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.

Let's look at specific examples possible schemes inclusion of a person in 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 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 the body resistance to be 1 kOhm, and the electrical network to be 380/220 V, then 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 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. Danger of defeat electric shock in this case, it 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 R H 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 dry ground. 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 long time not in use), 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 comes into contact with electrically conductive objects connected to the ground with his second hand (the body of a grounded machine, metal or reinforced concrete structure buildings, damp wooden wall, water pipe, 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 circuit resistance is equal to the body resistance 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, no matter how the current flows, 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.

Security questions

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

2. Name your sources electrical hazard in production.

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?

Great Encyclopedia Oil and Gas. Schemes for connecting a person to an electrical circuit

6.2.3. Schemes for connecting a person to a current circuit

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 voltage (Upr).

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 circuit, neutral mode and other factors, has the greatest value:

where is the linear voltage, i.e. voltage between phase wires of the network, V;

Uph - 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;

Rh - human body resistance, Ohm.

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

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 networks approved for use are considered three-phase current voltage up to 1000 V: four-wire with solidly grounded neutral and three-wire with insulated neutral.

6.2.4. Three-phase networks with solidly grounded neutral

In a three-phase, four-wire network with a solidly grounded neutral, the calculation of the touch voltage Upr and the current Ih 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.

r1 ≠ r2 ≠ r3 ≠ rn; С1 ≠ С2 ≠ С3 ≠ Сн ≠ 0,

where r1, r2, r3, rn - insulation resistance of phase L and neutral (combined) PEN wires, Ohm;

C1, C2, C3, Cn - dispersed capacitances of phase L and neutral (combined) PEN wires relative to ground, F.

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

where w is the angular frequency, rad/s;

j is the 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 r0 is the 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

The current can be found by the formula

where is the complex voltage of phase 1 (phase voltage), V;

The 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, it can be expressed as follows:

Keeping in mind that for a symmetrical three-phase system

where Uph is the phase voltage of the source (module), V;

a is 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:

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

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.

Y1 = Y2 = Y3 = Yн = 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)

and the current is equal

According to the requirements of the PUE, the resistance value r0 should not exceed 8 ohms, while the resistance of the human body Rh does not fall below several hundred ohms. Consequently, without a big error in equations (6.4) and (6.5), we can neglect the value of r0 and assume that when touching one of the phases of a three-phase four-wire network with a grounded neutral, a person is practically under the phase voltage Uph, and the current passing through it is equal to the quotient of Uph divided by Rh.

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 full The conductivity of the wires relative to the ground is very small compared to the grounding conductivity of the network neutral.

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

A solid ground fault in a network with a solidly grounded neutral changes the phase voltage relative to the 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 rzm, and a person touches phase conductor L1, equation (6.2) will take the following form:

Here we also assume that Y1, Y2 and Yн are small compared to Y0, i.e. are equal to zero.

Having made the appropriate transformations and taking into account that

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 Upr is equal to

The current passing through a person is determined by the formula

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 wire-to-ground resistance rzm is 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 r0 equal to zero, then from equation (6.6) we obtain that Unp = Uph, i.e. The voltage under which a person will be will be equal to the phase voltage.

However, in practical conditions, the resistances rzm and r0 are always greater than zero, therefore the voltage under which a person touches a serviceable phase wire of a three-phase network with a grounded neutral during an emergency is always less than the linear one, but more than the phase one, i.e.

> Upr > Uph. (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). Thus, for small values of rzm and r0 compared to Rh, the first term in the denominator can be neglected. Then the fraction for any ratio rzm and r0 will always be greater than one, but less, i.e. we obtain expression (6.8).

studfiles.net

Analysis of the danger of electric shock in various electrical networks

The passage of current through a person is a consequence of his touching no less than two points of an electrical circuit, between which there is a certain potential difference (voltage).

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

diagrams for connecting a person to an electrical circuit;

mains voltage;

diagrams of the network itself;

network neutral mode;

degree of insulation of live parts from the ground;

capacitance of live parts relative to ground.

Classification of networks with voltage 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 those isolated from the ground and those with a grounded wire.

Isolated from the ground

With grounded wire

These networks are widely used in national economy, starting with low-voltage power supply of portable tools and ending with 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, subways, 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.

The neutral point of the current source is the point at which the voltages relative to all phases are equal in absolute value.

The zero point of the current source is a grounded neutral point.

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

1. Three-wire network with isolated neutral

2. Three-wire connector with grounded neutral

3. Four-wire network with isolated neutral

4. Four-wire network with grounded neutral

For voltages up to 1000V, circuits “1” and “4” are used in our country.

Schemes for connecting a person to an electrical circuit

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

Single-phase contact - between phase and ground. This assumes that there is an electrical connection between the network and ground.

For more information about schemes for connecting a person to a circuit, see P.A. Dolin. Basic safety precautions in electrical installations.

Single-phase networks

Isolated from the ground

The better the insulation of the wires relative to the ground, the less the danger of single-phase contact with the wire. Human contact with a wire with high electrical insulation resistance is more dangerous.

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

With grounded wire

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

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

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

Three-phase networks

With isolated neutral

The danger of contact is determined by the total electrical resistance of the wires relative to the ground; with increasing resistance, the danger of contact decreases.

The touch voltage is almost equal to the line voltage of the network. The most dangerous case.

