Diagrams of possible inclusion in the current circuit of a heavy-duty railway. Single-phase and two-phase connection of a person in various electrical networks. Possible schemes for connecting a person to an electrical circuit

The connection of a person to the electrical network can be single-phase or two-phase. Single-phase connection is the connection of a person between one of the phases of the network and the ground. The strength of the damaging current in this case depends on the neutral mode of the network, human resistance, shoes, floor, and phase insulation relative to the ground. Single-phase switching occurs much more often and often causes electrical injuries in networks of any voltage. With a two-phase connection, a person touches two phases of the electrical network. With a two-phase switching on, the strength of the current flowing through the body (striking current) depends only on the network voltage and the resistance of the human body and does not depend on the neutral mode of the network supply transformer. Electrical networks are divided into single-phase and three-phase. A single-phase network can be isolated from the ground or have a grounded wire. In Fig. 1 shows possible options for connecting a person to single-phase networks.

Thus, if a person touches one of the phases of a three-phase four-wire network with a solidly grounded neutral, then he will be practically under phase voltage (R3≤ RF) and the current passing through the person during normal operation of the network will practically not change with changes in insulation resistance and capacitance wires relative to ground.

The effect of electric current on the human body

Passing through the body, electric current has thermal, electrolytic and biological effects.

The thermal effect manifests itself in burns of the skin or internal organs.

During electrolytic action, due to the passage of current, decomposition (electrolysis) of blood and other organic liquid occurs, accompanied by the destruction of red blood cells and metabolic disorders.

The biological effect is expressed in irritation and excitation of living tissues of the body, which is accompanied by spontaneous convulsive contraction of muscles, including the heart and lungs.

There are two main types of electric shock:

§ electrical injuries,

§ electric shocks.

Electric shocks can be divided into four degrees:

1. convulsive muscle contractions without loss of consciousness;

2. with loss of consciousness, but with preservation of breathing and heart function;

3. loss of consciousness and disturbance of cardiac activity or breathing (or both);

4. clinical death, i.e. lack of breathing and blood circulation.

Clinical death is a transition period between life and death, begins from the moment the activity of the heart and lungs stops. A person in a state of clinical death does not show any signs of life: she has no breathing, no heartbeat, no reaction to pain; The pupils of the eyes are dilated and do not react to light. However, it should be remembered that in this case the body can still be revived if help is given to it correctly and in a timely manner. The duration of clinical death can be 5-8 minutes. If help is not provided in a timely manner, biological (true) death occurs.

The result of electric shock to a person depends on many factors. The most important of them are the magnitude and duration of the current, the type and frequency of the current and the individual properties of the organism.

Determination of the current spreading resistance of single grounding conductors and the procedure for calculating the protective grounding loop for stationary process equipment (GOST 12.1.030-81. CCBT. Protective grounding, grounding)

Implementation of grounding devices. A distinction is made between artificial grounding devices, intended exclusively for grounding purposes, and natural ones - third-party conductive parts that are in electrical contact with the ground directly or through an intermediate conducting medium, used for grounding purposes.

For artificial grounding electrodes, vertical and horizontal electrodes are usually used.

The following can be used as natural grounding conductors: water supply and other metal pipes laid in the ground (with the exception of pipelines of flammable liquids, flammable or explosive gases); casing pipes of artesian wells, wells, pits, etc.; metal and reinforced concrete structures of buildings and structures that have connections to the ground; lead sheaths of cables laid in the ground; metal sheet piles for hydraulic structures, etc.

The calculation of protective grounding aims to determine the basic parameters of grounding - the number, dimensions and order of placement of single grounding conductors and grounding conductors, at which the touch and step voltages during the phase closure to the grounded body do not exceed permissible values.

