There are various schemes for including a person in electrical circuit current:
Single-phase contact - touching the conductor of one phase of an existing electrical installation;
Two-phase contact - simultaneous contact with the conductors of two phases of an existing electrical installation;
Touching non-current-carrying parts of electrical installations that are energized as a result of damage to the insulation;
Switching on step voltage - switching between two points of the earth (soil) that are under different potentials.
Consider the most characteristic schemes for including a person in an electric current circuit.
Single-phase touch in a network with a solidly grounded neutral. The current flowing through the human body ( I h) with a single-phase touch (Fig. 6) closes in the circuit: phase L 3 - human body - base (floor) - neutral grounding - neutral (zero point).
Rice. 6. Scheme of single-phase touch in the network
with solidly grounded neutral
According to Ohm's law:
Where R o - neutral grounding resistance,
R osn - base resistance.
If the base (floor) is conductive, then R base ≈ 0
Given the fact that R O " R h, That
U h = U f
Such contact is extremely dangerous.
Single-phase contact in a network with isolated neutral. The current flowing through the human body (Fig. 7) will close in circuits: phase L 3 - human body - floor and then returns to the network through phase isolation L 2 and L 1 , i.e. then the current follows the circuits: phase isolation L 2 - phase L 2 - neutral (zero point) and phase isolation L 1 - phase L 1 - neutral (zero point). Thus, in the circuit of current flowing through the human body, phase isolations are switched on in series with it. L 2 and L 1 .
Rice. 7. Scheme of single-phase touch in the network
with isolated neutral
Phase insulation resistance Z has active ( R) and capacitive components ( WITH).
R- characterizes the imperfection of the insulation, i.e. the ability of insulation to conduct current, although much worse than metals;
WITH- the capacitance of the phase relative to the ground is determined by the geometric dimensions of an imaginary capacitor, the "plates" of which are phases and grounds.
At R 1 = R 2 = R 3 = R f and WITH 1 = WITH 2 = WITH 3 = WITH F current flowing through the human body:
Where Z- impedance of the insulation of the phase wire relative to the ground.
If the capacitance of the phases is neglected WITH f = 0 (aerial networks of small extent), then:
whence it follows that the magnitude of the current depends not only on the resistance of the person, but also on the resistance of the insulation of the phase conductor to earth.
If, for example, R 1 = R 2 = R 3 = 3000 Ohm, then
; U h= 0.0111000 = 110 V
Biphasic touch. With a two-phase touch (Fig. 8), regardless of the neutral mode, a person will be under the line voltage of the network U l and according to Ohm's law:
at U l=380V: I= 380/1000 = 0.38 A = 380 mA.
Rice. 8. Scheme of two-phase human touch
Two-phase contact is extremely dangerous, such cases are relatively rare and are usually the result of working under voltage in electrical installations up to 1000 V, which is a violation of the rules and regulations.
Touching a metal case that is energized. Touching the body of the electrical installation (Fig. 9), in which the phase ( L 3) closed on the case, tantamount to touching the phase itself. Therefore, the analysis and conclusions for the cases of single-phase contact, considered earlier, fully apply to the case of a ground fault.
Rice. 9. Scheme of a person touching a metal
hull under tension
Since the resistance of the electrical circuit R Since the magnitude of the electric current passing through a person depends significantly, the severity of the lesion is largely determined by the scheme for including a person in the circuit. The circuits formed when a person contacts a conductor of circuits depend on the type of power supply system used.
The most common electrical networks in which the neutral wire is grounded, that is, it is short-circuited with a conductor to the ground. Touching the neutral wire is practically not dangerous to humans, only the phase wire is dangerous. However, it is difficult to figure out which of the two wires is zero - they look the same in appearance. You can figure it out using a special device - a phase determiner.
Let's take a look at specific examples possible schemes inclusion of a person in an electrical circuit by touching the conductors.
Two-phase inclusion in the circuit. The rarest, but also the most dangerous, is a person's touching two phase wires or current conductors connected to them (Fig. 2.29).
In this case, the person will be under the action of linear voltage. A current will flow through a person along the “hand-hand” path, i.e., the resistance of the circuit will include only the resistance of the body (D,).
|
If we take the resistance of the body to 1 kOhm, and the electrical network with a voltage of 380/220 V, then the strength of the current passing through the person will be equal to
This is a deadly current. The severity of an electrical injury or even a person's life will depend primarily on how quickly he gets rid of contact with the current conductor (breaks the electrical circuit), because the exposure time in this case is decisive.
Much more often there are cases when a person with one hand comes into contact with a phase wire or part of a device, an apparatus that is accidentally or intentionally electrically connected to it. The danger of electric shock in this case depends on the type of electrical network (grounded or isolated neutral).
Single-phase connection to a circuit in a network with a grounded neutral(Fig. 2.30). In this case, the current passes through the person along the “arm-leg” or “arm-arm” path, and the person will be under phase voltage.
In the first case, the resistance of the circuit will be determined by the resistance of the human body (I_, shoes (R o 6), grounds (R w), on which a person is standing, by the neutral grounding resistance (R H), and a current will flow through a person
Neutral resistance R H small, and it can be neglected in comparison with other circuit resistances. To assess the magnitude of the current flowing through a person, we will take a mains voltage of 380/220 V. If a person is wearing insulating dry shoes (leather, rubber), he is standing on a dry wooden floor, the resistance of the circuit will be large, and the current strength according to Ohm's law is small.
For example, the floor resistance is 30 kOhm, leather shoes 100 kOhm, human resistance is 1 kOhm. Current passing through a person
This current is close to the threshold perceptible current. A person will feel the flow of current, stop working, eliminate the malfunction.
