Analysis of the risk of electric shock in various networks. Analysis of schemes for connecting a person to an electrical circuit Analysis of the danger of electric shock in electrical networks

The passage of current through a person is a consequence of his touching at least two points of the electrical circuit, between which there is a certain potential difference (voltage).

The danger of such touching is ambiguous and depends on a number of factors:

circuits for connecting a person to an electrical circuit;

mains voltage;

diagrams of the network itself;

network neutral mode;

the degree of isolation of live parts from the ground;

capacitance of live parts relative to the ground.

Classification of networks with voltages up to 1000 V

Single-phase networks

Single-phase networks will be divided into two-wire and single-wire.

Two-wire

Two-wire networks are divided into isolated from ground and with a grounded wire.

Isolated from the ground

With grounded wire

These networks are widely used in the national economy, from low voltage power supply of portable tools to power supply of powerful single-phase consumers.

Single-wire

In the case of a single-wire network, the role of the second wire is played by the ground, rail, etc.

Single-phase network. Single wire

These networks are mainly used in electrified transport (electric locomotives, trams, metro, etc.).

Three-phase networks

Depending on the neutral mode of the current source and the presence of a neutral or neutral conductor, they can be performed according to four schemes.

Neutral point of the current source - a point at which the voltages relative to all phases are the same in absolute value.

Zero point of the current source - grounded neutral point.

A conductor connected to a neutral point is called a neutral conductor (neutral), and to a zero point is called a neutral conductor.

1. Three-wire network with isolated neutral

2. Three-wire sit with grounded neutral

3. Four-wire network with isolated neutral

4. Four-wire network with grounded neutral

At voltages up to 1000V in our country, circuits "1" and "4" are used.

Schemes for including a person in an electrical circuit

Two-phase touch - between two phases of the electrical network. As a rule, the most dangerous because there is a line voltage. However, these cases are quite rare.

Single phase touch - between phase and ground. In this case, it is assumed that there is an electrical connection between the network and the ground.

For more details on the schemes for including a person in a circuit, see P.A. Dolin. Fundamentals of safety in electrical installations.

Single-phase networks

Isolated from the ground

Normal mode

The better the insulation of the wires to ground, the less the danger of single-phase touching the wire.
It is more dangerous for a person to touch a wire with a high electrical insulation resistance.

Emergency mode

When the wire is shorted to ground, a person who touches a serviceable wire is under a voltage equal to almost the full voltage of the line, regardless of the insulation resistance of the wires.

With grounded wire

Touching an ungrounded wire

In this case, the person finds himself under almost full voltage from the network.

Touching a grounded wire

Under normal conditions, touching a grounded wire is practically harmless.

Touching a grounded wire. Emergency operation

In the event of a short circuit, the voltage on a grounded conductor can reach dangerous values.

Three-phase networks

With isolated neutral

Normal mode

The danger of touching is determined by the electrical impedance of the wires to ground, with increasing resistance, the risk of touching decreases.

Emergency mode

The contact voltage is practically equal to the line voltage of the mains. The most dangerous case.

With earthed neutral

Normal mode

In this case, a person is practically under the phase voltage of the network.

Emergency mode

The magnitude of the touch voltage lies between the line-to-line and phase-to-neutral voltages, depending on the ratio between the earth fault resistance and the earth resistance.

Electrical safety measures

Elimination of human contact with live parts.
It is released through the location of live parts in inaccessible places (at a height, in cable ducts, ducts, pipes, etc.)

Use of low voltages (12, 24, 36 V).
For example, for powering hand tools in rooms with increased risk of electric shock.

The use of double insulation.
For example, the implementation of the body of an electrical installation from a dielectric.

The use of personal protective equipment.
Before using PPE, it is imperative to make sure that they are in good working order, integrity, and also check the timing of the previous and subsequent verification of the instrument.

Basic protective equipment provide immediate protection against electric shock.
Additional protective equipment cannot provide security on their own, but can help with the use of fixed assets.

Insulation control of equipment and networks.
- Output control.
- Planned.
- Extraordinary, etc.