With grounded neutral

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

The magnitude of the touch voltage lies between the linear and phase voltages and depends on the ratio between the ground fault resistance and the grounding resistance.

Electrical safety measures

Avoiding human contact with live parts. It is implemented by placing live parts in inaccessible places (at a height, in cable ducts, ducts, pipes, etc.)

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

Application of funds personal protection. Before using PPE, you must ensure that it is in good working order, intact, and also check the timing of the previous and subsequent instrument verification.

Basic protective equipment provides immediate protection against electric shock. Additional protective equipment cannot ensure safety on its own, but can help when using basic equipment.

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

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

When grounding is impossible, for protection purposes, the potential of the base on which the person stands and the equipment is equalized by increasing it. For example, connecting a repair basket to a phase conductor of a power line.

Grounding conductors are divided into: a. Artificial, intended for grounding purposes directly. b. Natural metal objects found in the ground for other purposes, which

jurik-phys.net

Schemes for connecting a person to an electrical circuit

During the operation of electrical installations, the possibility of a person touching live parts that are energized cannot be excluded. In most cases, dangerous contact with live wires occurs when a person is standing on the ground and wearing shoes. P has some electrical conductivity.

In the conditions of a tourist complex. The most typical two schemes for connecting the human body in an electrical circuit: Between two wires 1 between wire and ground. In three-phase networks AC The first circuit is called two-phase connection, and the second is called single-phase. In the hotel industry, in addition to three-phase AC networks, single-phase ones are widely used to power various household appliances (vacuum cleaners, refrigerators, irons).

The diagram for connecting a person to a single-phase two-wire network, isolated from the ground, is shown in Fig. 41

Fig. 41. A person touches the wire of a single-phase two-wire network during its operating mode: a - normal, b - emergency. A, N - wire designation

Similar networks are obtained using isolation transformers. Under normal operating conditions and high-quality insulation of the wires, touching one of them reduces the risk of electric shock

In emergency mode (Fig. 41, b), when one of the wires is shorted to ground, the insulation is shunted by the resistance of the wire to ground, which, as always, is so small that it can be taken equal to zero. To create single-phase two-wire networks with a grounded wire, single-phase transformers are used, and to obtain a voltage of 220. Intra-phase networks are connected to the phase and neutral wires. In both cases, an electrical circuit arises, one of the sections of which is the human body. The current path through the human body in the first case may be “arm - leg”, and in the second - “arm - arm”. Other cases of a person being included in an electrical circuit are also possible, for example, touching live parts with the face, head, neck or switching on the path current "leg - leg leg".

Three-phase four-wire networks with grounded neutral. With a two-phase (two-pole) touch, a person is exposed to the full operating voltage of the installation. With single-pole contact, which happens more often, the current depends not only on the installation voltage and the resistance of the human body, but also on the neutral mode, the state of insulation of the network, the floor, the person’s shoes.

Let's consider the features of various electrical networks. In the tourist complex, four wire networks with a tightly grounded neutral with a voltage of up to 1000 V are common, for example 380/220. B. The power source is a three-phase step-down transformer, the secondary windings of which are connected by a star. The neutral of the secondary winding of the step-down transformer (for example, 1000/400. V) is tightly grounded, which determines the mode in which the voltage of any phase of the secondary network relative to the ground does not exceed phase voltage, i.e. for a transformer with a voltage of 400.V it will be no more than 230.V (for the consumer 220.V). In addition, in case of insulation failure between the primary and secondary windings when the neutral is workingly grounded high voltage, goes to the secondary network in relation to the ground, is significantly reduced due to the low neutral grounding resistance (2,4,8. Ohms or more for voltages of 660, 380 and 220. In a three-phase network (State Standard 121030-81)0-81)) .

A simplified diagram explaining single-pole human contact with a four-wire network with solid grounding of the neutral of the power source (transformer or generator) is presented in Fig. 42

Figure 42. Single-phase connection of a person to a network with a tightly grounded neutral of the power supply (transformer)

Due to the low resistance of the spreading current of the working neutral grounding compared to the resistance of the human body, it is equal to zero. The touch of a person standing on the ground (or on a grounded structure, floor) causes a closed electrical circuit: power source winding - line wire - human body - earth - wire - working grounding - source winding. In the “human body” section of the circuit, it is subject to a phase voltage of 220. V. If the person’s shoes are electrically conductive, then the floor or structure on which it stands will also be electrically conductive, and almost all of the voltage will be applied to the person along the way. “arm - legs” If, under unfavorable conditions, the resistance of the human body is 1000 Ohm, then a current of 220 mA will pass through it, which is deadly for it. If the total resistance of the shoes and the floor turns out to be equal to the resistance of the human body, then the current through it will be less. For example, with a high resistance of the “shoes - floor” section (10,000 ohms), the current through a person will be 20 mA, which is much less dangerous, but in this case pain, convulsions, and in some cases the inability of the victim to independently free himself from the action of the current. This proves that a single-phase human touch to a network with a tightly grounded neutral is always safe.

In the practice of operating electrical installations, there may be cases of short circuits to the ground of live parts, for example, through the housing of an electrical receiver or a metal wiring structure. If such a short circuit turns out to be dead, that is, there is a low transition resistance, then the installation through a single-phase short circuit is switched off by maximum flow protection (the fuse link blows out or the automatic switch turns off). After this, the normal operation of the other electrical network is restored.

