the main » Android » The inclusion of a person in the electrical network. Schemes for connecting a person to an electrical network Analysis of schemes for connecting a person in an electrical circuit

The inclusion of a person in the electrical network. Schemes for connecting a person to an electrical network Analysis of schemes for connecting a person in an electrical circuit

All cases of electric shock to a person as a result of an electric shock are a consequence of touching at least two points of the electrical circuit, between which there is a potential difference. The danger of such a touch largely depends on the characteristics of the electrical network and the scheme for including a person into it. Having determined the amperage / hr passing through a person taking these factors into account, appropriate protective measures can be selected to reduce the risk of injury.

Two-phase inclusion of a person in the current circuit (Fig. 8.1, a). It happens quite rarely, but it is more dangerous compared to single-phase, since the highest voltage in this network is applied to the body - linear, and the current, A, passing through a person does not depend on the network circuit, its neutral mode and other factors, i.e. e.

I \u003d Ul / Rch \u003d √ 3Uph / Rch,

where Ul and Uf are linear and phase voltage, V; Rch is the resistance of the human body, Ohm (according to the Rules for the Installation of Electrical Installations in the calculations, Rh is taken equal to 1000 Ohm).

Cases of two-phase contact can occur when working with electrical equipment without removing the voltage, for example, when replacing a blown fuse at the entrance to a building, using dielectric gloves with ruptured rubber, connecting a cable to unprotected clamps of a welding transformer, etc.

Single-phase inclusion. The current passing through a person is influenced by various factors, which reduces the risk of injury compared to two-phase contact.

Fig. 8.1. Schemes of the possible inclusion of a person in a three-phase current network:

a - two-phase touch; b - single-phase contact in a network with a grounded neutral; c - single-phase contact in a network with an isolated neutral

In a single-phase two-wire network, isolated from the ground, the current strength, A, passing through a person, when the insulation resistance of the wires is equal to the ground r1 \u003d r2 \u003d r, is determined by the formula

Ich \u003d U / (2Rh + r),

where U is the mains voltage, V; r - insulation resistance, Ohm.

In a three-wire network with an isolated neutral at r1 \u003d r2 \u003d r3 \u003d r, the current will go from the point of contact through the human body, shoes, floor and imperfect insulation to other phases (Figure 8.1, b). Then

Ich \u003d Uph / (Ro + r / 3),

where Ro is the total resistance, Ohm; RO \u003d Rh + Rop + Rp; Rob - shoe resistance, cm: for rubber footwear Rab ≥ 50 000 Ohm; Rn - floor resistance, Ohm: for a dry wooden floor, Rp \u003d 60,000 Ohm; d - insulation resistance of wires, Ohm (according to the PUE, it should be at least 0.5 megohm per phase of the network section with voltage up to 1000 V).

In three-phase four-wire networks, the current will go through the person, his shoes, the floor, the grounding of the neutral of the source and the neutral wire (Figure 8.1, c). The strength of the current, A, passing through a person,

Ich \u003d Uph (Rо + Rн),

where RH is the neutral grounding resistance, Ohm. Neglecting the resistance RH, we get:

At agricultural enterprises, they mainly use four-wire electrical networks with a dead-grounded neutral voltage of up to 1000 V. Their advantage is that through them two operating voltages can be obtained: linear Ul \u003d 380 V and phase Uph \u003d 220 V. Such networks are not presented. high requirements for the quality of insulation of wires and they are used with a large branching network. Somewhat less often, a three-wire network with an isolated neutral is used at voltages up to 1000V - it is safer if the insulation resistance of the wires is maintained at a high level.

Touch voltage. It occurs as a result of touching live electrical installations or metal parts of equipment.

Step voltage. This is the voltage Ush on the human body when the legs are positioned at the points of the current spreading field from the ground electrode or from the wire that fell to the ground, where the feet are located, when a person walks in the direction of the ground electrode (wire) or away from it (Fig. 8.2).

If one leg is at a distance x from the center of the ground electrode system, then the other is at a distance x + a, where a is the step length. Usually, in the calculations, a \u003d 0.8 m is taken.