Protective network separation.
Allows to reduce the capacity of lines near consumers of electrical energy.

Protective grounding is a deliberate electrical connection of metal non-current-carrying parts that may be energized to the ground or its equivalent (popularly about grounding at geektimes.ru).

In networks up to 1000 V, protective grounding is used in networks with isolated neutral.
The principle of operation is to reduce the touch voltage to a safe value.

When grounding is not possible, in order to protect, the potential of the foundation on which the person and equipment stands are equalized by raising. For example, the connection of the repair basket with the phase conductor of the power transmission line.

Earthing switches are divided into:
a. Artificial, intended for direct grounding purposes.
b. Natural metal objects in the ground for other purposes, which can be used as grounding conductors. Exceptions for the criterion of explosion and fire hazard (gas pipelines, etc.).

The grounding resistance should be no more than a few ohms. In this case, over time, as a result of corrosion, the resistance of the ground electrode increases. Therefore, its value must be periodically monitored (winter / summer).

Protective grounding is a deliberate connection of metal non-current-carrying parts that may be energized with a multi-grounded neutral protective conductor.

Scope - electrical installations with grounded neutral with voltages up to 1000V.

The principle of operation is the transformation of a short circuit to the equipment case into a single-phase short circuit, with the subsequent shutdown of the equipment when the maximum permissible current is exceeded.

Overcurrent protection is realized either by means of circuit breakers or fuses. Particular attention must be paid to the choice of the thickness of the neutral protective conductor, sufficient to carry the short-circuit current.

The use of RCDs (residual current devices).

This type of protection is triggered when the incoming and outgoing currents in the monitored circuit do not match in magnitude, i.e. when there is a leakage current. For example, when a person touches a phase conductor, part of the current goes past the main circuit to the ground, which causes a power outage of the equipment in the controlled circuit. More details,.

Analysis of the risk of electric shock in various networks

The defeat of a person with an electric current is possible only with his direct contact with the points of the electrical installation, between which there is voltage, or with a point whose potential is different from the potential of the earth. The analysis of the danger of such a touch, assessed by the value of the current passing through a person or by the touch voltage, depends on a number of factors: the scheme for connecting a person to the power grid, its voltage, neutral mode, isolation of live parts, their capacitive component, etc.

When studying the causes of electric shock, it is necessary to distinguish between direct contact with live parts of electrical installations and indirect. The first, as a rule, occurs in case of gross violations of the rules for the operation of electrical installations (PTE and PTB), the second - as a result of emergency situations, for example, during insulation breakdown.

Schemes for including a person in an electrical circuit can be different. However, the most common are two: between two different wires - a two-phase connection and between one wire or the body of an electrical installation, one phase of which is broken, and the ground - a single-phase connection.

Statistics show that the greatest number of electrical injuries occurs with single-phase switching, and most of them are in networks with a voltage of 380/220 V. Two-phase switching is more dangerous, since in this case a person is under line voltage, while the current strength passing through a person is will be (in A)

where Ul is the line voltage, i.e. voltage between phase wires, V; Uf - phase voltage, i.e. voltage between the beginning and the end of one winding (or between phase and neutral wire), V.

As can be seen from Fig. 8.1, the danger of two-phase connection does not depend on the neutral mode. Neutral is the point of connection of the windings of a transformer or generator, not connected to the grounding device or connected to it through devices with high resistance (network with isolated neutral), or directly connected to the grounding device - network with a solidly grounded neutral.

With a two-phase switch on, the current passing through the human body will not decrease when the person is isolated from the ground using dielectric galoshes, boats, rugs, floors.

With a single-phase connection of a person to the network, the current strength is largely determined by the neutral mode. For the case under consideration, the current passing through a person will be (in A)

, (8.3)

where w is the frequency; C - phase capacitance relative to ground

Fig. 8.1. Connecting a person to a three-phase network with an isolated neutral:
a - two-phase inclusion; b - single-phase inclusion; Ra, Rt, Rc - electrical resistance of phase insulation relative to ground. Ohm; Ca, Cb, Cc - the capacitance of the wires relative to the ground, F, Ia, Ib, IC currents flowing to the ground through the phase insulation resistance (leakage currents)

To simplify the formula, it is assumed that Ra \u003d Rb \u003d Rc \u003d Rfrom, and Ca \u003d Cb \u003d Cc \u003d C.