The maximum permissible levels of touch voltage and current during emergency operation of industrial and household electrical installations in tourist complexes with a voltage of up to 1000. V and a frequency of 50. Hz should not exceed the values given in Table 41 (State Standard 121038-82-82).

tables 41

Maximum permissible levels of touch voltage and current

Standardized value	Current duration, s	Standardized value

Three-phase networks with neutral isolated from ground

Accommodation electrical energy to the second stage of power supply manufacturing enterprises, cities and villages is carried out using cable (in cities) or overhead (in villages) lines at the rated voltage of electrical receivers (step-down transformers of enterprises, residential areas) at 6-10 or 35 kV. These electrical networks are made with neutrals isolated from the ground. I phases of power sources (transformers of district substations of the power system) or neutrals grounded through significant inductive resistances are switched on to reduce the capacitance of the current components of a single-phase fault to ground.

During a single-phase ground fault in a network with a neutral isolated from the ground, a current flows at the point of the ground fault, which is caused by the operating voltage of the installation and the conductivity of the phases relative to the ground

networks with an isolated neutral are quite effective even if their length is relatively short. In this case, we can take the capacitance of the wires relative to the ground equal to zero, and the resistance of the wires is quite large

Figure 43 shows the connection of a person in three-phase networks with an isolated neutral

Fig. 43. Human contact with the wire of a three-phase 3-wire network with an insulated neutral during normal operation:. A.V. C - wire designation

In networks with an insulated neutral, during normal operation, the danger of electric shock to a person who touches one of the phases depends on the resistance of the conductor relative to the ground, i.e. As resistance increases, the danger decreases.

Protective grounding is one of the protective measures against electric shock to a person when touching metal non-current-carrying parts with damaged insulation (for example, a short circuit to the body). The purpose of such grounding is to make an intentional electrical connection to the ground or. TI is the equivalent of metal non-current-carrying parts that may be energized using grounded devices (a set of ground electrode and grounding conductors). One or more metal electrodes (for example, steel rods, pipes) that are located in the ground serve as a grounding electrode, providing a sufficiently low transition resistance. The resistance of a grounded device is called the total resistance, consisting of the current spreading resistance of the grounding conductor and the resistance of the grounded conductor.

Let's consider the effect of protective grounding. If the body of the electric motor (cable sheath apparatus) does not have a reliable connection to the ground and, as a result of insulation damage, has contact with the conductive part, then a single-phase connection of a person to the current circuit will occur.

In the network, when there is a short circuit to the frame, a single-phase ground fault occurs

Due to the relatively small current that flows to the ground, the protection will not turn off and will continue to operate in emergency mode. But a current will flow through the body of a machine or apparatus with damaged insulation, and a voltage relative to the ground will appear between the body 1 and the ground (Fig. 44.4).

Fig. 44. Short circuit to the housing of an electric motor connected to a network with an isolated neutral

A person will be exposed to touch voltages that can be significant and depend on where the person's feet are located, as well as the electrical conductivity (resistance) of the shoes. As always, the touch voltage is less than the voltage relative to the ground.

Thus, the magnitude of the voltage of the grounded frame relative to the ground, and therefore the touch voltage, depends on the resistance of the earth, and the touch voltage depends on the resistance of the grounded device. In order for the contact voltage to be as low as possible, it is necessary to have a low resistance of the grounded device. Electrical installations are not grounded at a voltage of 42 V and below, 1110 V and below. DC in all premises and working conditions without increased danger.

Parts of electrical equipment to be grounded. The following are subject to grounding: housings of electrical machines, transformers, devices; drives of electrical devices and secondary windings of welding transformers; frames of distributed panels, control panels, lighting and power cabinets; metal structures distributed devices cable lines. The following are not subject to grounding: reinforcement of suspension and support insulators; brackets and lighting fixtures when installing them on wooden supports and structures; electrical equipment that is installed on metal grounded structures, if reliable electrical contact is ensured in places where metal non-current-carrying parts of the electrical equipment contact them. The housings of electrical measuring instruments and relays installed on switchboards and in cabinets are also not subject to grounding 1. Wall of switchgear chambers; housings of electrical receivers with double or reinforced insulation, for example, electric drills, washing machines, electric razor

siltation in electrical installations and networks with voltages up to 1000. V is the intentional electrical connection of metal non-current-carrying elements of an installation, normally insulated from current-carrying parts that are not energized (electrical equipment housings, cable structures), with a neutral protective conductor.

The neutral protective conductor in electrical installations with voltages up to 1000. V is the conductor connecting the neutralized parts (electrical equipment housings) with a tightly grounded neutral point of the winding of the current source (generator or transformer) or its equivalent (GOST 121030-811. Gosstandart 121009-76-76) .

In electrical installations with a tightly grounded neutral wire, in the event of a short-circuit to grounded metal structural non-string conductors, automatic shutdown of the equipment from damage must be ensured. Jenny insulation, since this creates a single-phase short circuit.

Neutral protective wires are grounded directly in power supplies, i.e. at substations or power plants. In addition to the main working grounding of the neutral, it is necessary to re-ground the neutral wire in the network, which reduces the resistance of the neutral grounding and serves as a backup grounding in the event of a break in the neutral grounding of the wire (Figure 45.5).

Figure 45. Schematic diagram protective silting: 1 - electrical installation, 2 - maximum jet protection

Repeated grounding on overhead lines is done every 250 m of their length, at their ends, at branches and branches from high-voltage lines with a branch length of 200 m 1 more, as well as at the inputs of overhead lines. Budina.