The maximum voltage in this case arises at the point of the current short circuit to the ground, and with distance from it it decreases according to the law of hyperbola. It is considered that at a distance of 20 m from the point of short circuit, the earth potential is zero.

Step voltage, V,

Fig. 8.2. Step voltage generation circuit

Even with a small step voltage (50 ... 80 V), an involuntary convulsive contraction of the leg muscles can occur and, as a consequence, a person falls to the ground. At the same time, he simultaneously touches the ground with his hands and feet, the distance between which is greater than the length of the stride, so the acting stress increases. In addition, in this position of a person, a new pathway for the passage of current is formed, affecting the vital organs. This creates a real threat of fatal defeat. As the stride length decreases, the step voltage decreases. Therefore, in order to get out of the zone of action of the step voltage, one should move by jumping on one leg or on two closed legs, or in as short steps as possible (in the latter case, a voltage of no more than 40 V is considered permissible).

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

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

The voltage between two conductive parts or between a conductive part and earth when a person or animal touches them at the same time is called tension of touch (U etc).

Two-phase touch, all other things being equal, is more dangerous, since the highest voltage in this 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, is of greatest importance:

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

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

R h - resistance of the human body, Ohm.

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

Cases of two-phase contact occur very rarely and cannot serve as a basis for assessing networks for safety conditions. They are usually found in installations up to 1000 V as a result of work under voltage, the use of faulty protective equipment, as well as the operation of equipment with unshielded bare current-carrying parts (open circuit breakers, unprotected terminals of welding transformers, etc.).

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

6.2.4. Three-phase networks with solidly grounded neutral

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

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

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

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

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

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

;
;
;

where w - angular frequency, rad / s;

j - imaginary unit equal to (
).

Fig. 6.2. Human touching the 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 - zero (combined) wire.

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

;
,

where r 0 - neutral grounding resistance, Ohm.

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

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

, (6.1)

The current is found by the formula

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

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

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

Bearing in mind that for a symmetrical three-phase system

;
;
,

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

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

we will have the equality

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

. (6.2)

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

. (6.3)

In normal operation of the network, the conductivity of the phase and neutral wires relative to the ground, in comparison with the conductivity of the neutral grounding, has very small values \u200b\u200band, with some assumption, can be equated to zero, i.e.

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

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

or (in valid form)

, (6.4)

and the current is

(6.5)

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

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

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

A dead earth fault in a network with a dead-grounded neutral changes little the phase voltage relative to earth.

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

Here we also assume that Y 1 , Y 2 and Y n small compared to Y 0 , i.e. equated to zero.

Making the appropriate transformations and taking into account that

,
and
,

we get the touch voltage in real form

To simplify this expression, assume that

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

. (6.6)

The current passing through a person is determined by the formula

. (6.7)

Fig. 6.3. Human touching the phase wire of a three-phase four-wire network with a grounded neutral in emergency mode: a - network diagram; b - vector diagram of stresses.

Let's consider two typical cases.

If the resistance of the wires to ground r gm is assumed to be zero, then equation (6.6) takes the form

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

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

However, in practical terms of resistance r gm and r 0 is always greater than zero, therefore the voltage under which a person turns out to touch during the emergency mode to a serviceable phase conductor of a three-phase network with a grounded neutral is always less than the linear one, but more than the phase one, i.e.

\u003e U etc \u003e U f . (6.8)

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

Great Encyclopedia of Oil and Gas. Schemes for including a person in an electrical circuit

6.2.3. Schemes for connecting a person to a current circuit

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

The voltage between two conductive parts or between a conductive part and the ground when a person or animal touches them at the same time is called the contact voltage (Upr).

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

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

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

Rh - resistance of the human body, Ohm.

Fig. 6.1. Cases of a person touching live parts that are energized: a - two-phase connection: b and c- single-phase connections

6.2.4. Three-phase networks with solidly grounded neutral

In a three-phase four-wire network with a deafly earthed 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 by a symbolic (complex) method.