Under industrial conditions, the insulation of the phases, made of dielectric materials and having a finite value, in the process of aging, humidification, dust coating changes for each phase unequally. Therefore, the calculation of safe conditions, which is greatly complicated, must be carried out taking into account the real values \u200b\u200bof resistance R and capacitances C for each phase. If the capacitance of the phases relative to the ground is small, i.e. Ca \u003d Cb \u003d Cc \u003d 0 (for example, in air networks of short length), then

Ich \u003d Uph / (Rch + Riz / 3), (8.4)

If the capacitance is large (Ca \u003d Cb \u003d Cc is not equal to 0) and Rf is large (for example, in cable lines), then the strength of the current flowing through the human body will be determined only by the capacitive component:

, (8.5)

where Хс \u003d 1 / wС - capacitive resistance, Ohm.

From the above expressions it can be seen that in networks with an isolated neutral, the risk of shock to a person is less, the less the capacitive and the higher the active component of the phase wires relative to the ground. Therefore, in such networks, it is very important to constantly monitor R from to identify and eliminate damage.

Fig. 8.2. Inclusion of a person in a three-phase network with an isolated neutral in emergency mode. Explanations in the text

If the capacitive component is large, then the high insulation resistance of the phases does not provide the necessary protection.

In the event of an emergency (Fig. 8.2), when one of the phases is closed to ground, the current passing through a person will be equal to (in A)

If we assume that Rsm \u003d 0 or Rsm<< Rч (что бывает в реальных аварийных условиях), то, исходя из приведенного выражения, человек окажется под линейным напряжением, т. е. попадет под двухфазное включение. Однако чаще всего R3M не равно 0, поэтому человек будет находиться под напряжением, меньшим Uл, но большим Uф, при условии, что Rиз/3 >\u003e Rzm.

A ground fault also significantly changes the voltage of the live parts of the electrical installation relative to the ground and the grounded structures of the building. A ground fault is always accompanied by current spreading in the ground, which, in turn, leads to the emergence of a new type of human injury, namely, contact voltage and step voltage. Such closure can be accidental or deliberate. In the latter case, the conductor in contact with the ground is called a ground electrode or electrode.

In the volume of the earth, where the current flows, a so-called "" "field (zone) of current spreading" appears. Theoretically, it extends to infinity, however, in real conditions, already at a distance of 20 m from the ground electrode, the spreading current density and potential are practically zero.

The nature of the potential spreading curve significantly depends on the shape of the ground electrode system. So, for a single hemispherical ground electrode, the potential on the earth's surface will change according to the hyperbolic law (Fig. 8.3).

Fig. 8.3. Distribution of potential on the earth's surface around the hemispherical ground electrode (f - change in the potential of the ground electrode on the earth's surface; fz - the maximum potential of the ground electrode at the current strength of the earth fault I3; r - radius of the ground electrode)

Fig. 8.4. Contact voltage with a single earthing switch (f3 is the total soil resistance to current spreading from the earthing switch):
1 - potential curve; 2 - curve characterizing the change in Upr with distance from the ground electrode; 3 - phase breakdown to the case

Depending on the location of the person in the spreading zone and his contact with the electrical installation b, the body of which is grounded and energized, the person can get under the touch voltage Upr (Fig. 8.4), which is defined as the potential difference between the point of the electrical installation, which is touched by the person f3, and the point of the ground on which it stands - phosn (in B)

Upr \u003d f3 - fosn \u003d f3 (1 - fosn / f3), (8.7)

where the expression (1 - phosn / f3) \u003d a1 is the touch voltage coefficient, which characterizes the shape of the potential curve.