When powered by cable lines voltage 380/220. Re-grounding of the neutral wire is carried out in rooms where grounding of electrical appliances is planned, including in these rooms there must be a line for re-grounding of the neutral wire, to which the objects required for grounding are connected.

To re-ground the neutral wire, natural grounding conductors should be used whenever possible, with the exception of direct current networks, where re-grounding should be done using only natural grounding conductors. The resistance of the grounding device of each of the repeated groundings should not be more than 10 Vm.

Considering that the neutral wire, even with an uneven load, carries a current that is significantly less than in the phase wires, the cross-section of the neutral working wire for the four leading lines is chosen to be approximately equal. Half the cross-section of phase wires. In single-phase branches from main lines, the phase-zero intersection of the neutral wire should be the same as the phase wire, since it carries a current equal to the current of the phase wire.

The resistance of the grounded wires must be so small that when a phase is shorted to the housing, the single-phase short circuit current is sufficient to instantly trigger the maximum current protection according to no. The PUE of the phase-zero circuit current when shorted to the housing must be at least 3 times higher than the rated current of the corresponding fuse.

When protecting an electrical installation circuit breaker The neutral wires are selected in such a way that the phase-zero loop provides a short-circuit current that does not exceed the insertion current of the circuit breaker by 1.4 times.

In two leading branches, phase - zero, feeding single-phase electrical receivers, a protective device (fuse, single-pole switches) is installed only on the phase wire, if in this connection there are parts that must be grounded. For the purpose of electrical safety, when installing lamp sockets, the phase wire is connected to the central contact of the socket (heel), and the neutral wire is connected to the threaded part of the socket. This will prevent an accident if you accidentally touch the lamp base (for example, when replacing it) without disconnecting from the network. When grounding, you should connect a separate branch from the neutral wire to the illuminated fixture, and not use a conductive neutral wire for this purpose.

uchebnikirus.com

Presentation on the topic: TYPES OF ELECTRICAL NETWORKS

Ground 01 is generally an equipotential conductor.

UАЗ UВЗ UСЗ - phase voltages relative to the ground.

a – phase operator of a three-phase system, taking into account phase shift

Electrical parameters characterizing the connection of the network to the ground:

insulation resistance,

capacity relative to ground,

grounding.

INSULATION RESISTANCE

Ri is an indicator of the ability of insulating structures to pass electric current under the influence of a direct voltage applied to these structures

CAPACITANCE RELATIVE TO GROUND

Possible schemes for connecting a person to an electrical circuit

1.Double-pole touch.

2. Unipolar touch.

3. Residual charge.

5. Electrical breakdown of the air gap.

6.Induced charge.

7. Static electricity charge.

studfiles.net

Scheme - inclusion - man

Page 1

Schemes for connecting a person to a current circuit can be different.

Schemes for connecting a person to a current circuit can be different. However, the most typical are two of them: between two wires and between a wire and ground. In relation to the most common three-phase AC networks, the first circuit is usually called two-phase, and the second - single-phase.

Schemes for connecting a person to a current circuit can be different. However, the most typical are two of them: between two wires and between a wire and ground.

In Fig. Figure 4.13 shows a diagram of connecting a person to a single-phase network with an isolated neutral.

The touch voltage depends on the network voltage, its circuit, the neutral mode, the circuit of connecting a person to the electrical circuit, and the degree of insulation of live parts from the ground.

A single-phase (single-pole) touch occurs much more often than a two-phase touch, so this scheme for connecting a person to the electrical network has received the main attention.

In the conditions of technological workshops, touch voltage depends on the network voltage, its circuit, neutral mode, the circuit of connecting a person to the electrical circuit, and the degree of insulation of live parts from the ground.

In the conditions of technological workshops, touch voltage depends on the network voltage, its circuit, neutral mode, the circuit of a person being connected to the circuit, and the degree of insulation of live parts from the ground. The resistance of a human electrical circuit includes the resistance of the person’s body, the resistance of shoes, the floor or soil on which he stands. Whenever a person is connected single-phase to the circuit, he touches the floor or ground, so the resistance of the supporting surface significantly affects the value of the current passing through the person. At the same time, during the operation of the equipment, one cannot fully rely on the protective properties of the supporting surfaces, which, in the event of damage, can lose electrical resistance, which is very high in normal condition.

Schemes for connecting a person to an electrical circuit can be bipolar and single-pole.

Electrical installations produce, transform, distribute and consume electricity. When using them, a person may find himself in the range of an electromagnetic field or in direct contact with live parts, as a result of which an electric current will flow through his body. This can lead to injury to a person. The danger of injury depends on the magnitude of the current, the duration of exposure, the type of current (direct or alternating), frequency, path of the current (scheme for connecting a person to the electrical circuit), environment and a number of other factors.

Pages: 1

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Analysis of the danger of electric shock in various electrical networks. Electrical safety

Cases of electric shock to a person are possible only when an electrical circuit is closed through the human body or, in other words, when a person touches at least two points of the circuit, between which there is some voltage.

The danger of such a touch, assessed by the magnitude of the current passing through the human body, or the voltage of the touch, depends on a number of factors: the circuit diagram of the person being connected to the circuit, the network voltage, the circuit diagram of the network itself, the mode of its neutral, the degree of insulation of live parts from the ground, as well as capacitance values of live parts relative to the ground, etc.

