The connection point of the power supply transformer (generator) is called a neutral point or neutral.Power supply neutral can be isolated and grounded. Groundedit is called the generator neutral (transformer) attached to the grounding device directly or through a small resistance (for example, through current transformers). Isolatedthe neutral of the generator or a transformer is called, which is not attached to the grounding device or attached to it through a large resistance (alarm, measurement devices, protection, ground-grinding reactors).

Manifesting a person with electric shock occurs when the electrical circuit is closed through the human body. This occurs in the case of a man's touch at least two points of the electrical circuit, between which there is some voltage. The inclusion of a person in a chain can occur in several schemes: between wire and ground, called single-phase inclusion; between two wires - two-phase inclusion .

Single-phase inclusionit is a direct contact of a person with parts of electrical installation or equipment, normally or randomly located under voltage. With single-phase inclusion in the network with an isolated and grounded neutral, a person falls under a phase voltage, which is 1.73 times less than linear, and exposed to current, which depends on the phase voltage of the installation, resistance of the telechel, shoes, gender, neutral grounding, isolation.

For single-phase inclusion three-phase four-wire network with grounded neutralthe strength of the current passing through the human body can be expressed as:

I h \u003d u f / (r h + r p + r o + r n) \u003d\u003e i h r h \u003d u f r h / (r h + r p + r o + r n)

where u f-phase voltage. IN; R h is pressed the human body, Ohm; R P is the resistance of the floor on which there is a person. Ohms; R O isolate shoes. Ohms; R n - resistance to grounding neutors. Ohms; U pr - tension tension, V.

As an example, two cases of one-phase inclusion of a person in a three-phase four-wire electrical circuit with a grounded neutral with a linear voltage are considered.

U f \u003d 380V; U l \u003d 220 V \u003d U F \u003d 1.73 U F

Case with unfavorable conditions.The person who touched one phase is on the cheese of soil or conductive (metallic) floor, its shoes are raw or has metal nails. In accordance with this, resistance are accepted: human body \u003d 1000 ohms; soil or floor R n \u003d 0; Shoes R O \u003d 0. The grounding resistance of the neutral R H \u003d 4 Ohms (calculated due to a minor value can be neglected).

Through the body of a person will pass deadly current:

I h \u003d u f / r h \u003d u l / (1.73 r h) \u003d 220/1000 \u003d 0.22 A \u003d 220 mA;

U PR \u003d U F \u003d 220 V.

Case with favorable conditions.The person is on a wooden dry floor resistance R n \u003d 100000 ohm, on his feet dry tok-cutting (rubber) shoes with resistance r o = \u003d 45000 ohms. Then through the body of a person will pass a threshold, a long-permissible current to a person:

I h \u003d 220 / (1000 + 100000 + 45000) \u003d 220/146000 \u003d 0.0015a \u003d 1.5th

U PR \u003d 220 * 1000/146000 \u003d 1.5V

These examples illustrate the value of the insulating properties of the floor and shoes to ensure the safety of persons working under possible electric shock contact.

Two-phase inclusion is the simultaneous touch of a person to two different phases of the same network under voltage. At the same time, the person turns out to be included on the full linear voltage of the installation. The strength of the current acting on a person depends on linear voltage andresistance to human body R. C. . With two-phase inclusion, the insulation resistance of the wires does not have a protective effect:

I h \u003d 1.73 U f / r h \u003d 380/1000 \u003d 0.38a \u003d 380ma u pr \u003d i h r h \u003d 380 V

This value of the current (voltage) is fatally dangerous for human life. In this case, the neutral mode for two-phase inclusion practically does not matter. Cases of two-phase inclusion are relatively rare: they are most likely when working under voltage, when the current-in-part parts of different phases are located at a small distance from each other.

According to technological requirements, preference is often given to the four-wire network, it uses two voltages - linear and phase. Thus, from the four-wire network 380, it is possible to feed as a force load - three-phase, including its between phase wires to a linear voltage of 380 V and lighting, including its between phase and zero wires, i.e., on the phase voltage of 220 V., is significantly cheaper than the electrical installation Due to the use of a smaller number of transformers, a smaller cross section of wires and T p.

Networks with a grounded neutral are used where it is impossible to ensure good insulation of electrical installations (due to high humidity, aggressive medium, etc.) or it is impossible to quickly find and eliminate the insulation damage when the capacitive current currents of the network due to significant branchedness reach large values, life-threatening man. Such networks include networks of large industrial enterprises, urban distributive, etc. The existing opinion on a higher degree of reliability of networks with isolated neutral is not enough reasonable. Statistical data indicate that, according to the terms of the reliability of the work, both networks are almost the same.