To calculate grounding, the following information is required:

1) characteristics of the electrical installation - type of installation, types of main equipment, operating voltages, methods of grounding neutrals of transformers and generators, etc.;

2) electrical installation plan indicating the main dimensions and placement of equipment;

3) the shapes and sizes of the electrodes from which it is planned to construct the designed group grounding system, as well as the expected depth of their immersion into the ground;

4) data from measurements of soil resistivity in the area where the ground electrode is to be constructed, and information about the weather (climatic) conditions under which these measurements were made, as well as characteristics of the climatic zone. If the earth is assumed to be two-layer, then it is necessary to have measurement data on the resistivity of both layers of the earth and the thickness of the top layer;

5) data on natural grounding conductors: what structures can be used for this purpose and their resistance to current spreading, obtained by direct measurement. If for some reason it is impossible to measure the resistance of the natural ground electrode, then information must be provided that allows this resistance to be determined by calculation;

6) calculated ground fault current. If the current is unknown, then it is calculated using the usual methods;

7) calculated values of permissible touch (and step) voltages and protection duration, if the calculation is made based on touch (and step) voltages.

Grounding calculations are usually made for cases where the ground electrode is placed in homogeneous ground. In recent years, engineering methods for calculating grounding systems in multilayer soil have been developed and began to be used.

When calculating grounding conductors in homogeneous soil, the resistance of the upper layer of the earth (layer of seasonal changes), caused by freezing or drying out of the soil, is taken into account. The calculation is made using a method based on the use of grounding conductivity utilization factors and is therefore called the utilization factor method. It is performed with both simple and complex designs of group grounding conductors.

When calculating grounding systems in a multilayer earth, a two-layer earth model is usually adopted with the resistivities of the upper and lower layers r1 and r2, respectively, and the thickness (thickness) of the upper layer h1. The calculation is made by a method based on taking into account the potentials induced on the electrodes that are part of the group grounding system, and is therefore called the method of induced potentials. Calculation of grounding conductors in multi-layer earth is more labor-intensive. At the same time, it gives more accurate results. It is advisable to use it in complex designs of group grounding systems, which usually take place in electrical installations with an effectively grounded neutral, i.e. in installations with voltages of 110 kV and above.

When calculating a grounding device by any method, it is necessary to determine the required resistance for it.

The required resistance of the grounding device is determined in accordance with the PUE.

For installations with voltages up to 1 kV, the resistance of the grounding device used for protective grounding of exposed conductive parts in an IT type system must meet the following conditions:

where Rз is the resistance of the grounding device, ohm; Upred.add – touch voltage, the value of which is assumed to be 50 V; Iз – total ground fault current, A.

As a rule, it is not necessary to accept a grounding device resistance value of less than 4 ohms. A grounding device resistance of up to 10 Ohms is allowed if the above condition is met, and the power of transformers and generators supplying the network does not exceed 100 kVA, including the total power of transformers and (or) generators operating in parallel.

For installations with voltages above 1 kV above 1 kV, the resistance of the grounding device must correspond to:

0.5 Ohm with an effectively grounded neutral (i.e. with large earth fault currents);

250/Iz, but not more than 10 Ohms with an isolated neutral (i.e. with low ground fault currents) and the condition that the ground electrode is used only for electrical installations with voltages above 1000 V.

In these expressions, Iз is the calculated ground fault current.

During operation, there may be an increase in the resistance to the spreading of the ground electrode current above the calculated value, therefore it is necessary to periodically monitor the value of the ground electrode resistance.

Ground loop

The ground loop is classically a group of vertical electrodes of small depth connected by a horizontal conductor, mounted near an object at a relatively small mutual distance from each other.

As grounding electrodes in such a grounding device, a steel corner or reinforcement 3 meters long was traditionally used, which was driven into the ground using a sledgehammer.

A 4x40 mm steel strip was used as a connecting conductor, which was laid in a pre-prepared ditch 0.5 - 0.7 meters deep. The conductor was connected to the mounted grounding conductors by electric or gas welding.

To save space, the ground loop is usually “rolled” around the building along the walls (perimeter). If you look at this ground electrode from above, you can say that the electrodes are mounted along the contour of the building (hence the name).

Thus, a ground loop is a ground electrode consisting of several electrodes (groups of electrodes) connected to each other and mounted around the building along its contour.