Schemes for connecting a person to a circuit can be different. However, the most typical are two connection schemes: between two wires and between one wire and ground (Fig. 68). Of course, in the second case, an electrical connection is assumed between the network and the ground.

In relation to alternating current networks, the first circuit is usually called two-phase connection, and the second - single-phase.

Two-phase switching on, i.e. a person touching two phases at the same time, is usually more dangerous, since the highest voltage in a given network is applied to the human body - linear, and therefore more current will flow through the person:

where Ih is the current passing through the human body, A; UЛ = √3 Uф - linear voltage, i.e. the voltage between the phase wires of the network, V; Uph - phase voltage, i.e. the voltage between the beginning and end of one winding (or between the phase and neutral wires), V.

Rice. 68. Cases of including a person in a current circuit: a - two-phase connection; b, c - single-phase inclusions

It is not difficult to imagine that two-phase connection is equally dangerous in a network with both isolated and grounded neutrals.

With a two-phase switching on, the danger of injury will not decrease even if the person is reliably isolated from the ground, that is, if he has rubber galoshes or boots on his feet, or stands on an insulating (wooden) floor, or on a dielectric mat.

Single-phase switching occurs much more often, but is less dangerous than two-phase switching, since the voltage under which a person finds himself does not exceed the phase voltage, i.e., 1.73 times less than linear. Accordingly, the current passing through a person is less.

In addition, the magnitude of this current is also influenced by the neutral mode of the current source, the insulation resistance and capacitance of the wires relative to the ground, the resistance of the floor on which a person stands, the resistance of his shoes and some other factors.

In a three-phase three-wire network with an insulated neutral, the current passing through a person when touching one of the phases of the network during its normal operation (Fig. 69, a) is determined by the following expression in complex form (A):

where Z is the complex impedance of one phase relative to ground (Ohm):

here r and C are, respectively, the insulation resistance of the wire (Ohm) and the capacitance of the wire (F) relative to the ground (for simplicity, taken to be the same for all wires of the network).

Rice. 69. Human contact with a wire of a three-phase three-wire network with an insulated neutral: a - in normal mode; b - in emergency mode

The current in real form is (A):

, (35)

If the capacitance of the wires relative to the ground is small, i.e. C = 0, which usually occurs in short-length overhead networks, then equation (35) will take the form

, (36)

If the capacitance is large and the insulation conductivity is insignificant, i.e. r ≈ ∞, which usually occurs in cable networks, then according to expression (35) the current through a person (A) will be:

, (37)

where xc = 1/wC - capacitance, Ohm.

From expression (36) it follows that in networks with an isolated neutral, which have insignificant capacitance between the wires and the ground, the danger to a person who touches one of the phases during normal operation of the network depends on the resistance of the wires relative to the ground: with increasing resistance, the danger decreases.

Therefore, it is very important in such networks to ensure high insulation resistance and monitor its condition in order to timely identify and eliminate faults that have arisen.

However, in networks with large capacitance relative to ground, the role of wire insulation in ensuring touch safety is lost, as can be seen from equations (35) and (37).

During emergency operation of the network, i.e., when there is a short circuit of one of the phases to the ground through a low resistance of the circuit, the current through a person touching the healthy phase (Fig. 69, b) will be (A):

, (38) and touch voltage (V):

, (39)

If we accept that rzm = 0 or at least assume that gzm< Rh (так обычно бывает на практике), то согласно выражению (39)

, (40)

i.e. the person will be under linear voltage.

In actual conditions, gzm > 0, therefore the voltage under which a person who touches a serviceable phase of a three-phase network with an isolated neutral during an emergency will be significantly greater than the phase voltage and somewhat less than the linear voltage of the network. Thus, this case of touching is many times more dangerous than touching the same phase of the network in normal mode

work [see equations (36) and (39), keeping in mind that r/3>rzm].

In a three-phase four-wire network with a grounded neutral, the insulation conductivity and capacitive conductivity of the wires relative to the ground are small compared to the neutral grounding conductivity, therefore, when determining the current through a person touching the network phase, they can be neglected.

Under normal operation of the network, the current through a person will be (Fig. 70, a):

, (41)

where g0 is the neutral grounding resistance, Ohm.

Rice. 70. Human contact with a phase wire of a three-phase four-wire network with a grounded neutral: a - in normal mode; b - in emergency mode

In regular networks r0< 10 Ом, сопротивление тела человека Rh не опускается ниже нескольких сотен Ом. Следовательно, без большой ошибки в уравнении (41) можно пренебречь значением г0 и считать, что при прикосновении к одной из фаз трехфазной четырехпроводной сети с заземленной нейтралью человек оказывается практически под фазным напряжением Uф, а ток, проходящий через него, равен частному от деления Uф на Rh

It follows that touching a phase of a three-phase network with a grounded neutral during its normal operation is more dangerous than touching a phase of a normally operating network with an isolated neutral [cf. equations (36) and (41)], but it is less dangerous to touch an undamaged phase of a network with an isolated neutral during an emergency period [cf. equations (38) and (41)], since rzm can in some cases differ little from r0.

Useful information:

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Lecture notes

HARDEN

Head department OP KHNURE

prof. Dzyundzyuk B.V.