Fig. 8.4 it can be seen that the touch voltage will be maximum when a person is removed from the ground electrode by 20 m or more (electrical installation c) and is numerically equal to the potential of the ground electrode Upr \u003d f3, while a1 \u003d I. If a person stands directly above the ground electrode (electrical installation a), then Unp \u003d 0 and a1 \u003d 0. This is the safest case.

Expression (8.7) allows you to calculate Unp without taking into account additional resistances in the human-ground circuit, i.e., without taking into account the resistance of the shoes, the resistance of the supporting surface of the legs and the resistance of the floor. All this is taken into account by the coefficient a2, therefore, in real conditions, the magnitude of the touch voltage will be even less.

The defeat of a person by electric current as a result of electric impact, i.e., the passage of current through a person, is a consequence of his touching 2 points of the electrical circuit, between which there is some voltage. The danger of such a touch is assessed, as is known, by the current passing through the human body or by the voltage under which he finds himself. It should be noted that the touch voltage depends on a number of factors: the circuit for connecting a person to the electrical circuit, the voltage of the network, the circuit of the network itself, the mode of its neutral, the degree of isolation of the live parts from the ground, as well as the capacitance of the live parts relative to the ground, etc.

Consequently, the above danger is not unambiguous: in one case, the inclusion of a person in an electrical circuit will be accompanied by the passage of small currents through it and will not be very dangerous, in other cases the currents can reach significant values \u200b\u200bthat can lead to death. This article examines the dependence of the danger of including a person in an electrical circuit, that is, the values \u200b\u200bof the touch voltage and current flowing through a person, on the listed factors.

This dependence must be known when assessing a particular network according to safety regulations, choosing and calculating appropriate protection measures, in particular grounding, neutralization, protective shutdown, network isolation monitoring devices, etc.

In this case, in all cases, except for those specifically mentioned, we will assume that the resistance of the foundation on which the person stands (ground, floor, etc.), as well as the resistance of his shoes, are insignificant and therefore they can be taken equal to zero.

So, the most typical schemes for connecting a person to an electrical circuit when accidentally touching live conductors are:

1. Connection between two phase conductors of the circuit,

2. Connection between phase and earth.

Of course, in the second option, it is assumed that the considered network is electrically connected to the ground due to, for example, grounding the neutral of the current source or due to poor insulation of the wires relative to the ground, or due to the presence of a large capacitance between them.

Two-phase touch is considered the most dangerous, since in this case a line voltage of 380 volts is applied to the human body, and the current passing through the body does not depend on the network circuit and its neutral mode.

Two-phase touching occurs very rarely and is mainly associated with working under voltage:

On electrical panels, assemblies and overhead lines;

When using faulty personal protective equipment;

On equipment with unshielded live parts, etc.

Single-phase contact is usually considered less dangerous, since the current passing through the person in this case is limited by the influence of a number of factors. But it happens in practice much more often than biphasic. Therefore, the topic of this article is to analyze only cases of single-phase contact in the networks under consideration.

In case of electric shock to a person it is necessary to take measures to free the victim from the current and immediately begin to provide him with first aid.

Free a person from the action of the current it is necessary as soon as possible, but precautions must be taken. If the victim is at a height, measures should be taken to prevent him from falling.

Touching an energized person, it is dangerous, and when conducting rescue operations, it is necessary to strictly observe certain precautions against possible electric shock to persons carrying out these operations.

The easiest way to release the victim from the current is disconnection of an electrical installation or that part of it that a person touches... When the unit is turned off, the electric light may go out, therefore, in the absence of daylight, it is necessary to have another light source ready - a lantern, a candle, etc.

After releasing the victim from the currentit is necessary to establish the degree of damage and, in accordance with the condition of the victim, provide him with medical assistance. If the victim has not lost consciousness, it is necessary to provide him with rest, and in the presence of injuries or injuries (bruises, fractures, dislocations, burns, etc.), he must be given first aid before the arrival of a doctor or taken to the nearest medical institution.

