authors recommend taking 0.8 meters to a person, 1 meter to animals).Leave the hazardous area as short( goose) steps as possible.
Effect of electric current on the human body
It is not recommended to approach the scene of the accident closer than 4-5 meters at a voltage of 1000 volts. In other cases, it is dangerous to approach at 8-10 meters. Step voltage represents some danger. It is considered relatively harmless if the potential difference does not exceed 40 volts between the stops. In addition to the obvious effect on the nervous system, as a result, convulsive muscle contractions( biological effect), the electric current causes a number of specific injuries:
- Thermal action is accompanied by increased tissue warming. Electrical burns are divided into:
- Current, caused by direct contact of the conductor of the circuit under voltage up to 2 kV, and the skin. There is the Joule-Lenz law, according to which the heat released is proportional to the product of the square of the effective current value and the electrical resistance( of the human body).Burns usually I or II degree. Not very strong. It is dangerous when the current path passes through the body( avoid contact of the opposite arm, legs, body with grounded objects).Redness will remain on the local area - electrical signs( expressed marks of various skin formations).
- Arc. The temperature of the arc is high( at least 3500 degrees Celsius).In fact, the air turned into a plasma. The arc is formed between the high-voltage wire and the skin. The result without horror is hard to imagine. Surely burn III-IV degree. Like a welding electrode, the conductor melts, metallizes the skin, spreading. Of course, it causes a burn at the same time.
- The electrolytic effect of current is not described in detail by the literature for obvious reasons. During the destructive process, decompose into the constituent fluids of the human body. Including blood. Those interested will be referred to the “war of currents,” marching in America between the Edison corporation and the allies of Nikola Tesla. Thirsty to prove superiority people went to a lot. The first electric chair appeared( see Tesla Coil).
The biological effects of current are caused by various injuries of skeletal muscles, bones, ligaments. Myofibrill contractions achieve great strength. Therefore, the propulsion apparatus is under great threat.
In addition to injuries, medicine is singled out as a separate category, electric shocks. No need to treat lightly, referring to this group, only from the fact that visible damage to the body is not observed. Electric shocks are divided into IV severity. Moreover, the latter is characterized by a state of clinical death( lack of pulse in the arteries, respiration).Accordingly, from others requires a thorough knowledge of the rules of behavior.
If a person has fallen in the range of the step voltage, a current is surely flowing through the body. Anyone who has directly touched the victim is at great risk. It is necessary to correctly calculate the vector of the potential difference gradient, in practice it is not easy to do( not everyone understands the words spoken).In other words, you need to take the points of the body, between which the voltage drop is zero. Assess( correctly execute) will be able to less people than understood what was said. Therefore, to act in practice will have otherwise.
Cut the power supply is not always possible. It’s not a fact that a leak was noticed at the substation, the reactors will allow automation to respond correctly rarely. It is not possible to eliminate the danger, it is necessary to evaluate the epicenter( the place of contact of the phase, the soil), hook the victim( with a hook), and slowly begin to drag it beyond the reach of the step voltage( 20 meters from the epicenter).Move the "goose" step.
The danger of step voltage
The step voltage is detected on the ground when the phase of the power line closes on the ground, or due to the potential being skidded by a conductive object( rail track, a faulty, broken ground loop, improperly equipped, not enough deeply driven into the metal rod of the lightning rod).The situation is aggravated: when a person is defeated, it falls to the ground, the current will flow through the body. Internal organs will suffer. Since the conventional network frequency( 50 Hz) does not protect a person from internal damage. Warned Nikola Tesla, indicating a lower security limit of 700 Hz.