At voltage above 1 000V, up to 35 kV networks for technological reasons are isolated neutral, and above 35 kV - grounded.

Premises according to the degree of danger can be attributed: to the 1st class - office premises and laboratories with accurate devices, assembly goals of instrumentation plants, time plants, etc.; to the 2nd class - warehouse unheated premises, staircases with conductive floors, etc.; By the 3rd class - all the sets of machine-building factories: electroplated, batteries, etc., the sections of work outside the premises.

Analysis of the danger of the lesion is practically reduced to the determination of the value of the current flowing through the human body in various conditions in which it may be during the operation of electrical installations, or tensions. The danger of lesion depends on a number of factors: the scheme of inclusion of a person into an electrical circuit, the network voltage, the scheme of the network itself, the mode of its neutral, the degree of insulation of current parts from the ground, the capacity of the current-carrying parts relative to the Earth, etc.

What are the inclusion schemes in the electrical circuit?

The most characteristic are two inclusion schemes: between the two phases of the electrical network, between the same phase and the earth. In addition, it is possible to touch grounded inactive units, as well as the inclusion of a person under step tension.

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

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

The grounded is the generator neutral (transformer) attached to the grounding device directly or through a small resistance (for example, through current transformers).

Isolated called a generator or transformer neutral, not attached to the grounding device or attached to it through a large resistance (alarm, measurement, protection, grounding reactors).

What is the basis for choosing a neutral regime?

The choice of network scheme, and therefore, the current source neutral modes are based on the technological requirements and safety conditions.

At voltage up to 1000, both three-phase network schemes were obtained in widespread: three-wire with an isolated neutral and four-wire with a grounded neutral.

According to technological requirements, preference is often given to the four-wire network, it uses two operating stresses - linear and phase. Thus, from the four-wire network 380 V, it is possible to feed as a power load - three-phase, including its between phase wires to a linear voltage of 380 V and lighting, including its between phase and zero wires, i.e., on the phase voltage of 220 V. It becomes much cheaper than the electrical installation due to the use of a smaller number of transformers, a smaller cross section of the wires, etc.

By security conditions, one of the two networks are chosen based on the position: according to the terms of contact with the phase wire during the normal operation of the network, the network with an isolated neutral network is safer, and in the emergency period - a network with a grounded neutral. Therefore, it is advisable to apply networks with isolated neutral when it is possible to maintain a high level of network insulation and when the network capacity relative to the Earth is insignificant. These may be little branched networks that are not exposed to the aggressive environment and under constant supervision of qualified personnel. An example is the network of small enterprises, 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 medium, etc.) or it is impossible to quickly find and eliminate the insulation damage when the capacitive current currents of the network due to significant branchedness reach large values, life-threatening man. Such networks include networks of large industrial enterprises, city distribution and pr.

An existing opinion on a higher degree of reliability of networks with isolated neutral is not enough reasonable.

Statistical data indicate that, according to the terms of the reliability of the work, both networks are almost the same.

At a voltage above 1000, up to 35 kV networks for technological reasons are isolated neutral, and above 35 kV - grounded.

Since such networks have a large wire capacity relative to the Earth, the person is equally dangerous to the network to the network of both insulated and with a grounded neutral. Therefore, the neutral mode of the network above 1000 in the security conditions is not selected.

What is the danger of two-phase touch?

Under a two-phase touch, a simultaneous touch of two phases of electrical installation, which is under voltage is understood (Fig. 1).

Fig. 1. A diagram of a two-phase touch of a person to the AC network

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

• U l - linear voltage, i.e. voltage between network phase wires;
• R people - human body resistance.

In a network with a linear voltage U L \u003d 380 V, with the resistance of the human body, R person \u003d 1000 ohm current passing through the human body will be equal to:

This current for a person is deadly dangerous. With a two-phase touch current passing through the human body, it practically does not depend on the network neutral mode. Consequently, a two-phase touch is equally dangerous both on a network with isolated and grounded neutral (subject to equal linear stresses of these networks).

Cases of human touching to two phases occur relatively rarely.

What is characterized by a single-phase touch?

A single-phase touch is a touch of a single phase of electrical installation, which is substituted.

It occurs many times more often than a two-phase touch, but less dangerous, because the voltage under which the person does not exceed the phase. Accordingly, the current passing through the human body is less. In addition, the current source of the current source, the resistance of the network wires relative to the Earth, the resistance of the floor (or base) has a large influence on this current, the resistance of the floor (or base).