"____" ________2014

from the discipline “Fundamentals of Defense Practices”

Topic 2.2: “Use people’s minds with electric shock”

Lecturer - Art. wickle cafe OP

Mamontov O. V.

2.2.1 Schemes for connecting a person to an electric current circuit

According to the PUE, the risk of electric shock is possible when a person or animal directly or indirectly touches live parts of electrical installations.

Direct touch is the electrical contact of people or animals with live parts that are energized, or approaching them at a dangerous distance.

Indirect contact is the electrical contact of people or animals with an exposed conductive part that becomes energized as a result of insulation failure.

If a person simultaneously touches two points between which there is an electrical voltage, and a closed circuit is formed, a current passes through his body. The value of this current depends on the touch pattern, i.e. which parts of the electrical installation the person touches, as well as the parameters of the electrical network. Without touching on the network parameters, let’s consider the schemes for connecting a person.

Two-phase (two-pole) touching live parts

In Fig. 1 a and 1 b show direct contact with two poles of a single-phase network. In this case, the current through the human body is equal to

Operating (phase) network voltage, V; - human body resistance, Ohm.

In a three-phase (see Fig. 1.b) network, the current through the human body is determined by the linear voltage

Figure 1 - Two-phase (two-pole) direct contact in a single-phase network (a) and in a three-phase network (b)

2) Single-phase (single-pole) touching live parts

If a person, standing on the ground, touches one of the poles or one of the phases, the current is closed through it to the ground and then through the insulation resistance and capacitance of the phases relative to the ground (see Fig. 2 a) or the neutral grounding (see Fig. 2 b) .

In a network with an isolated neutral (Fig. 2 a), the current value depends on the insulation resistance and phase capacitance relative to the ground (will be discussed below). In a network with a grounded neutral (Fig. 2 b), the current value is equal to

where is the neutral grounding resistance.

Figure 2 - Single-phase (single-pole) direct contact in a three-phase network with an isolated neutral (a) and in a three-phase network with a grounded neutral (b)

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§ 3. Danger of electric shock.

Scheme of a single-phase connection of a person to a three-phase current network with a grounded neutral.

Electric shock occurs when an electrical circuit closes through the human body. This occurs when a person touches at least two points of an electrical circuit, between which there is some voltage. The inclusion of a person in a circuit can occur in several ways: between the wire and the ground, called single-phase connection; between two wires - two-phase connection. These schemes are most typical for three-phase AC networks. It is also possible to switch between two wires and ground at the same time; between two points on the earth having different potentials, etc.

Single-phase connection of a person to the network represents direct contact of a person with parts of an electrical installation or equipment that are normally or accidentally energized. In this case, the degree of danger of injury will vary depending on whether the electrical network has a grounded or insulated neutral, as well as depending on the quality of the insulation of the network wires, its length, operating mode and a number of other parameters.

When connected single-phase to a network with a grounded neutral, a person comes under phase voltage, which is 1.73 times less than linear, and is exposed to current, the magnitude of which is determined by the value of the phase voltage of the installation and the resistance of the human body (Fig. 69). An additional protective effect is provided by the insulation of the floor on which a person stands and shoes.

Rice. 69. Scheme of single-phase connection of a person to a three-phase current network with a grounded neutral

Thus, in a four-wire three-phase network with a grounded neutral, the current circuit passing through a person includes the resistance of his body, as well as the resistance of the floor, shoes and the grounding of the neutral of the current source (transformer, etc.). In this case, the current value

where U l - linear voltage, V; R t - human body resistance, Ohm; R p - resistance of the floor on which the person is located, Ohm; R rev - resistance of a person’s shoes, Ohm; R 0 - neutral grounding resistance, Ohm.

As an example, consider two cases of single-phase connection of a person to a three-phase four-wire electrical network with a grounded neutral at U l = 380 V.

A case of adverse conditions. A person who touches one phase is on damp ground or a conductive (metal) floor, his shoes are damp or have metal nails. In accordance with this, we accept resistance: human body R t = 1000 Ohm, soil or floor R p = 0; shoes R rev = 0.

The neutral grounding resistance R0 = 4 Ohms is not taken into account due to its insignificant value. A current will pass through the human body

being life-threatening.

A case of favorable conditions. A person is on a dry wooden floor with a resistance of R p = 60,000 Ohm, and has dry, non-conductive (rubber) shoes on his feet with a resistance of R rev = 50,000 Ohm. Then a current will pass through the human body

which is long-term acceptable for humans.

In addition, dry floors and rubber shoes have significantly greater resistance in comparison with the values accepted for the calculation.

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

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.

The most common electrical networks are those in which the neutral wire is grounded, that is, 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 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).

During the operation of electrical installations, the possibility of a person touching live parts that are energized cannot be excluded. In most cases, touching live parts is dangerous when a person is standing on the ground, and shoes P have some electrical conductivity.

In the conditions of a tourist complex, the most typical two schemes for connecting the human body in an electrical circuit: Between two wires 1 between the wire and the ground. In three-phase AC networks, the first circuit is called two-phase connection, and the second is called single-phase. In the hotel industry, in addition to three-phase AC networks, single-phase ones are widely used to power various household appliances(vacuum cleaners, refrigerators, irons).

The diagram for connecting a person to a single-phase two-wire network, isolated from the ground, is shown in Fig. 4.1.

Rice. 4.1. Human contact with a wire of a single-phase two-wire network during its operating mode: a - normal; b - emergency; A, N - designation of wires.