The circuits for connecting to the current circuit can be different. However, the most typical connection schemes are: between two phases and between one phase and ground (Fig. 1). Of course, in the second case, an electrical connection is assumed between the network and the ground.

The first circuit corresponds to a two-phase touch, and the second to a single-phase touch.

The voltage between two conductive parts or between a conductive part and the ground when simultaneously touched by a person or animal is called touch tension (U etc).

Two-phase touch, all other things being equal, is more dangerous, since the highest voltage in a given network is applied to the human body - linear, and The current through a person, being independent of the network diagram, neutral mode and other factors, is of greatest importance:

Where
- line voltage, i.e. voltage between phase wires of the network, V;

U f - phase voltage, i.e. voltage between the beginning and end of one winding of a current source (transformer or generator) or between the phase and neutral wires of the network, V;

R h- resistance of the human body, Ohm.

Rice. 6.1. Cases of human contact with live parts that are energized: a - two-phase inclusion: b and c - single-phase inclusion

Cases of two-phase touch occur very rarely and cannot serve as a basis for assessing networks for security conditions. They usually occur in installations up to 1000 V as a result of working under voltage, the use of faulty protective equipment, as well as the operation of equipment with unprotected bare live parts (open switches, unprotected clamps of welding transformers, etc.).

Single-phase touch, other things being equal, is less dangerous than two-phase touch, since the current passing through a person is limited by the influence of many factors. However, single-phase contact occurs much more often and is the main scheme in which people are electrocuted in networks of any voltage. Therefore, only cases of single-phase touch are analyzed below. In this case, both three-phase current networks with voltages up to 1000 V approved for use are considered: four-wire with a solidly grounded neutral and three-wire with an insulated neutral.

6.2.4. Three-phase networks with solidly grounded neutral

In a three-phase four-wire network with a solidly grounded neutral, calculation of touch voltage U etc , And current I h passing through a person, in the case of touching one of the phases (Fig. 6.2), is easiest to perform using the symbolic (complex) method.

Let's consider the most general case, when the insulation resistance of the wires, as well as the capacitance of the wires relative to the ground, are not equal to each other, i.e.

r 1 ≠ r 2 ≠ r 3 ≠ r n ; WITH 1 ≠ WITH 2 ≠ WITH 3 ≠ WITH n ≠ 0,

Where r 1 , r 2 , r 3 , r n- insulation resistance of phase L and neutral (combined) PEN wires, Ohm;

C 1 , C 2 , C 3 , C n - dispersed capacitances of phase L and neutral (combined) PEN wires relative to the ground, F.

Then the total conductivities of the phase and neutral wires relative to the ground in complex form will be:

;
;
;

Where w- angular frequency, rad/s;

j - imaginary unit equal to (
).

Rice. 6.2. Human contact with a phase wire of a three-phase four-wire network with a grounded neutral during normal operation: a - network diagram; b - equivalent circuit; L1, L2, L3, - phase conductors; PEN - neutral (combined) wire.

The total grounding conductances of the neutral and the human body are equal, respectively

;
,

Where r 0 - neutral grounding resistance, Ohm.

The capacitive component of human conductivity can be neglected due to its small value.

When a person touches one of the phases, for example, phase conductor L1, the voltage under which he will be determined by the expression

, (6.1)

The current can be found by the formula

Where - complex voltage of phase 1 (phase voltage), V;

- complex voltage between the neutral of the current source and the ground (between points 00" on the equivalent circuit).

Using the well-known two-node method, can be expressed as follows:

Keeping in mind that for a symmetrical three-phase system

;
;
,

Where U f - phase voltage of the source (module), V;

A - phase operator taking into account the phase shift, where

we will have equality

Substituting this value into (6.1), we obtain the required equation for touch voltage in complex form acting on a person who touches phase conductor L1 of a three-phase four-wire network with a grounded neutral:

. (6.2)

We obtain the current passing through a person if we multiply this expression by Y h :

. (6.3)

Under normal operating conditions of the network, the conductivity of the phase and neutral wires relative to the ground compared to the conductivity of the neutral grounding has very small values and, with some assumption, can be equated to zero, i.e.