What is the danger of single-phase touch on the network with a grounded neutral?

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

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

• U f - phase voltage network, in;
• R people - human body resistance, Ohm;
• R about - the resistance of the man's shoes, Ohm;
• R P - floor resistance (base), on which a person stands, ohms;
• R O is the resistance to grounding the current source neutral, Ohm.

Under the most unfavorable conditions (a person who touched the phase has a conductive shoes on the legs - raw or chopped with metal nails, it stands on crude ground or on a conductive base - metal floor, on grounded metal structures), i.e. when R is \u003d 0 and R n \u003d 0, the equation takes the form:

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

However, under these conditions, a single-phase touch, despite the smaller current, is very dangerous. So, in the network with a phase voltage U F \u003d 220 V with R in person \u003d 1000 ohm current, passing through the human body, it will be:

Such a current is deadly dangerous for a person.

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

• 45,000 - human shoes resistance, Ohm;
• 100 000 - Paul resistance, Ohm.

The current of such strength is not dangerous for a person.

From the given data it can be seen that the insulating floors and non-conductive shoes have great importance for the safety of working in electrical installations.

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

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

Taking into account the resistance of the shoes R O and Floor or the base R n, on which the person included consistently resist the human body of 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. Scheme of a person's touch to one phase of a three-phase network with an isolated neutral

With the most unfavorable case, when a person has a conductive shoe shoes and stands on a conductive floor, i.e., with Rf \u003d 0 and R n \u003d 0, the equation will significantly simplify:

For this case, on a network with a phase voltage U F \u003d 220 V and the insulation resistance of the phase R out of \u003d 90 000 Ohm with R in person \u003d 1000 ohm current passing through a person, will be equal to:

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

What network is more secure - with an isolated or grounded neutral?

All other things being equal, the touch of a person to one phase of the network with an isolated neutral is less dangerous than in the network with a grounded neutral. However, this conclusion is valid only for normal (trouble-free) working conditions, if there are insignificant capacity relative to the Earth.

In the case of an accident, when one of the phases is closed to Earth, the network with an isolated neutral may be more dangerous. This is explained by the fact that with such an accident in the network with an isolated neutral, the voltage of the intact phase relative to the Earth may increase from phase to linear, while in the network with a grounded neutral incidence will be insignificant.

However, modern electrical networks due to their branching and considerable length create a greater capacitive conductivity between phase and land. In this case, the danger of a person's touch to one and two phases is almost the same. Each of these touches is very dangerous, since the current passing through the human body reaches very large values.

What is the stress step?

Under the stress of the step it is understood as the voltage between the two points of the current circuit, which are one from the other at a distance of the step, on which the person is simultaneously. Step value is usually taken equal to 0.8 m.

For some animals (horses, cows) the magnitude of the step voltage is greater than for people, and the flow path captures the chest. For these reasons, they are more susceptible to stepping voltage.

Stepper voltage occurs around the location of the current from the damaged electrical installation into the ground. The greatest value will be near the location of the transition, and the smallest - at a distance of more than 20 m, i.e. outside, limiting the field of expansion of the current in the soil.

At a distance of 1 m from the earthinger, 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, which can practically be zero.

Such dots of the surface of the soil are considered to be outside the zone of the current leveling and are called "Earth".

The risk of step voltage increases, if a person who has exposed to its effect falls. And then the tensile step increases, as the path of the current is no longer through the legs, but through the whole body.

Cases of lesions of people due to the effects of the step voltage relatively rare. They can occur, for example, near the wires falling on Earth (at such moments, it is impossible to allow people and animals to turn off the line to the place of the lines of falling the wire). The most dangerous steps voltage at the lightning strike.

Once in the stepping voltage zone, it follows from it by small steps towards the opposite place of the alleged closure on the ground, and in particular the wire lying on Earth.

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§ 3. Danger of human damage by electric shock.

Scheme of single-phase inclusion of a person in a three-phase current network with a grounded neutral.

The defeat of the person current occurs when the electrical circuit is closed through the human body. This occurs in the case of a man's touch at least two points of the electrical circuit, between which there is some voltage. The inclusion of a person in a chain can occur in several schemes: between wire and ground, called single-phase inclusion; Between two wires - two-phase inclusion. These schemes are most characteristic of three-phase AC networks. It is also possible to include between two wires and land at the same time; between two points of land having different potentials, etc.