If the victim has lost consciousness, but breathing is preserved, it is necessary to lay him evenly and comfortably on a soft bedding - a blanket, clothes, etc., unfasten the collar, belt, remove embarrassing clothing, cleanse the mouth of blood, mucus, provide fresh air, Allow to smell ammonia, sprinkle with water, grind and warm the body.

In the absence of signs of life (in case of clinical death, there is no breathing and pulse, the pupils of the eyes are dilated due to oxygen starvation of the cerebral cortex) or in case of intermittent breathing, the victim should be quickly released from the clothing that restrains breathing, cleansed the mouth and do artificial respiration and heart massage.

The analysis of the risk of injury is practically reduced to determining the value of the current flowing through the human body in various conditions in which it may find itself during the operation of electrical installations, or the touch voltage. The danger of injury depends on a number of factors: the scheme for connecting a person to an electrical circuit, the voltage of the network, the circuit of the network itself, the mode of its neutral, the degree of isolation of live parts from the ground, the capacity of live parts relative to the ground, etc.

What are the schemes for connecting a person to an electrical circuit?

The most typical are two connection schemes: between two phases of the electrical network, between one phase and ground. In addition, it is possible to touch grounded non-current-carrying parts that are energized, as well as to turn on a person under a step voltage.

What is called the neutral of a transformer (generator) and what are the modes of its operation?

The connection point of the windings of the supply transformer (generator) is called the neutral point, or neutral. The neutral of the power supply can be isolated and earthed.

Grounded is the neutral of the generator (transformer), connected to the grounding device directly or through a low resistance (for example, through current transformers).

An isolated neutral is the neutral of a generator or transformer that is not connected to the grounding device or connected to it through a large resistance (signaling devices, measurements, protection, grounding arc suppression reactors).

What is the basis for choosing a neutral mode?

The choice of the network diagram, and therefore the neutral mode of the current source, is made based on the technological requirements and safety conditions.

At voltages up to 1000 V, both three-phase network schemes are widely used: three-wire with insulated neutral and four-wire with grounded neutral.

According to technological requirements, preference is often given to a four-wire network, it uses two operating voltages - line and phase. So, from a four-wire 380 V network, you can supply both a power load - three-phase, including it between the phase wires for a line voltage of 380 V, and lighting, including it between the phase and neutral wires, that is, for a phase voltage of 220 V. the electrical installation becomes much cheaper due to the use of a smaller number of transformers, a smaller cross-section of wires, etc.

According to safety conditions, one of the two networks is selected based on the position: according to the conditions of touching the phase wire during the normal operation of the network, the network with an isolated neutral is safer, and in the emergency period - the network with a grounded neutral. Therefore, it is advisable to use networks with an isolated neutral when it is possible to maintain a high level of network isolation and when the capacity of the network relative to the ground is insignificant. These can be sparsely branched networks that are not exposed to an aggressive environment and are under constant supervision of qualified personnel. An example is the networks of small businesses, mobile installations.

Networks with a grounded neutral are used where it is impossible to ensure good insulation of electrical installations (due to high humidity, aggressive environment, etc.) or it is impossible to quickly find and eliminate insulation damage when the capacitive currents of the network, due to its significant branching, reach high values \u200b\u200bthat are dangerous to life person. Such networks include networks of large industrial enterprises, urban distribution networks, etc.

The existing opinion about a higher degree of reliability of networks with isolated neutral is not sufficiently substantiated.

Statistical data indicate that in terms of reliability of operation, both networks are practically the same.

At voltages above 1000 V up to 35 kV, networks for technological reasons have an isolated neutral, and above 35 kV - grounded.

Since such networks have a large wire capacitance relative to the ground, it is equally dangerous for a person to touch a network wire with both an isolated and a grounded neutral. Therefore, the mains neutral mode above 1000 V is not selectable for safety reasons.

What is the danger of two-phase touching?

By two-phase touching is meant the simultaneous touching of two phases of an electrical installation under voltage (Fig. 1).