Y 1 = Y 2 = Y 3 = Y n = 0

In this case, equations (6.2) and (6.3) will be significantly simplified. So, the touch voltage will be equal

or (in actual form)

, (6.4)

and the current is equal

(6.5)

According to the requirements of the PUE, the resistance value r 0 should not exceed 8 ohms, the resistance of the human body R h , does not fall below several hundred ohms. Therefore, without a big error in equations (6.4) and (6.5), we can neglect the value r 0 and assume that when touching one of the phases of a three-phase four-wire network with a grounded neutral, a person finds himself practically under phase voltageU f , and the current passing through it is equal to the quotient of divisionU f onR h .

Another conclusion follows from equation (6.5): the current passing through a person who touches the phase of a three-phase four-wire network with a grounded neutral during its normal operation practically does not change with changes in the insulation resistance and capacitance of the wires relative to the ground, if the condition remains that the total conductivities of the wires relative to the ground are very small compared to the conductivity network neutral grounding.

In this case, the safety of the resistance of shoes, soil (floor) and other resistance in the human electrical circuit significantly increases.

A solid ground fault in a network with a solidly grounded neutral changes the phase voltage relative to ground little.

In emergency mode, when one of the phases of the network, for example phase conductor L3 (Fig. 6.3, a), is shorted to ground through a relatively low active resistance r zm, and a person touches the phase conductor L1, equation (6.2) will take the following form:

Here we also assume that Y 1 ,Y 2 And Y n small compared to Y 0 , i.e. are equal to zero.

Having made the appropriate transformations and taking into account that

,
And
,

we obtain the touch voltage in real form

To simplify this expression, let us assume that

As a result, we finally obtain that the voltage U etc equals

. (6.6)

The current passing through a person is determined by the formula

. (6.7)

Rice. 6.3. Human contact with a phase wire of a three-phase four-wire network with a grounded neutral during emergency mode: a - network diagram; b - vector diagram of voltages.

Let's consider two typical cases.

If the resistance of the wires to ground is r zm considered equal to zero, then equation (6.6) will take the form

Consequently, in this case the person will be under the influence of the linear voltage of the network.

2. If we take the neutral grounding resistance equal to zero r 0 , then from equation (6.6) we obtain that U n.p. = U f , those. The voltage under which a person will be will be equal to the phase voltage.

However, in practical conditions of resistance r zm And r 0 is always greater than zero, so the voltage under which a person touches a serviceable phase wire of a three-phase network with a grounded neutral during an emergency mode is always less than linear, but more than phase, i.e.

>U etc >U f . (6.8)

This situation is illustrated by the vector diagram shown in Fig. 6.3, b and corresponding to the case under consideration. It should be noted that this conclusion also follows from equation (6.6). So, for small values r zm And r 0 compared with R h , the first term in the denominator can be neglected. Then the fraction for any ratio r zm And r 0 will always be greater than one, but less
, i.e. we obtain expression (6.8).

The first circuit corresponds to a two-phase touch, and the second to a single-phase touch.

The voltage between two conductive parts or between a conductive part and the ground when simultaneously touched by a person or animal is called touch tension (U etc).

Where
- line voltage, i.e. voltage between phase wires of the network, V;

U f - phase voltage, i.e. voltage between the beginning and end of one winding of a current source (transformer or generator) or between the phase and neutral wires of the network, V;

R h- resistance of the human body, Ohm.

Rice. 6.1. Cases of human contact with live parts that are energized: a - two-phase inclusion: b and c - single-phase inclusion

6.2.4. Three-phase networks with solidly grounded neutral

Let's consider the most general case, when the insulation resistance of the wires, as well as the capacitance of the wires relative to the ground, are not equal to each other, i.e.

r 1 ≠ r 2 ≠ r 3 ≠ r n ; WITH 1 ≠ WITH 2 ≠ WITH 3 ≠ WITH n ≠ 0,

Where r 1 , r 2 , r 3 , r n- insulation resistance of phase L and neutral (combined) PEN wires, Ohm;

C 1 , C 2 , C 3 , C n - dispersed capacitances of phase L and neutral (combined) PEN wires relative to the ground, F.