Such networks are obtained using isolation transformers. Under normal operating conditions and high-quality insulation of the wires, touching one of them reduces the risk of electric shock.

In emergency mode (Fig. 4.1, b), when one of the wires is locked to the ground, its insulation turns out to be shunted by the resistance of the wire to ground, which, as always, is so small that it can be taken equal to zero. To create single-phase two-wire networks with a grounded wire, single-phase transformers are used, and to obtain a voltage of 220 Intra-phase networks are connected to the phase and neutral wires. In both cases, an electrical circuit arises, one of the sections of which is the human body. The current path through the human body in the first case can be “arm - leg”, and in the second - “arm - arm”. Other cases of a person being included in an electrical circuit are also possible, for example, touching live parts with the face, head, neck, or being included in the leg-to-leg current path.

Three-phase four-wire network with grounded neutral. In case of two-phase (two-pole) contact, the person is under the full operating voltage of the installation. With single-pole contact, which happens more often, the current depends not only on the installation voltage and the resistance of the human body, but also on the neutral mode, the state of insulation of the network, floors, and the person’s shoes.

Let's consider the features of various electrical networks. In the tourist complex, there are four leading networks with a tightly grounded neutral with a voltage of up to 1000 V, for example 380/220 V. The power source is a three-phase step-down transformer, the secondary windings of which are connected by a star. A tightly grounded neutral of the secondary winding of a step-down transformer (for example, 1000/400 V) determines a mode in which the voltage of any phase of the secondary network relative to the ground does not exceed the phase voltage, that is, for a transformer with a secondary voltage of 400 V it will be no more than 230 V (at the consumer 220 V). In addition, in the event of an insulation failure between the primary and secondary windings when the neutral is workingly grounded, the highest voltage goes to the secondary network in relation to the ground, and is significantly reduced due to the low neutral grounding resistance (2,4,8 Ohms or more for voltages of 660, 380 and 220 V three-phase network (State Standard 12.1.030-81)).

A simplified diagram that explains single-pole human contact with a four-wire network with solid grounding of the neutral of the power source (transformer or generator) is shown in Fig. 4.2.

Rice. 4.2. Single-phase connection of a person to a network with a tightly grounded neutral of the power supply (transformer).

Due to the low resistance of the spreading current of the working neutral grounding compared to the resistance of the human body, it is equal to zero. The touch of a person standing on the ground (or on a grounded structure, floor) causes a closed electrical circuit: power source winding - line wire - human body - earth - wire - working ground - source winding. In the “human body” section of the circuit, the phase voltage of the 220 V network acts on it. If the person’s shoes are electrically conductive, then the floor or structure on which it stands will also be electrically conductive, and almost all of the voltage will be applied to the person along the “hand” path - legs ". If, under unfavorable conditions, the resistance of the human body is 1000 Ohms, then a current of 220 mA will pass through it, which is fatal to it. If the total resistance of the shoes and the floor turns out to be comparable to the resistance of the human body, then the current through it will be less. For example, with a high resistance of the “shoes - floor” section (10,000 Ohms), the current through the person will be 20 mA. that is, much less dangerous, but causes pain, convulsions, and in some cases the inability of the victim to independently free himself from the action of the current. This proves that single-phase human contact with a network with a tightly grounded neutral is always dangerous.

In practice, the operation of electrical installations may result in cases of short-circuiting of live parts to the ground, for example, through the housing of an electrical receiver or a metal wiring structure. If such a short circuit turns out to be dead, that is, there is a low transition resistance, then the installation through a single-phase short circuit is switched off by maximum ripple protection (the fuse link blows out or the circuit breaker turns off). After this, the normal operation of the other electrical network is restored.

The maximum permissible levels of touch voltage and current during emergency operation of industrial and household electrical installations in tourist complexes with a voltage of up to 1000 V and a frequency of 50 Hz should not exceed the values specified in table. 4.1 (State Standard 12.1.038-82).

Table 4.1.

Maximum permissible levels of touch voltage and current

Standardized value	Current duration, s	Standardized value

Three-phase networks with neutral isolated from earth.

The placement of electrical energy at the second stage of power supply to industrial enterprises, cities and towns is carried out using cable (in cities) or overhead (in towns) lines at the rated voltage of electrical receivers (step-down transformers of enterprises, residential areas) at 6, 10 or 35 kV. These electrical networks are made with neutrals isolated from the ground. I phases of power sources (transformers of regional substations of the power system) or neutrals grounded through significant inductive resistances are switched on to reduce the capacitance of the component current of a single-phase ground fault.

In the event of a single-phase ground fault in a network with a neutral isolated from the ground, a current will flow at the point of the ground fault caused by the operating voltage of the installation and the conductivity of the phases relative to the ground.

Networks with an isolated neutral are quite effective even if their length is relatively short. In this case, we can take the capacitance of the wires relative to the ground equal to zero, and the resistance of the wires is quite large.

In Fig. 4.3 shows the connection of a person in three-phase networks with an isolated neutral.

Rice. 4.3. Human contact with a wire of a three-phase 3-wire network with an insulated neutral during normal operation. A. B, C - designation of wires.

In networks with an isolated neutral, during normal operation, there is a danger of electric shock to a person who touches one of the phases. depends on the resistance of the conductor relative to the ground, that is, as the resistance increases, the danger decreases.