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 ≠ rн; C1 ≠ C2 ≠ C3 ≠ Sn ≠ 0,

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

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

Then the admittances 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 ().

Fig. 6.2. Human touch to the 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 - zero (combined) wire.

The admittances of the grounding 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, to the phase conductor L1, the voltage under which he finds himself will be determined by the expression

The current is 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 earth (between the 00 "points on the equivalent circuit).

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

Bearing in mind that for a symmetrical three-phase system

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

a is the phase operator taking into account the phase shift, where

we will have the equality

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

During normal operation of the network, the conductivity of the phase and neutral wires relative to the ground, in comparison with the conductivity of the neutral grounding, has very small values \u200b\u200band, with some assumption, can be equated to zero, i.e.

Y1 \u003d Y2 \u003d Y3 \u003d Yн \u003d 0

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

or (in valid form)

and the current is

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 large error in equations (6.4) and (6.5), we can neglect the value of r0 and assume that when one of the phases of a three-phase four-wire network with a grounded neutral is touched, a person is practically under the phase voltage Uph, and the current passing through it, is equal to the quotient of dividing Uph by Rh.

Equation (6.5) implies another conclusion: 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 a change in the insulation resistance and capacitance of the wires relative to the ground, if the condition remains that complete the conductivity of the wires to earth is very small compared to the conduction of the neutral of the network.

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

A dead earth fault in a network with a dead-grounded neutral changes little the phase voltage relative to earth.

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

Here we also assume that Y1, Y2 and Yн are small in comparison with Y0, i.e. equated to zero.

Making the appropriate transformations and taking into account that

we get the touch voltage in real form

To simplify this expression, assume that

As a result, we finally get that the voltage Upr is equal to

The current passing through a person is determined by the formula

Fig. 6.3. A person's touch to the phase wire of a three-phase four-wire network with a grounded neutral in emergency mode: a - network diagram; b - vector diagram of stresses.

Let's consider two typical cases.

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

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

2. If we take equal to zero the neutral grounding resistance r0, then from equation (6.6) we obtain that Unp \u003d Uph, i.e. the voltage under which the person will be will be equal to the phase voltage.

However, in practical conditions, the resistances rm and r0 are always greater than zero, therefore, the voltage under which a person turns out to touch during the emergency mode to a working phase wire of a three-phase network with a grounded neutral is always less than the linear one, but more than the phase one, i.e.

\u003e Upr\u003e Uph. (6.8)

This position is illustrated by the vector diagram shown in Fig. 6.3, b and corresponding to the case under consideration. It should be noted that this conclusion also follows from equation (6.6). So, for small values \u200b\u200bof rsm and r0 in comparison with Rh, the first term in the denominator can be neglected. Then the fraction for any ratios rsm and r0 will always be greater than one, but less, i.e. we obtain expression (6.8).

studfiles.net

Analysis of the risk of electric shock in various electrical networks

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

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

circuits for connecting a person to an electrical circuit;

mains voltage;

diagrams of the network itself;

network neutral mode;

the degree of isolation of live parts from the ground;

capacitance of live parts relative to the ground.

Classification of networks with voltages up to 1000 V

Single-phase networks

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

Two-wire

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

Isolated from the ground

With grounded wire

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

Single-wire

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

Single-phase network. Single wire

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

Three-phase networks

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

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

The zero point of the power source is the grounded neutral point.

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

1. Three-wire network with isolated neutral

2. Three-wire sit with grounded neutral

3. Four-wire network with isolated neutral

4. Four-wire network with grounded neutral

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

Schemes for including a person in an electrical circuit

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

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

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

Single-phase networks

Isolated from the ground

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

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

With grounded wire

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

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

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

Three-phase networks

With isolated neutral

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

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

With earthed neutral

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

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

Electrical safety measures

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

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

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

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

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

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

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

Earthing switches are divided into: a. Artificial, intended for direct grounding purposes. b. Natural metal objects found in the ground for other purposes, which

jurik-phys.net

Schemes for including a person in an electric circuit

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

In a tourist complex. The most typical two schemes for switching on the human body in an electrical circuit: Between two wires 1 between wire and ground. In three-phase AC networks, the first circuit is called two-phase switching, and the second is single-phase. In the hotel industry, in addition to three-phase AC networks, single-phase ones are widely used to power various household appliances in (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. Human touching the wire of a single-phase two-wire network during its operation mode: a - normal b - emergency ,. A, N - designation of wires