Single-phase inclusion of a person in the network It is a direct contact of a person with parts of electrical installation or equipment, normally or randomly located under voltage. At the same time, the degree of danger of lesion will be different depending on whether the electrical network has a grounded or isolated neutral, as well as depending on the quality of the insulation of the wires of the network, its length, operation mode, and a number of other parameters.

With a single-phase inclusion in a network with a grounded neutral, a person falls under a phase voltage, which is 1.73 times less than linear, and is exposed to current, the value of which is determined by the value of the phase voltage of the installation and resistance of the human body (Fig. 69). An additional protective effect has a floor insulation on which a person is standing and shoes.

Fig. 69. Single-phase inclusion scheme in a three-phase current network with grounded neutral

Thus, in a four-wire three-phase network with a grounded neutral circuit current passing through a person, includes resistance of its body, as well as floor resistance, shoes and grounding of the current source (transformer, etc.). At the same time the current

where u l is a linear voltage, in; R T is the resistance of the human body, Ohm; R P is the resistance of the floor on which the person is located; R about - the resistance of the man's shoes, Ohm; R 0 - neutral grounding resistance, Ohm.

As an example, we consider two cases of single-phase inclusion of a person in a three-phase four-wire electrical network with a grounded neutral at u l \u003d 380 V.

Case with unfavorable conditions. A person who touched one phase is on the cheese of soil or a current-conducting (metallic) floor, its shoes are raw or has metal nails. In accordance with this, we take resistance: human body R T \u003d 1000 Ohms, soil or floor R n \u003d 0; shoes R O \u003d 0.

The grounding resistance of the neutral r 0 \u003d 4 ohm into the calculation in view of a minor value is not accepted. Through the body of a person passes a current

being life-threatening.

Case with favorable conditions. The person is on a wooden dry floor resistance R n \u003d 60 000 Ohm, has a dry non-conductive (rubber) footwear on the feet of RF \u003d 50 000 Ohm. Then through the body of a person passes a current

being a long permissible person.

In addition, dry floors and rubber shoes have significantly greater resistance in comparison with the values \u200b\u200btaken to calculate.

These examples show the great importance of the insulating properties of the floor and shoes to ensure the safety of persons working in possible contact with electric power.

Personal circuits in the current chain can be different:

· Between two wires;

· Between wire and earth;

· Between two wires and land at the same time, etc.

However, the most characteristic are the first two schemes. In relation to three-phase AC networks, the first scheme is usually called a two-phase inclusion, and the second one is single-phase.

Two-phase inclusion, i.e. The touch of man at the same time to the two phases (Fig. 11.3.), as a rule, is more dangerous than single-phase, since the largest voltage is applied to the human body - linear, and therefore a larger current will go through a person, the strength of which is determined by the formula:

where I C is the strength of the current passing through the human body, and; U L \u003d 1.73 U F - linear voltage, i.e. voltage between network phase wires, in; U F - phase voltage, in; R h - the resistance of the human body, Ohm.

Fig. 11.3 Dual-phase inclusion scheme

man in a current chain in a three-phase network

It is easy to see that with a two-phase inclusion current passing through a person, it practically does not depend on the network neutral mode, therefore, two-phase inclusion is equally dangerous in the network both from isolated and grounded neutrals.

Single-phase inclusion occurs significantly more often, it is less dangerous than two-phase, since the voltage under which the person turns out, does not exceed the phase, i.e. Less linear 1.73 times. In addition, the value of the current source is also affected by the value of the current, the resistance of the floor on which the person is standing, the resistance of its shoes and some other factors.

In a network with a grounded neutral (Fig. 11.4), in series with resistance of the human body (R h), the resistance of the shoes (R vol), the resistance of the floor (R n) and the grounding resistance of the current source neutral (R o) are turned out to be included.

Fig. 11.4 Single-phase integration scheme in the current circuit in a three-phase four-wire network with grounded neutral

Taking into account these resisters, the strength of the current (I h) passing through a person will be separated by the formula:

I h \u003d ,

where r h is the resistance of the human body, Ohm; R about - resistance of shoes, Ohm; R n - Paul resistance, Ohm; R o - resistance to grounding neutral, Ohm.

In the network with an isolated neutral (Fig.

11.5.), Current passing through a person returns to a current source through insulation of wires, which has greater resistance. The value of the current flow passing through a person is determined for this case by the formula:

I h \u003d ,

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

In a network with an isolated neutral security, the safety conditions are in direct dependence not only from the resistance of the floor and shoes, but also from the resistance of the insulation of wires relative to the Earth: the better isolation, the smaller the current flowing through the person.