Protective grounding is one of the protective measures against electric shock to a person when touching metal non-conductive parts with damaged insulation (for example, a short circuit to the body). The purpose of such grounding is to intentionally make an electrical connection to the ground or TE equivalent of metallic non-conductive parts that may be energized, using grounded devices (a combination of ground electrode and grounding conductors). One or more metal electrodes (for example, steel rods, pipes) that are located in the ground serve as a grounding electrode, providing a sufficiently low transition resistance. The resistance of a grounded device is called the total resistance, consisting of the resistance to the spreading of the grounding current and the resistance of the grounded conductors.

In the network, when there is a short circuit to the frame, a single-phase ground fault occurs.

Due to the relatively small current flowing to the ground, the installed protection will not turn off and will continue to operate in emergency mode. But current will flow through the body of a machine or device with damaged insulation, and a voltage relative to the ground will appear between body 1 and ground (Fig. 4.4).

Rice. 4.4. Short circuit to the housing of an electric motor connected to a network with an isolated neutral.

The person who will be exposed to touch voltage, which can be significant and depends on where the person's feet are located, as well as the electrical conductivity (resistance) of the shoes. As always, the touch voltage is less than the voltage relative to the ground.

Thus, the magnitude of the voltage of the grounded frame relative to the ground, and therefore the touch voltage, depends on the resistance of the earth, and the touch voltage depends on the resistance of the grounded device. In order for the touch voltage to be as low as possible, it is necessary to have a low resistance of the grounded device. Electrical installations are not grounded at a voltage of 42 V and below alternating current 1 110 V and below direct current in all premises and operating conditions without increased danger.

Parts of electrical equipment to be grounded. The following are subject to grounding: housings of electrical machines, transformers, devices; drives of electrical devices and secondary windings of welding transformers; frames of distributed panels, control panels, lighting and power cabinets; metal structures of distributed devices of cable lines. The following are not subject to grounding: reinforcement of suspension and support insulators; brackets and lighting fixtures when installing them on wooden supports and structures; electrical equipment, installed on metal grounded structures, if reliable electrical contact is ensured at the points of contact of metal non-current-carrying parts of the electrical equipment with them. The housings of electrical measuring instruments and relays installed on switchboards, in cabinets and in the walls of switchgear chambers are also not subject to grounding; housings of electrical receivers with double or reinforced insulation, for example, electric drills, washing machines, electric shavers.

Siltation in electrical installations and networks with voltages up to 1000 V is the deliberate electrical connection of metal non-current-carrying elements of an installation, normally insulated from current-carrying parts that are not energized (electrical equipment housings, cable structures), with a neutral protective conductor.

The neutral protective conductor in electrical installations with voltages up to 1000 V is the conductor connecting the neutralized parts (electrical equipment housings) with a tightly grounded neutral point of the winding of the current source (generator or transformer) or its equivalent (Gosstandart 12.1.030-811 Gosstandart 12.1.009-76).

In electrical installations with a tightly grounded neutral wire, when there is a short circuit to grounded metal structural non-conductive parts, automatic shutdown of equipment with damaged insulation must be ensured, since this results in a single-phase short circuit.

Neutral protective grounding wires directly in power sources, that is, at substations or power plants. In addition to the main working grounding of the neutral, repeated grounding of the neutral wire in the network should be performed, which reduces the overall resistance of the neutral grounding and serves as a backup grounding in the event of a break in the neutral grounding of the wire (Fig. 4.5).

Rice. 4.5. Schematic diagram of protective siltation: 1 - electrical installation; 2 - maximum jet protection

When power is supplied via cable lines with a voltage of 380/220 V, re-grounding of the neutral wire is carried out when entering the premises in which grounding of electrical appliances is provided. Inside these premises there must be a line for re-grounding the neutral wire, to which objects required for grounding are connected.

To re-ground the neutral wire, natural grounding conductors should be used whenever possible, excluding direct current networks, where re-grounding should be done using only artificial grounding conductors. The resistance of the grounding device of each repeated grounding should not exceed 10 Ohms.

Considering that the neutral wire, even with an uneven load, carries a current that is significantly less than in the phase wires, the cross-section of the neutral working wire for the four leading lines is chosen to be approximately half the intersection of the phase wires. In single-phase branches from main lines, the phase - zero intersection of the neutral wire should be the same as the phase wire, since a current passes through it that is equal to the current of the phase wire.

The resistance of the grounded wires must be so small that when a phase is shorted to the housing, the single-phase short circuit current is sufficient to instantly trigger the overcurrent protection. According to the PUE. The current of the phase-zero circuit when shorted to the body must be at least 3 times higher than the rated current of the corresponding fuse.

When protecting an electrical installation with an automatic switch, the neutral wires are selected in such a way that in the phase-zero loop, a short-circuit current is provided that does not exceed the insertion current of the switch by 1.4 times.

In the two leading branches, phase - zero, which feed single-phase electrical receivers, a protective device (fuse, single-pole switches) is installed only on the phase wire, if this branch contains parts that must be grounded. For the purpose of electrical safety, when installing lamp sockets, the phase wire is connected to the central contact of the socket (heel), and the neutral wire is connected to the threaded part of the socket. This will prevent an accident from accidentally touching the lamp base (for example, during P replacement) without disconnecting from the network. When grounding, separate branches from the neutral wire should be connected to the illuminated fittings, and not use a conductive neutral wire for this purpose.