Similar networks are obtained using isolation transformers. During normal operation and good insulation of the wires, touching one of them reduces the risk of electric shock

In emergency mode (Figure 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 occurs, one of which is the human body. The path of current through the human body in the first case can be "hand - foot", and in the second - "hand - hand" Possible other cases of a person being included in an electrical circuit, for example, touching live parts with a face, head, neck or switching on the path current "leg - leg leg".

Three-phase four-wire networks with earthed neutral. With a two-phase (two-pole) touch, a person is under the full operating voltage of the installation. With a single-pole touch, which is 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 the network insulation, the floor, the person's shoes.

Consider the features of various electrical networks. In the tourist complex, there are four wire networks with a tightly grounded neutral voltage up to 1000. V, 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 is tightly grounded (for example, 1000/400. V) determines the mode when 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 a consumer 220. V). In addition, in the event of a breach of insulation between the primary and secondary windings with a working neutral grounding, a high voltage goes to the secondary network in relation to the ground, significantly decreases due to the low neutral grounding resistance (2.4.8. Ohm and more for a voltage of 660, 380 and 220. In a three-phase network (Gosstandart 121030-81) 0-81)).

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

Fig. 42. Single-phase connection of a person to a network with a tightly grounded neutral of power supplies (transformer)

Due to the low spreading resistance of the current of the working neutral ground in comparison with 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) determines a closed electrical circuit: the winding of the power source - the line wire - the human body - the earth - the wire - the working ground - the winding sources. On the section of the "human body" circuit, the phase voltage of the network 220. V acts on it. "hand - legs" If in unfavorable conditions the resistance of the human body is 1000. Ohm, then a current equal to 220 mA will pass through it, which is deadly for it. If the resistance of the shoes and the floor in the sum is equal to the resistance of the human body, then the current through it will be less. For example, with a high resistance of the section "shoes - floor" (10,000 Ohm), the current through a person will be 20 mA, it is much less dangerous, but there is pain, convulsions, and in some cases the victim's inability to free himself from the action of the current. This proves that a single-phase touch of a person to the network with a tightly grounded neutral is always heavenly.

In the practice of operating electrical installations, there may be cases of ground fault of current-carrying parts, for example, through the body of the electrical receiver or the metal structure of the electrical wiring. If such a short circuit I turns out to be deaf, that is, a small transition resistance, then the installation through a single-phase short circuit is turned off by the maximum stream protection (the fuse blows out or the circuit breaker turns off). After that, the normal operation of the other mains 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 value given in Table 41 (Gosstandart 121038-82-82).

table 41

Maximum permissible levels of touch voltage and current

Normalized value	Duration of current action, s	Normalized 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 villages is carried out using cable (in cities), or overhead (in villages) lines at a nominal voltage of electrical receivers (step-down transformers of enterprises, residential areas) at 6 10 or 35 kV. These electrical networks are made with neutrals I phases of power sources (transformers of regional substations of the power system) isolated from the ground or neutrals grounded through significant inductive resistances, are switched on to reduce the capacity of the current components of a single-phase earth fault.

In the event of a single-phase earth fault in a network with an insulated neutral from earth, a current will flow at the point of the earth fault, which is caused by the operating voltage of the installation and the phase conductivity with respect to earth

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

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

Fig. 43. Human touching a wire of a three-phase 3-wire network with isolated neutral during normal operation :. A. B ,. C - designation of wires

In networks with an insulated neutral, during normal operation, the risk of electric shock to a person who touched one of the phases depends on the resistance of the conductor to earth, i.e. 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-current-carrying parts with damaged insulation (for example, a short circuit to the case). The purpose of such grounding is to deliberately connect electrically to ground or. TI is the equivalent of metal non-current-carrying parts that may be energized using earthed devices (a set of earthing switches and grounding conductors). One or more metal electrodes (for example, steel rods, pipes), which are in the ground, provide a sufficiently small contact resistance as a ground electrode. The resistance of a grounded device is called the total resistance, which consists of the spreading resistance of the ground electrode current and the resistance of the grounded conductor.