Fig. 11.5 Scheme of single-phase inclusion of a person in a current chain in a three-phase network with an isolated neutral

Thus, with other things being equal, the single-phase inclusion of a person on the network with an isolated neutral is less dangerous than in the network with a grounded neutral. This conclusion is equitable to the day of normal (trouble-free) working conditions. In the case of an accident, when one of the phases is closed to the Earth, the network with an isolated neutral may be more dangerous, since due to aging isolation, moisturizing and under other adverse conditions, the insulation resistance is reduced. As a result, the tension between any intact phase and the ground can increase from the phase to the linear, while in the network with a grounded neutral voltage of intact phases relative to the Earth almost does not increase, i.e. It remains within the phase.

Thus, human safety is ensured by high insulation quality, which is monitored in the process of prophylactic tests. Periodic insulation control is to determine the insulation resistance of each phase relative to the Earth and between the phases on each site, between two consistently installed fuses, devices, or for the last fuse.

The electrical insulation of the power or lighting wiring is considered sufficient if its resistance between the wire of each phase and the ground, or between different phases on a plot bounded by consistently included fuses, is at least 0.5 MΩ (according to the rules of the electrical installation device).

The degree of damage is influenced by the current, the current, voltage, the current, the path of passing through the human body, the individual characteristics of the human body, its psychological state, the presence in the body of alcohol and narcotic substances, the parameters of the microclimate, the time of person in the influence of electric current.

Passing through the human body, the email has 4 types of exposure:

Thermal action - manifested in the burns of individual parts of the body, heating to high temperatures of blood vessels, blood, nerves, hearts, and brain, which causes a serious disorder of organs.

Electrolytic action - decomposition of organic fluid (lymph and blood) with a violation of its composition.

Mechanical action - (dynamic) bundle, body tissue rupture (heart muscles, vessels) as a result of an electrodynamic effect; Instant explosionless formation of steam from superheated tissue fluid and blood.

Biological - manifests itself in violation of biological processes occurring in the body, accompanied by irritation (destruction) of nervous and other tissues and burns, discontinuation of the activities of respiratory and blood circulation.

Exposure to electric current can lead to local injuries or overall electric shock (electric).

TO local these include eclectic burns, leather metallization, mechanical damage, electrophthalmia (inflammation of the outer shells of the eyes).

TO common: Electric blow, in which affects (or the threat of lesion is created), the entire body due to violation of the normal activity of vital organs. General injuries are accompanied by the excitation of various groups of human body muscles, which can lead to cramps, paralysis of heart respiratory organs, stopping the heart.

35. Factors affecting the severity of electric shock

Factors determining the danger of electric shock:

1. Electrical:

Voltage;

Current;

Its frequency;

Human electrical resistance.

2. Neelectric:

Individual features of a person;

Current duration;

His way through man.

3. State environment .

4. Electric current of the least power causing an annoying sense by man called threshold tangible current. It is about 1.1 mA for a frequency current of 50 Hz, and for DC - 6 mA.

36. Single-phase and two-phase inclusion of a person in various electrical networks

The defeat of the person current occurs when the electrical circuit is closed through the human body. This occurs in the case of a man's touch at least two points of the electrical circuit, between which there is some voltage. The inclusion of a person in a chain can occur in several schemes: between wire and ground, called single-phase inclusion; Between two wires - two-phase inclusion. These schemes are most characteristic of three-phase AC networks. It is also possible to include between two wires and land at the same time; between two points of land having different potentials, etc.

Single-phase inclusion of a person in the network It is a direct contact of a person with parts of electrical installation or equipment, normally or randomly located under voltage. At the same time, the degree of danger of lesion will be different depending on whether the electrical network has a grounded or isolated neutral, as well as depending on the quality of the insulation of the wires of the network, its length, operation mode, and a number of other parameters. With a single-phase inclusion in a network with a grounded neutral, a person falls under phase voltage, which is 1.73 times less than linear, and exposed to current, the value of which is determined by the value of the phase voltage of the installation and resistance of the human body. Additional protective action has an insulation of the floor on which There is a person and shoes.

Two-phase touch is, as a rule, more dangerous, since the largest voltage is applied to the human body (for a three-phase network - linear), and the current // g, passing through the human body, turns out to be independent of the neutral mode (for a three-phase network) or from availability Grounding one of the wires in a single-phase network and is of the greatest value. Cases of two-phase touches occur very rarely.