Fig. 1. Scheme of two-phase human touch to an alternating current network

Two-phase touching is more dangerous. With a two-phase touch, the current passing through the human body along one of the most dangerous paths for the body (hand-hand) will depend on the voltage applied to the human body, equal to the line voltage of the network, as well as on the resistance of the human body:

• U l - line voltage, i.e. the voltage between the phase conductors of the network;
• R people - the resistance of the human body.

In a network with a line voltage U l \u003d 380 V with a resistance of the human body R people \u003d 1000 Ohm, the current passing through the human body will be equal to:

This current is deadly for humans. With a two-phase touch, the current passing through the human body is practically independent of the neutral mode of the network. Therefore, two-phase contact is equally dangerous both in networks with isolated and grounded neutral (provided that the line voltages of these networks are equal).

Cases of a person touching two phases are relatively rare.

What is the characteristic of single-phase touch?

Single-phase touch is called touching one phase of an electrical installation that is energized.

It occurs many times more often than two-phase touch, but less dangerous, since the voltage under which a person finds himself does not exceed the phase one. Accordingly, the current passing through the human body turns out to be less. In addition, this current is greatly influenced by the neutral mode of the current source, the insulation resistance of the network wires relative to the ground, the resistance of the floor (or base) on which the person stands, the resistance of his shoes and some other factors.

What is the danger of single-phase contact in a network with earthed neutral?

Fig. 2. Scheme of a person's touch to one phase of a three-phase network with a grounded neutral

In a network with a grounded neutral (Fig. 2), the current circuit passing through the human body includes the resistances of the human body, his shoes, the floor (or base) on which the person stands, as well as the grounding resistance of the neutral of the current source. Taking into account the indicated resistances, the current passing through the human body is determined from the following expression:

• U f - phase voltage of the network, V;
• R people - resistance of the human body, Ohm;
• R about - resistance of human shoes, Ohm;
• R p - resistance of the floor (base) on which the person stands, Ohm;
• R o - grounding resistance of the neutral of the current source, Ohm.

Under the most unfavorable conditions (a person who touches the phase has conductive shoes on his feet - damp or lined with metal nails, stands on damp ground or on a conductive base - a metal floor, on a grounded metal structure), i.e. when R ob \u003d 0 and R p \u003d 0, the equation takes the form:

Since the neutral resistance R o is usually many times less than the resistance of the human body, it can be neglected. Then

However, under these conditions, single-phase contact, despite the lower current, is very dangerous. So, in a network with a phase voltage U f \u003d 220 V at R people \u003d 1000 Ohm, the current passing through the human body will have a value:

Such a current is deadly to humans.

If the person has non-conductive shoes on their feet (for example, rubber overshoes) and stands on an insulating base (for example, on a wooden floor), then

• 45,000 - resistance of human footwear, Ohm;
• 100,000 - floor resistance, Ohm.

A current of such strength is not dangerous to humans.

It can be seen from the above data that insulating floors and non-conductive footwear are of great importance for the safety of those working in electrical installations.

What are the features of single-phase contact in an isolated neutral network?

In a network with an insulated neutral (Fig. 3), the current passing through the human body to the ground returns to the current source through the insulation of the wires of the network, which in good condition has a high resistance.

Taking into account the resistance of the shoe R about and the floor or base R p on which the person stands, included in series with the resistance of the human body R people, the current passing through the human body is determined by the equation:

where R from is the insulation resistance of one phase of the network relative to the ground, Ohm.

Fig. 3. Diagram of a person's touch to one phase of a three-phase network with an isolated neutral

In the most unfavorable case, when a person has a conducting shoe and stands on a conductive floor, that is, when R about \u003d 0 and R p \u003d 0, the equation will be much simpler:

For this case, in a network with a phase voltage U f \u003d 220 V and an insulation resistance of the phase R of \u003d 90,000 Ohms at R people \u003d 1000 Ohms, the current passing through a person will be equal to:

This current is significantly less than the current (220 mA) calculated by us for the case of single-phase contact under similar conditions, but in a network with a grounded neutral. It is determined mainly by the insulation resistance of the wires to ground.