Consider the action of protective grounding. If the body of the electric motor (cable sheath device) 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 a short to the frame occurs, a single-phase earth fault occurs

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

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

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

Thus, the size of the magnitude of the voltage of the grounded housing with respect 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. The electrical installation is not grounded at a voltage of 42. V and below alternating current 1110 V and below DC in all rooms and operating conditions without increased danger.

Parts of electrical equipment to be grounded. Subject to grounding: cases of electrical machines, transformers, apparatus; drives of electric devices and secondary windings of welding transformers; frames of distributed boards, control boards, lighting and power cabinets; metal structures of distributed devices of cable lines. Not subject to earthing: fittings for suspension and support insulators; brackets and lighting fixtures when installed on wooden poles and structures; electrical equipment, which is installed on metal grounded structures, if in places of contact in connection with them metal non-current-carrying parts of electrical equipment ensured reliable electrical contact. The housings of electrical measuring instruments and relays installed on boards, in cabinets are also not subject to grounding. 1. Wall of chambers of switchgears; housings of electrical receivers with double or reinforced insulation, for example, electric drills, washing machines, electric shavers.

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

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

In electrical installations with a tightly grounded neutral wire, when shorting to zeroed metal structural non-string conductors, automatic disconnection of equipment from damage should be ensured. Jenny is insulated because this results in a single-phase short circuit.

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

Fig. 45. Schematic diagram of 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. Budin.

With power supply through cable lines with a voltage of 380/220. The re-grounding of the neutral wire is carried out in the introduction to the premises in which the device for neutralizing electrical appliances is provided, and among these premises there must be a line for re-grounding the neutral wire, to which the proper grounding of the object is connected.

To re-ground the neutral conductor, if possible, use natural grounding conductors, with the exception of DC networks, where repeated grounding should be using only physical grounding conductors. The resistance of the grounding device of each of the repeated groundings should not be more than 10. Vm.

Considering that the current passes through the neutral wire even with an uneven load, much 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 cross section of phase conductors. In single-phase branches from the mains, the phase-zero crossing of the neutral wire must be the same as that of the phase one, since a current equal to the current of the phase wire must pass through it.

The resistance of the grounded wires must be so small that when the phase is closed to the case, the single-phase short-circuit current is sufficient for instantaneous operation of the overcurrent protection according to no. PUE of the current of the phase - zero circuit when shorting to the case 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 chosen so that the phase-zero loop provides a short-circuit current that does not exceed the current insertion of the circuit breaker by 1.4 times.

In the two leading branches, the phase - zero supplying single-phase electrical receivers, the protective device (fuse, single-pole switches) is installed only on the phase wire, if there are parts in this alkaline vidal that must be neutralized. For the purpose of electrical safety, when installing lamp holders, the phase wire is connected to the central contact of the cartridge (heel), and the zero wire is connected to the threaded part of the cartridge. This will prevent accidents if accidentally touching the lamp base (for example, when replacing it) without disconnecting from the mains. When grounding to the illuminated fittings, separate branches from the neutral wire should be connected, and not use a conductive neutral wire for this purpose.

uchebnikirus.com

Presentation on the topic: TYPES OF ELECTRIC NETWORKS

Ground 01 is generally an equipotential conductor.

UАЗ UВЗ UСЗ - phase voltage relative to earth.

a - phase operator of a three-phase system, taking into account the 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 transmit an electric current under the action of a constant voltage applied to these structures

CAPACITY RELATED TO EARTH

Possible schemes for including a person in an electrical circuit

1.Bipolar touch.

2.Single pole touch.

3.Residual charge.

