Touch voltage

Contact voltage is the potential difference between two points of a circuit that a person took. Step voltage is not included in the definition.

Electrical injuries

They say that birds sit on bare wires and do not fall from the discharge, the resistance of the skin of their legs is great, the current flows mainly through the wire. If a person tries to take the power line at a similar distance with two hands, the outcome will be disastrous.

Electrical Injury - Damage caused by a person’s electrical contact.

Electrical injury can be divided into local and general. The first type accounts for one-fifth of the total number of industrial accidents, the second - more than half. Other effects are reduced to the usual blows( stimulation of body tissues, involuntary contraction of muscles), as a rule, do without consequences. Local electrical injuries are accompanied by burns, metallization of the skin from molten metal, eye damage and electrical signs( relatively harmless marks on the skin of various nature).A strong electrical shock can stop the heart and lungs.

The most common phenomena among local injuries are burns. They account for two thirds of all symptoms. The most at risk are electricians involved in the operation of existing installations. Electric shocks can be divided into 5 groups:

1. Unpleasant sharp loss of orientation, instantaneous cramp.
2. Reflexes of the body, accompanied by sharp pain.
3. Loss of consciousness from electric shock with no other visible consequences.
4. Disruption of cardiac activity with a simultaneous loss of consciousness. Shortness of breath.
5. Clinical death.

As it is easy to guess, even a short-term contact with bare parts of electrical equipment leads to unpleasant consequences. Electrical shocks accounted for five out of six of the total deaths recorded in enterprises.

How to calculate the voltage of the contactWe consider the case of the potential from the leakage current short circuit. Obviously, the potential of the soil decreases exponentially with increasing distance to the ground. At a distance of 20 meters from the point of immersion in the soil becomes zero.

Belyavin proposes to consider the issue of danger as if a man took hold of the grounding point. Then the least danger( oddly enough) when he stands nearby. Although the step voltage at the same time maximum, you can not far apart legs, so as not to receive a fatal blow. Indeed, the potential of the conductor differs little from the ground, the area is shorted by the ground loop. In this case, it is necessary to immediately release the structure, which is under the current, in a “goose step” to carefully leave the scene of the accident.

Step voltage

The situation is much worse if a person is standing half a meter or a meter from the point of immersion of the protective or working( neutral) neutral conductor in the ground. This is more dangerous than taking a step a similar distance. Because the resistance between the hand and the metal is small, and the contours of the legs are connected in parallel, which increases the danger of breakdown( rubber shoes do not save).But even worse is the case when a piece of grounding is laid in the air parallel to the ground, but does not touch it at any point, except the one where the neutral conductor enters the ground. In the latter case, the potential difference is the highest. Situation Description:

1. A man stands on the ground 20 meters from the entrance of the grounding conductor to the ground. Here the potential created by the flowing current is already zero.
2. By accident or an oversight, a structure running parallel to the earth on dielectric supports( pipeline, hedge) with low electrical resistance( metal) was connected to the point of entry of the neutral conductor into the ground and is located 20 meters from it.
3. A man stands on the ground, took a hand on the iron according to claim 2. Immediately finds himself under the phase voltage of the network.220 V will not be able to penetrate the sole of the shoe, if you stand barefoot, or accidentally lean on the ground with your second hand, the touch voltage will be the maximum possible and extremely dangerous( characteristic of the current path depending on the damage is given in the conclusion of the step voltage).

So, the damage is the least if the person is at the grounding point. In this case, the step voltage is maximum, you need to move away from the source carefully. At a point at a distance of 20 meters, it is permissible to walk freely, but if you accidentally take up the conductor, according to claim 2, the consequences are predicted to be the hardest.

Skeptics will say that in the situation described above, the fact that the voltage between the resistance of the neutral conductor, above and below the ground, is not taken into account. In fact, everything is taken into account. The resistance of iron( especially copper) is much lower than the resistance of the ground( the work function of electrons from the surface of the contour into the soil).The last parameter is directly proportional to the resistance of the soil, and back to the geometric dimensions of the contour.

Safety Requirements

According to current regulations, the touch voltage should not exceed 65 V, which is considered a safe value for a long( over three seconds) touch. Then the permissible threshold increases with a drop in the interval:

1. 0.1 s - 740 V.
2. 0.2 s - 370 V.