Which network is safer - isolated or grounded neutral?

All other things being equal, human touch to one phase of a network with an isolated neutral is less dangerous than in a network with a grounded neutral. However, this conclusion is valid only for normal (trouble-free) operating conditions of networks, in the presence of insignificant capacity relative to the ground.

In the event of an accident, when one of the phases is shorted to ground, a network with an isolated neutral may be more dangerous. This is explained by the fact that with such an accident in a network with an isolated neutral, the voltage of the undamaged phase relative to earth may increase from phase to linear, while in a network with a grounded neutral the voltage increase will be insignificant.

However, modern electrical networks, due to their ramification and considerable length, create a large capacitive conductance between the phase and the ground. In this case, the danger of a person touching one and two phases is practically the same. Each of these touches is very dangerous, since the current passing through the human body reaches very high values.

What is stride voltage?

The step voltage is understood as the voltage between two points of the current circuit, located at a step distance from one another, on which a person stands at the same time. The step size is usually taken equal to 0.8 m.

For some animals (horses, cows), the step voltage is greater than for humans, and the current path engulfs the chest. For these reasons, they are more susceptible to step voltage lesions.

Step voltage arises around the place where the current passes from the damaged electrical installation to the ground. The largest value will be near the transition point, and the smallest - at a distance of more than 20 m, that is, outside the limits limiting the field of current spreading in the soil.

At a distance of 1 m from the earthing switch, the voltage drop is 68% of the total voltage, at a distance of 10 m - 92%, at a distance of 20 m the potentials of the points are so small that they can practically be equal to zero.

Such points of the soil surface are considered to be outside the current spreading zone and are called "ground".

The danger of striding stress increases if the exposed person falls. And then the tension of the step increases, since the path of the current no longer passes through the legs, but through the whole body.

Cases of injury to people due to stride stress are relatively rare. They can occur, for example, near a wire that has fallen to the ground (at such moments, before the line is disconnected, people and animals must not be allowed close to the place where the wire fell). The most dangerous are step voltages when struck by lightning.

Once in the step voltage zone, you should leave it in small steps in the direction opposite to the place of the alleged ground fault, and in particular the wire lying on the ground.

Since the resistance of the electrical circuit Rthe magnitude of the electric current passing through a person depends significantly, the severity of the lesion is largely determined by the scheme for including the person in the circuit. The circuits formed when a person contacts a conductor of circuits depend on the type of power supply system used.

The most common electrical networks in which the neutral wire is grounded, that is, it is short-circuited by a conductor to the ground. Touching the neutral wire practically does not pose a danger to humans, only the phase wire is dangerous. However, it is difficult to figure out which of the two wires is zero - they look the same in appearance. 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 the touch of a person to 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. A current will flow through the person along the "hand-hand" path, that is, the resistance of the circuit will include only the resistance of the body (D,).

If we take the body resistance of 1 kOhm, and the electrical network with a voltage of 380/220 V, then the strength of the current 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 gets rid of contact with the current conductor (breaks the electrical circuit), because the exposure time in this case is decisive.

Much more often there are cases when a person comes into contact with one hand with a phase wire or part of a device, an apparatus that is accidentally or intentionally electrically connected to it. The danger of electric shock in this case depends on the type of electrical network (grounded or isolated 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 "hand-to-foot" or "hand-to-hand" path, and the person will be under phase voltage.

In the first case, the resistance of the circuit will be determined by the resistance of the human body (I_,shoe (R o 6),foundations (R w),on which a person stands, with a neutral grounding resistance (R H),and a current will flow through the person

Neutral resistance R His small and negligible compared to other circuit resistances. To assess the magnitude of the current flowing through a person, we take the mains voltage 380/220 V. If a person is wearing insulating dry shoes (leather, rubber), he stands on a dry wooden floor, the resistance of the circuit will be large, and the current strength according to Ohm's law is small.

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

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

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

This current can cause disruption in the functioning of the lungs and heart, and with prolonged exposure, death.