5. Electrical breakdown of the air gap.

6. Induced charge.

7.Static charge.

studfiles.net

Scheme - turn on - person

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 common are two: between two wires and between a wire and ground. With regard to the most common three-phase AC networks, the first circuit is usually called two-phase connection, and the second single-phase.

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

In fig. 4.13 shows a diagram for connecting a person to a single-phase network with an isolated neutral.

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

Single-phase (single-pole) touch occurs much more often than two-phase, therefore, the main attention is paid to this scheme for connecting a person to an electrical network.

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

In the conditions of technological workshops, the touch voltage depends on the voltage of the network, its circuit, the neutral mode, the circuit for connecting a person to the circuit, the degree of isolation of live parts from the ground. The resistance of a person's electrical circuit includes the resistance of a person's body, the resistance of shoes, the floor or the ground on which he stands. With any single-phase connection of a person to the circuit, he touches the floor or the ground, therefore, 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 a normal state.

Circuits for connecting a person to an electrical circuit can be two-pole and single-pole.

Electrical installations generate, convert, distribute and consume electricity. During their operation, a person may find himself in the sphere of action 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 the defeat of the person. The risk of injury depends on the magnitude of the current, the duration of exposure, the type of current (direct or alternating), frequency, current path (circuits for connecting a person to an electrical circuit), the environment and a number of other factors.

Pages: 1

www.ngpedia.ru

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 by the touch voltage, depends on a number of factors: the circuit for connecting a person to the circuit, the voltage of the network, the circuit of the network itself, the mode of its neutral, the degree of isolation of live parts from the ground, as well as on the value of the capacitance of live parts relative to the ground, etc.

Schemes for including a person in a chain 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, it is assumed that there is an electrical connection between the network and the ground.

With regard to AC networks, the first circuit is usually called two-phase switching, and the second one-phase.

Two-phase switching on, that is, a person touching two phases at the same time, as a rule, is more dangerous, since the highest voltage in this 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; UL \u003d √3 Uf - line voltage, i.e. voltage between phase wires of the network, V; Uf - phase voltage, i.e. voltage between the beginning and end of one winding (or between phase and neutral wires), V.

Fig. 68. Cases of switching a person into a current circuit: a - two-phase switching; b, c - single-phase inclusions

It is easy to imagine that a two-phase connection is equally dangerous in a network with both isolated and grounded neutrals.

With two-phase switching on, the risk 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 it is less dangerous than two-phase switching, since the voltage under which a person finds himself does not exceed the phase voltage, that is, it is 1.73 times less than the linear one. Accordingly, the current passing through a person turns out to be less.

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

In a three-phase three-wire network with an isolated 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 of the impedance of one phase relative to earth (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, they are taken to be the same for all wires in the network).

Fig. 69. Human touch to a wire of a three-phase three-wire network with an isolated neutral: a - in normal mode; b - in emergency mode

Actual current is (A):

, (35)

If the capacitance of the wires relative to the ground is small, that is, C \u003d 0, which usually occurs in air networks of short length, 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 \u003d 1 / wC - capacitive resistance, Ohm.

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

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

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

In the emergency mode of operation of the network, that is, when there is a short circuit of one of the phases to ground through a low resistance gm, the current through a person who has touched a healthy phase (Fig. 69, b) will be (A):

, (38) and contact voltage (V):

, (39)

If we assume that rsm \u003d 0 or at least assume that rsm< Rh (так обычно бывает на практике), то согласно выражению (39)

, (40)

that is, the person will be under line voltage.

In actual conditions, gm\u003e 0, therefore, the voltage under which a person who touches the serviceable phase of a three-phase network with an isolated neutral during an emergency period will be much higher than the phase voltage and slightly less than the line voltage of the network. Thus, this case of touching is many times more dangerous than touching the same phase of the network during normal operation.

work [see. equations (36) and (39), bearing in mind that r / 3\u003e rsm].

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

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

, (41)

where r0 is the neutral grounding resistance, Ohm.