When these requirements are not met, protective clothing should be used. The case of simultaneous contact with the current-carrying part of the equipment and the grounding is recognized as especially dangerous. When carrying out preventive( repair) works, metal structures that are under ground potential, closer than 2 meters to serviced equipment, are covered with shields, insulating plates, etc.

Precaution at work

During prolonged leakage of current, the touch voltage is applied to metal structures directlybordering the grounding: pipes, fences, stairs, etc. As the voltage of a step, it quickly decreases with distance, but the safe zone cannot be clearly drawn, much depends onproperties of the dangerous section, its conductivity.

Separate pipelines are under cathodic protection by the method of formation of a negative potential with respect to soil. In this case, the site is clearly isolated from the grounding and is an increased danger. The boundary of the section usually lies on the border of the plant or building. Visually, it is possible to determine by the presence of an insulating flange in the pipeline. In case of an accident, it is recommended to eliminate the source of danger as quickly as possible.

Protection measures

In addition to working clothes, there are constructive considerations. To reduce the voltage step and touch, equalized potentials. This is achieved by introducing a grounding conductor into the soil at several points. Usually around the perimeter of a certain form. It turns out that in all places of entry the potential is equal, and the touch voltage is above all outside the specified line. Inside there is a danger caused by pseudo-random processes, but much lower than with a single circuit.

The shape of the perimeter depends on the conditions existing on the ground: the line, if this increases the safety of movement, or the grid, square, hexagon, etc. If you take European standards, you can see the design of the underground contour of the grounding plane in the form of a comb. This is done to reduce the spreading current: moving charges fall on a larger perimeter, which naturally reduces the potential difference( according to Ohm’s law for a section of the circuit).A similar idea was used in the above case. The longer the perimeter, the lower the voltage of touch.

So, the design of the grounding plays a big role in protecting personnel and bystanders from danger. In particular, the territory of the enterprise detects a cluster of random earths, united in a single chain. Including lightning circuit. All this is done to reduce the risk in case of an accident. Continue to focus: it is considered cases of leakage. In other situations, the current through the protective and working neutral conductor is very small. This is achieved as a good insulation, and a uniform load on all phases.

Weather and external conditions

Obviously, a damp room is more dangerous than a dry room. The condition of the air and other climatic conditions strongly influence the probability of being hit by the touch voltage. The aggravating effect, besides dampness, is exerted by:

1. Vapors of aggressive liquids and gases.
2. Conductive dust.
3. Uninsulated floors: brick, concrete, metal, soil.

Visual Aid

Increased temperature is an additional weakening factor as people sweat and skin resistance decreases. In addition, in the heat insulation cable is most at risk. According to these factors, the premises are usually divided as follows:

• Particularly dangerous: chemically aggressive or organic environment, plenty of moisture( especially wet), outdoor electrical installations.
• . With increased danger: conductive floors( see above);moisture or relative humidity over 75%;heat;abundance of grounded metal structures.
• No increased danger: with a normal climate, insulated floors( wood, polymers), not containing metal structures.

The above terms have a better color than a quantitative, safety textbooks offer further decoding of hazard classes. Specific terms:

• Hot room - with a temperature above 35 degrees Celsius.
• Dusty room - items guaranteed to be dusty. Particularly dangerous conductive dust.
• Dry room - with a relative humidity of not more than 60%.If there are no symptoms listed below, the room is called normal.
• Wet room - with a relative humidity of not more than 75%.Light condensate allowed but temporary.
• Wet room - relative air humidity over 75%.
• A particularly damp room is the maximum humidity, therefore there is guaranteed condensation on objects, walls, ceiling and floor.
• Chemically aggressive( organic) premises - containing aggressive or organic media and their vapors.

Measurement of the

The touch voltage is measured with an ammeter and a voltmeter. Estimated potential difference between the objects accessible to the touch and imitation of the soles of a person is a metal square plate with an area of ​​625 square meters lying on the ground.see. Body resistance is replaced by an equivalent resistor; a voltmeter is connected in parallel to measure voltage.

The current source is a test transformer that produces a voltage that can hypothetically arise on metal structures. If the voltage of the circuit is too high, the resistor value is taken higher, it will be necessary to measure the current. Then the resistance of the circuit is calculated and the graphs( straight line) are the values ​​for the "combat" conditions of this accident.

One of the points of the secondary winding is grounded. If this is not possible under the conditions, an isolation transformer is installed. And already the point of its secondary winding is grounded. This is necessary( in violation of safety) to achieve the "danger" of the maximum.