Then the total conductivities of the phase and neutral wires relative to the ground in complex form will be:

;
;
;

Where w- angular frequency, rad/s;

j - imaginary unit equal to (
).

The total grounding conductances of the neutral and the human body are equal, respectively

;
,

Where r 0 - neutral grounding resistance, Ohm.

The capacitive component of human conductivity can be neglected due to its small value.

When a person touches one of the phases, for example, phase conductor L1, the voltage under which he will be determined by the expression

, (6.1)

The current can be found by the formula

Where - complex voltage of phase 1 (phase voltage), V;

- complex voltage between the neutral of the current source and the ground (between points 00" on the equivalent circuit).

Using the well-known two-node method, can be expressed as follows:

Keeping in mind that for a symmetrical three-phase system

;
;
,

Where U f - phase voltage of the source (module), V;

A - phase operator taking into account the phase shift, where

we will have equality

. (6.2)

We obtain the current passing through a person if we multiply this expression by Y h :

. (6.3)

Y 1 = Y 2 = Y 3 = Y n = 0

In this case, equations (6.2) and (6.3) will be significantly simplified. So, the touch voltage will be equal

or (in actual form)

, (6.4)

and the current is equal

(6.5)

In this case, the safety of the resistance of shoes, soil (floor) and other resistance in the human electrical circuit significantly increases.

A solid ground fault in a network with a solidly grounded neutral changes the phase voltage relative to ground little.

Here we also assume that Y 1 ,Y 2 And Y n small compared to Y 0 , i.e. are equal to zero.

Having made the appropriate transformations and taking into account that

,
And
,

we obtain the touch voltage in real form

To simplify this expression, let us assume that

As a result, we finally obtain that the voltage U etc equals

. (6.6)

The current passing through a person is determined by the formula

. (6.7)

Rice. 6.3. Human contact with a phase wire of a three-phase four-wire network with a grounded neutral during emergency mode: a - network diagram; b - vector diagram of voltages.

Let's consider two typical cases.

If the resistance of the wires to ground is r zm considered equal to zero, then equation (6.6) will take the form

Consequently, in this case the person will be under the influence of the linear voltage of the network.

>U etc >U f . (6.8)

Since from the resistance of the electrical circuit R Since the magnitude of the electric current passing through a person significantly depends, the severity of the injury is largely determined by the circuit of connecting the person to the circuit. The patterns of circuits formed when a person comes into contact with a conductor depend on the type of power supply system used.

The most common electrical networks are those in which the neutral wire is grounded, i.e., short-circuited by a conductor to the ground. Touching the neutral wire poses virtually no danger to humans; only the phase wire is dangerous. However, it is difficult to figure out which of the two wires is neutral - they look the same. You can figure it out using a special device - a phase detector.

Using specific examples, we will consider possible schemes for connecting a person to an electrical circuit when touching conductors.

Two-phase connection to the circuit. The rarest, but also the most dangerous, is a person touching two phase wires or current conductors connected to them (Fig. 2.29).

In this case, the person will be under the influence of line voltage. Current will flow through the person along the “hand-to-hand” path, i.e. the resistance of the circuit will include only the resistance of the body (D,).

If we take a body resistance of 1 kOhm, and an electrical network with a voltage of 380/220 V, then the current strength passing through a person will be equal to

This is a deadly current. The severity of an electrical injury or even a person’s life will depend primarily on how quickly he frees himself from contact with the current conductor (breaks the electrical circuit), because the time of exposure in this case is decisive.

Much more often there are cases when a person comes into contact with a phase wire or part of a device with one hand, a device that is accidentally or intentionally electrically connected to it. The danger of electric shock in this case depends on the type of electrical network (with grounded or insulated neutral).