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

In ordinary 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 the intact phase of the network with isolated neutral during the emergency period [cf. Eqs. (38) and (41)], since rsm can in some cases differ little from r0.

Helpful information:

ohrana-bgd.narod.ru

Lecture notes

CONFIRMED

Head department OP KHNURE

prof. Dzyundzyuk B.V.

"____" ________2014

from the discipline "Fundamentals of protection of pratsi"

Topic 2.2: "Mind people with an electric strum"

Lecturer - Art. wickle cafe. OP

O. V. Mamontov

2.2.1 Schemes for connecting a person to an electric current circuit

According to the PUE, the risk of electric shock is possible with direct and indirect contact of a person or animals to parts of electrical installations that are energized.

Direct contact 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 is energized as a result of damage to the insulation.

If a person touches two points at the same time, between which there is an electric voltage, and a closed circuit is formed, a current passes through his body. The value of this current depends on the touch circuit, that is, which parts of the electrical installation the person touches, as well as the parameters of the electrical network. Without touching on the network parameters, we will consider the schemes for switching on 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

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

In a three-phase (see Fig. 1.b) network, the current through the human body is determined by the line 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 closes 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 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 earth (discussed below). In a network with a grounded neutral (Fig. 2 b), the current value is

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)

Fig. 3. Two-phase (two-pole) touch to live parts in the system IT .

U f- phase voltage; I h - the strength of the current flowing through the person;

R h- human resistance; L 1 , L 2 , L 3 - phase conductors.

Current strength ( I h , AND) flowing through a person is determined by the formula

where U l - line voltage, IN;

U f - phase voltage, IN;

R h - human resistance, Ohm.

For example, in a power grid with a line voltage of 380 IN (U f = 220 IN) with the resistance of the human body 1000 Ohm the strength of the current flowing through a person is:

This current strength is deadly to humans.

With a two-phase touch, the current passing through a person practically does not depend on the mode of operation of the neutral. The danger of touching is not diminished even if the person is reliably isolated from the ground.

Single phase touch(Fig. 4) occurs many times more often than two-phase, but it is less dangerous, since the voltage under which a person finds himself does not exceed the phase voltage, i.e. less than linear by 1.73 times and, in addition, the current flowing through a person,

Fig. 4. Single-phase (single-pole) touching live parts in the system IT .

r 1, r 2 , r 3 - insulation resistance of wires of the power supply network; from 1 , from 2 , from 3 - the capacity of the mains wires.

returns to the source (mains) through the insulation of the wires, which in good condition has high resistance.

Current strength ( I h , AND) flowing through a person, for this case is determined by the formula

where R p - transition resistance, Ohm (resistance of the floor on which the person stands and the shoe); Z - insulation resistance of the phase wire relative to earth, Ohm (active and capacitive components).

In the most unfavorable situation, when a person has conductive shoes and stands on a conductive floor ( R p ~ 0), the strength of the current flowing through the body is determined by the formula

if a U f = 220 IN, R h = 1 kOhm, Z = 90 kOhmthen I h = 220/(1000 + (90000 / 3)) = 0,007 AND (7 mA).

Three-phase four-wire AC power network with grounded neutral (in system TN ).

Single phase touch to live parts.

Fig. 5. Single-phase (single-pole) touching live parts

in system TN .

R 0- grounding resistance of the neutral of the mains.

In a four-wire AC electrical network with a solidly grounded neutral (system TN ) the current passing through a person returns to the source (power grid) not through the insulation of the wires, as in the previous case, but through the neutral grounding resistance ( R 0 ) current source (Fig. 5). The strength of the current passing through the human body is determined by the formula:

where R 0 - grounding resistance of the neutral of the current source, Ohm.

The resistance of the grounding device to which the neutral of the current source is connected at any time of the year should be no more than 2, 4 and 8 Ohm respectively at line voltages of 660, 380 and 220 IN... This resistance must be ensured taking into account the use of natural grounding electrodes, as well as grounding electrodes of repeated grounding. PEN - or PE - conductor of overhead power lines (VL) with voltage up to 1 kV... The resistance of the ground electrode located in the immediate vicinity of the neutral of the current source should be no more than 15, 30 and 60 Ohm respectively, at the same line voltages 660, 380 and 220 IN.