If a person stands on wet soil in dry and intact rubber boots, a current flows 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 drastically reduce the resistance of the rubber sole and make work dangerous.

Before you start working with electrical devices (especially those that are not in operation for a long time), they must be carefully inspected for damage to the insulation. Electrical devices must be cleaned of dust and, if wet,- dry. Do not operate wet electrical devices! It is better to store electrical tools, appliances, equipment in plastic bags to prevent dust or moisture from entering them. You have to work in shoes. If the reliability of an electrical device is in doubt, you need to play it safe.- place a dry wooden floor or rubber mat under your feet. Rubber gloves can be used.

The second path of current flow occurs when a person comes into contact with the other hand with electrically conductive objects connected to the ground (the body of a grounded machine tool, a metal or reinforced concrete structure of a building, a damp 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 low. Therefore, the resistance of the circuit is equal to the resistance of the body and a current will flow through the person

This amount of current is deadly.

When working with electrical devices, do not touch objects with your other hand that may be electrically connected to the ground. Working in damp rooms, in the presence of well-conductive objects connected to the ground near a person, presents 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 that the person touched) turned out to be closed to the ground, a redistribution of voltage occurs, and the voltage of the serviceable phases differs from the phase voltage of the network. Touching a working phase, a person gets under a voltage that is more than phase, but less than linear. Therefore, for any 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 insulated neutral are used to supply power to 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 resistances r AND,r in, r frominsulation of the wire of each phase and capacity C A, C c, C crefer to each phase __________________________

under much higher voltages, and therefore more dangerous. However, the main conclusions and recommendations for ensuring safety are practically the same.

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

Thus, good phase isolation is the key to ensuring safety. However, with extensive electrical networks, this is not easy to achieve. In extended 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, the phase capacitance cannot be neglected (CV0). Even with very good phase isolation (r \u003d oo), the current will flow through the person through the capacitive resistance 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 isolation.

If the insulation of any phase is violated, touching a network with an insulated neutral becomes more dangerous than a network with a grounded neutral wire. In emergency operating mode (Fig. 2.31, b)the current passing through a person who touches the good phase will flow through the earth fault circuit to the emergency phase, and its value will be determined by the formula:

Since the resistance of the short circuit D, the emergency phase on the ground is usually small, the person will be under line voltage, and the resistance of the formed circuit will be equal to the resistance of the person's circuit ____, which is very dangerous.

For these reasons, as well as because of the convenience 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 are most widespread.

We have considered far from all possible electrical circuits and touch options. In production, you can deal with more complex power supply circuits, especially earth.

To simplify the analysis, we take g A - g c= r c \u003d r,and C A= L B\u003d C c \u003d C

If a person touches one of the wires or any object that is electrically connected to it, the current will flow through the person, shoes, base and through the insulation and capacitance of the wires will flow to the other two wires. Thus, a closed electric circuit is formed, in which, in contrast to the previously considered cases, the insulation resistance of the phases 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 higher than the resistance of the circuit formed in the 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,

ω \u003d 2π - the circular frequency of the current, rad / s (for a current of industrial frequency \u003d 50 Hz, therefore ω \u003d 10Ol).

If the capacitance of the phases is small (this is the case for non-extended overhead networks), we can take C «0. Then the expression for the magnitude of the 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 insulation resistance of the phases is 300 kOhm, the current that passes through a person (for a 380/220 V network) will be equal to

A person may not even feel such a current.

test 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 \u200b\u200bdepend 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 the current thresholds.

7. What is the most dangerous path of electric current flowing through the human body?

8. Indicate the sources of the greatest electrical hazards in the workplace associated with your future profession.

9. Conduct a hazard analysis for grounded neutral electrical networks.

10. Give a hazard analysis of electrical networks with isolated neutral.

11. What touching live conductors is the most dangerous for a person?

12.Why touching an object that is electrically connected to the ground (such as a water pipe) with your hand while working with electrical devices will dramatically increase the risk of electric shock?

13.Why do I need to unplug the power plug when repairing electrical equipment?

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

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