Single-phase connection to a circuit in a network with a grounded neutral(Fig. 2.30). In this case, the current passes through the person along the “arm-legs” or “arm-arm” path, and the person will be under phase voltage.

In the first case, the circuit resistance will be determined by the resistance of the human body (I_, shoes (R o 6), grounds (Rzh), on which a person stands, the neutral grounding resistance (RH), and current will flow through the person

Neutral resistance RH is small and can be neglected compared to other circuit resistances. To estimate the magnitude of the current flowing through a person, we will assume a network voltage of 380/220 V. If a person is wearing insulating dry shoes (leather, rubber), he is standing on a dry wooden floor, the circuit resistance will be large, and the current strength, according to Ohm’s law, will be small.

For example, floor resistance is 30 kOhm, leather shoes are 100 kOhm, human resistance is 1 kOhm. Current passing through a person

This current is close to the threshold perceptible current. The person will feel the flow of current, stop working, and eliminate the malfunction.

If a person stands on wet ground with damp shoes or bare feet, a current will pass through the body

This current can cause damage to the lungs and heart, and with prolonged exposure, death.

If a person stands on wet soil wearing dry and intact rubber boots, a current passes through the body

A person may not even feel the impact of such a current. However, even a small crack or puncture in the sole of a boot can dramatically reduce the resistance of the rubber sole and make work dangerous.

Before you start working with electrical devices (especially those that have not been in use for a long time), they must be carefully inspected for damage to the insulation. Electrical devices must be wiped free of dust and, if they are wet,- dry. Wet electrical devices must not be used! It is better to store electric tools, instruments, and equipment in plastic bags to prevent dust or moisture from getting into them. You have to wear shoes when working. If the reliability of an electrical device is in doubt, you need to be on the safe side.- place a dry wooden floor or rubber mat under your feet. You can use rubber gloves.

The second path of current flow occurs when a person’s second hand comes into contact with electrically conductive objects connected to the ground (the body of a grounded machine tool, a metal or reinforced concrete building structure, a wet wooden wall, a water pipe, a heating battery, etc.). In this case, the current flows along the path of least electrical resistance. These objects are practically short-circuited to the ground, their electrical resistance is very small. Therefore, the resistance of the circuit is equal to the resistance of the body and current will flow through the person

This amount of current is deadly.

When working with electrical devices, do not use your other hand to touch objects that may be electrically connected to ground. Working in damp areas, in the presence of highly conductive objects connected to the ground near a person, poses an extremely high danger and requires compliance with increased electrical safety measures.

In emergency mode (Fig. 2.30, b), when one of the phases of the network (another phase of the network, different from the phase touched by a person) is shorted to ground, voltage redistribution occurs, and the voltage of the healthy phases differs from the phase voltage of the network. When touching a working phase, a person comes under voltage, which is greater than the phase voltage, but less than the linear one. Therefore, regardless of the path of current flow, this case is more dangerous.

Single-phase connection to a circuit in a network with an isolated neutral(Fig. 2.31). In production, three-wire electrical networks with an insulated neutral are used to supply power to power electrical installations. In such networks there is no fourth grounded neutral wire, and there are only three phase wires. In this diagram, rectangles conventionally show electrical resistance r A, r V, r With insulation of wires of each phase and capacitance S A, S v, S s each phase relative____________________

being under significantly higher voltages, and therefore more dangerous. However, the main conclusions and recommendations for ensuring safety are almost the same.

Even if we do not take into account the resistance of the human circuit (the person is standing on wet ground in damp shoes), the current passing through the person will be safe:

Thus, good phase insulation is the key to safety. However, with extensive electrical networks, this is not easy to achieve. In long and branched networks with a large number of consumers, the insulation resistance is low, and the danger increases.

For long electrical networks, especially cable lines, phase capacitance cannot be neglected (CV0). Even with very good phase insulation (r = oo), the current will flow through a person through the capacitance of the phases, and its value will be determined by the formula:

Thus, long electrical circuits of industrial enterprises with high capacitance are highly dangerous, even with good phase insulation.