Example... In the most unfavorable situation, considered above, with U f = 220 IN, R h = 1000 Ohm, R p ~ 0 Ohm R 0 = 30 Ohm the strength of the current flowing through the human body will be:

I h = 220/1000 + 30 = 0,214 AND (214 mA), which is deadly to humans.

If the shoes are not conductive (for example, rubber overshoes with a resistance of 45 kOhm) and the person stands on a non-conductive floor (for example, a wooden floor with a resistance of 100 kOhm), i.e. R p = 145 kOhm, then the strength of the current flowing through the human body will be:

I h = 220/1000 + 60 + 145000 = 0,0015 AND (1,5 mA), which does not pose a danger to humans.

Thus, all other things being equal, a person's touch to one phase wire of an electrical network with an isolated neutral is less dangerous than in an electrical network with a grounded neutral.

The above schemes for connecting a person to a three-phase alternating current electrical circuit are valid for normal (trouble-free) operating conditions of electrical networks.

In emergency mode operation of a three-phase AC electrical network, one of the phase wires, for example, an electrical network with a grounded neutral (in the system TN ) can be shorted to ground (when the protective grounding system is triggered, the phase conductor falls to the ground, etc.) through the resistance R zm (fig. 6).

Fig. 6. Single-phase (single-pole) touch to live parts in emergency operation of the power grid.

R zm - phase wire short-circuit resistance ( L 2 ) to the ground.

The strength of the current passing through the human body, touching in this situation one of the serviceable phase wires ( L 1 , L 3 ), is determined from the equation

where R zm - the resistance of the phase wire to ground, Ohm.

If at the same time R zm ~ 0 or much less and R 0 ,and R h , then it can be neglected, then the strength of the current passing through the human body will be determined by the formula

that is, a person will be included in the electrical circuit in two phases, and the second phase is connected to him through his legs and by an amount I h will have a significant effect on the transient resistance R p .

At voltages up to 1000 IN in industrial conditions, both of the above schemes of three-phase AC electrical networks are widespread: three-wire with isolated neutral (system IT ) and four-wire with grounded neutral (system TN ).

It is advisable to use an electrical network with an insulated neutral in cases where it is possible to maintain a high level of insulation resistance of phase wires and an insignificant capacity of the latter relative to ground. These are low-branch electrical networks that are not exposed to aggressive environments and are under constant supervision of qualified personnel. For example, in coal mines, only power grids with isolated neutral are used.

An electrical network with a grounded neutral should be used where it is impossible to ensure good insulation of wires (for example, due to high humidity or an aggressive environment), when it is impossible to quickly find or eliminate damage to the insulation, or when the capacitive currents of the electrical network, due to its significant branching, reach large values, dangerous to humans.

At voltages above 1000 IN for technological reasons, electrical networks with voltages up to 35 kV inclusively have isolated neutral, over 35 kV - grounded. Since such power grids have a large capacity of wires relative to the ground, it is equally dangerous for a person to touch their phase wires, regardless of the operating mode of the neutral of the energy source. Therefore, the operating mode of the neutral of the power grid with a voltage higher than 1000 IN not selected for safety reasons.

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

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

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

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

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

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

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

What is the basis for choosing a neutral mode?

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

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

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

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

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

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

The statistics indicate that in terms of operational reliability, both networks are practically the same.

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

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

What is the danger of two-phase touching?

Two-phase touch is understood to mean the simultaneous touch of two phases of an electrical installation under voltage (Fig. 1).

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

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

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

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

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

Cases of a person touching two phases are relatively rare.

What is the characteristic of single-phase touch?

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

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

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

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

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

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

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

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

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

Such a current is deadly to humans.

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

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

A current of such strength is not dangerous to humans.

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

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

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

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

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

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

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

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

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

Which network is safer - isolated or grounded neutral?

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

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

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

What is stride voltage?

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

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

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

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

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

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

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

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

Type