The step-step voltage formation pattern is shown in the figure. It can be seen: at a distance of 20 meters from the source of danger is reduced to zero. The potential difference between the epicenter is high, where the safety engineering recommends to move exclusively with a “goose” step. Putting the sock of one foot to the heel of the other. The potential difference decreases to zero. Safety instructions prohibit approaching the location of the leakage of electricity closer than 8 meters. In addition to this method of escape from the danger zone, two are invented:
- Jumping on one leg reduces the probability of an electric shock to zero. Each movement separately should not be too large. Some sources do not quite logically prohibit the procedure. It is necessary to be afraid of falling: the chance to survive depends on random factors. The probability of a lethal outcome, other unpleasant consequences is high.
- If the ground is flat, the shoes are comfortable, try jumping on two legs. Feet are put together, should not break away from each other. The danger is the same - to fall to the ground. A simple touch of the hand( awkward movement) to the soil can cause disastrous consequences. Jumps, as in the previous case, are as short as possible.
Behavioral rules for avoiding striking voltage are given as memos. Strictly avoid being guided by online reviews. Look through the safety tutorial. Some of the above methods are marked dangerous, unacceptable. Due to the elementary ignorance of the simplest laws of physics by the authors( not VashTehnik portal).
Exit the danger zone by stepping on dry, non-conductive objects. Boards. It is dangerous to attack bricks, reinforced concrete structures, earth( avoid puddles).Coatings, according to PUE, are considered unsafe, conduct electricity. Carefully navigate the sand. The underlying wet layer will be dangerous. Less soil resistance, less danger. Provided that the foot does not fall. Consider why it happens.
The occurrence of step voltage
Why there is a step voltage. When the phase wire contacts the ground, current begins to flow. According to the reference books, the soil has some definite resistance. Permanent parameter can not be considered, much depends on humidity. Obviously, with increasing depth the soil is wetter, it conducts electricity better. For this reason( no other), the steel stakes of the lightning system circuit are driven in at some minimum, pre-calculated distance.
Between the epicenter( the point of contact between the phase and the soil), the edge of a circle with a radius of 20 meters forms a resistive divider. The picture shows: the voltage drops nonlinearly. The equipotential surface of the leakage current is close to the shape of the ellipsoid of rotation. The charges spread in three directions. I got used to seeing a different alignment in physics lessons( recall the resistive electric circuit divider).
There the current moves along the wire. The path is one-dimensional. The ratio of the potentials is proportional to the resistance of the resistors taken. In the case of step voltage, the current moves in rectangular coordinates of the soil surface, leaving at the same time deep into. This explains the nonlinearity of the dependence represented by the figure: the danger drops sharply with distance from the center of the accident.
Regularities characteristic of ordinary physics: higher resistance, less current. Good for energy suppliers, the accident is cheaper. Those who feel the danger are important, the ratio of resistance of the ground and the human body. In wet soil, the currents are large, a small part of the branches to strike. A resistive divider is formed; the more a person resists electricity, the less damage will be.
It becomes clear why electricians wear special shoes with insulating soles. AC power is imperfect protection. Tensions in tens of kilovolts pierce through the soles, even if the person is standing on dry ground. The potential difference increases, following the width of the step. Safety guidelines unanimously recommend that the goose gait out of the danger zone.
For each individual, it is impossible to predict in advance the result of the step voltage action. Ultimately, it is determined by individual physiological features( body resistance).However, it is worth knowing some general patterns:
- The skin’s resistance is several times higher than the internal organs. If the epidermis is unhealthy, the underlying layers are damaged, the outcome of the collision with electricity is unfavorable.
- Physiologists identify the following most fatal paths for the passage of electric current through the body( it should be noted, this includes almost all possible trajectories of charge movement): arm-arm( worst version), any arm-legs, both arms-legs. In these cases( in descending order) a significant part of the current passes through the heart, fraught with death. It becomes clear why it is impossible to fall to the ground( so that the hands are under different potential).
At lower temperatures, the danger is lower. For loamy soils, soil moisture is 15 - 20% safe distance is 4 meters. For this reason, the soil is not allowed to warm up along cable lines in winter. In each case, other standards are also established. For example: horse transport should not work closer than 20 meters from the fenced area of the electric heating of the soil. The figure is familiar, figured above.