If the insulation of any phase is broken, touching a network with an isolated neutral becomes more dangerous than touching a network with a grounded neutral wire. In emergency mode (Fig. 2.31, b) the current passing through a person who has touched the serviceable phase will flow through the ground fault circuit to the emergency phase, and its value will be determined by the formula:

Since the closure resistance D, the emergency phase on earth, is usually small, the person will be under linear voltage, and the resistance of the resulting circuit will be equal to the resistance of the person’s circuit ____, which is very dangerous.

For these reasons, as well as because of ease of use (the ability to obtain voltages of 220 and 380 V), four-wire networks with a grounded neutral wire for a voltage of 380/220 V have become most widespread.

We have not considered all possible electrical network diagrams and touch options. In production, you may be dealing with more complex power supply circuits, especially ground circuits.

To simplify the analysis, let us assume g A - g c= g c = g, A S A= L B= C c = C

If a person touches one of the wires or any object electrically connected to it, current will flow through the person, the shoe, the base, and through the insulation and capacitance of the wires to the other two wires. Thus, a closed electrical circuit is formed, in which, unlike the previously considered cases, the phase insulation resistance is included. Since the electrical resistance of good insulation is tens and hundreds of kilo-ohms, the total electrical resistance of the circuit is much greater than the resistance of the circuit formed in a network with a grounded neutral wire. That is, the current through a person in such a network will be less, and touching one of the phases of the network with an isolated neutral is safer.

The current through a person in this case is determined by the following formula:

where is the electrical resistance of the human circuit,

co = 2nd - circular frequency of the current, rad/s (for industrial frequency current = 50 Hz, therefore co = YuOl).

If the phase capacitance is small (this is the case for short air networks), we can take C « 0. Then the expression for the amount of current through a person will take the form:

For example, if the floor resistance is 30 kOhm, leather shoes are 100 kOhm, the human resistance is 1 kOhm, and the phase insulation resistance is 300 kOhm, the current that passes through the person (for a 380/220 V network) will be equal to

A person may not even feel such a current.

Control questions

1. What types of electrical networks are most common in production?

2. Name the sources of electrical hazards at work.

3. What is touch voltage and step voltage? How do their values depend on the distance from the point where the current flows into the ground?

4. How are premises classified according to the degree of electrical hazard?

5. How does electric current affect a person? List and describe the types of electrical injuries.

6. What parameters of electric current determine the severity of electric shock? Specify current thresholds.

7. Which path of electric current flow through the human body is most dangerous?

8. Indicate the sources of the greatest electrical danger in production related to your future profession.

9. Do a hazard analysis of electrical networks with a grounded neutral.

10. Give an analysis of the dangers of electrical networks with an isolated neutral.

11.Which touching of live conductors is most dangerous for a person?

12. Why does touching objects electrically connected to the ground (for example, a water pipe) with your hand when working with electrical devices sharply increase the risk of electric shock?

13.Why do you need to remove the electrical plug from the socket when repairing electrical equipment?

14.Why do you need to wear shoes when working with electrical devices?

15.How can you reduce the risk of electric shock?

Since the magnitude of the electric current passing through a person significantly depends on the resistance of the electrical circuit R, the severity of the injury is largely determined by the way the person is connected to the circuit. The patterns of circuits formed when a person comes into contact with a conductor depend on the type of power supply system used.

Using specific examples, we will consider possible schemes for connecting a person to an electrical circuit when touching conductors.

Two-phase connection to an electrical circuit

The rarest, but also the most dangerous, is a person touching two phase wires or current conductors connected to them (Fig. 1).

In this case, the person will be under the influence of line voltage. Current will flow through the person along the “hand-to-hand” path, i.e. the circuit resistance will include only body resistance ()

If we assume a body resistance of 1 kOhm, and an electrical network with a voltage of 380-220 V, then the current strength passing through a person will be equal to

Much more common are cases when a person comes into contact with a phase wire or part of a device with one hand: a device that is accidentally or intentionally electrically connected to it. The danger of electric shock in this case depends on the type of electrical network (with grounded or insulated neutral).