If a person stands on damp ground with damp shoes or bare feet, current will flow 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 in 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 drastically reduce the resistance of the rubber sole and make work dangerous.
Before starting work on electrical devices (especially those that have not been in operation for a long time), they must be carefully inspected for insulation damage. Electrical devices must be wiped of dust and, if they are wet- dry. Wet electrical devices must not be operated! Electrical tools, appliances, equipment should be stored in plastic bags to prevent dust or moisture from entering them. You have to work in shoes. If the reliability of an electrical device is in doubt, you need to insure- place dry wooden flooring or a rubber mat under your feet. You can use rubber gloves.
The second current flow path occurs when with the second hand a person comes into contact with electrically conductive objects connected to the ground (a grounded machine body, a metal or reinforced concrete building structure, a wet wooden wall, a water pipe, a heating battery, etc.). In this case, the current flows along the path of least electrical resistance. These objects are almost short-circuited to the ground, their electrical resistance is very small. Therefore, the resistance of the circuit is equal to the resistance of the body and current will flow through the person
This amount of current is deadly.
When working with electrical devices, do not touch objects with your other hand that may be electrically connected to earth. Working in damp rooms, in the presence of well-conductive objects connected to the ground near a person, poses an exceptionally 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 (the other phase of the network, different from the phase that the person touched) turned out to be shorted to the ground, the voltage is redistributed, and the voltage of the serviceable phases differs from the phase voltage of the network. Touching a working phase, a person gets under voltage, which is more than phase, but less than linear. Therefore, for any path of current flow, this case is more dangerous.
Single-phase connection in a circuit in a network with an isolated neutral(Fig. 2.31). In production, for the power supply of power electrical installations, three-wire electrical networks with an isolated neutral are used. In such networks, there is no fourth grounded neutral wire, and there are only three phase wires. In this diagram, rectangles conditionally show electrical resistances r A, r V, r With wire insulation of each phase and capacitance C A, C B, C C each phase relative to __________________________
under significantly higher voltages, and therefore more dangerous. However, the main conclusions and recommendations for ensuring security 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 isolation is a guarantee of safety. However, with extensive electrical networks, this is not easy to achieve. In extended and branched networks with a large number of consumers, the insulation resistance is low, and the danger increases.
For extended electrical networks, especially cable lines, the capacitance of the phases cannot be neglected (CV0). Even with very good phase isolation (r = oo), the current will flow through the person through the capacitance of the phases, and its value will be determined by the formula:
Thus, extended electrical circuits of industrial enterprises with high capacitance are highly dangerous, even with good phase isolation.
If the insulation of any phase is broken, touching a network with an isolated neutral becomes more dangerous than a network with a grounded neutral wire. In emergency operation mode (Fig. 2.31, b) the current passing through a person who has touched the serviceable phase will flow through the earth fault circuit to the emergency phase, and its value will be determined by the formula:
Since the circuit resistance D, the emergency phase on the ground 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 human circuit ____, which is very dangerous.
For these reasons, and also because of the ease of use (the possibility of obtaining voltages of 220 and 380 V), four-wire networks with a grounded neutral wire for a voltage of 380/220 V are most widely used.
We have considered far from all possible schemes of electrical networks and touch options. In manufacturing, you may be dealing with more complex electrical circuits, such as ground.
To simplify the analysis, we take d A - d c= g c \u003d g, A S A= LB= C c = C
If a person touches one of the wires or any object electrically connected to it, the current will flow through the person, the shoes, the base, and through the insulation and capacitance of the wires will flow to the other two wires. Thus, a closed electrical circuit is formed, in which, in contrast to 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 \u003d 2n - circular frequency of the current, rad / s (for industrial frequency current \u003d 50 Hz, therefore co \u003d YuOl).
If the capacitance of the phases is small (this is the case for non-extended overhead networks), we can take C « 0. Then the expression for the magnitude of the current through a person will take the form:
For example, if the resistance of the floor is 30 kOhm, leather shoes 100 kOhm, the resistance of a person is 1 kOhm, and the phase insulation resistance is 300 kOhm, the current that passes through a person (for a 380/220 V network) will be equal to
A person may not even feel such a current.
Control questions
1. What types of electrical networks are most common in production?
2. Name the 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 drains into the ground?
4. How are the premises classified according to the degree of electrical hazard?
5. How does an electric current affect a person? List and describe the types of electrical injuries.
6. What parameters of electric current determine the severity of electric shock? Specify the threshold current values.
7. What is the most dangerous way for electric current to flow through the human body?
8. Indicate the sources of the greatest electrical hazard in the workplace associated with your future profession.
9. Make a hazard analysis of electrical networks with grounded neutral.
10. Give an analysis of the danger of electrical networks with an isolated neutral.
11. What is the most dangerous contact for a person with live conductors?
12. Why does touching objects electrically connected to the ground (for example, a water pipe) when working with electrical devices dramatically increase the risk of electric shock?
13. Why do I need to remove the electrical plug from the socket when repairing electrical equipment?
14. Why do I need to wear shoes when working with electrical devices?
15. How can the risk of electric shock be reduced?
Great Encyclopedia of Oil and Gas. Schemes for including a person in an electrical circuit
6.2.3. Schemes for including a person in a current circuit
Schemes of inclusion in the current circuit can be different. However, the most characteristic are the connection schemes: between two phases and between one phase and the 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 a two-phase contact, and the second - to a single-phase one.
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 touch voltage (Upr).
A two-phase touch, ceteris paribus, is more dangerous, since the greatest voltage in a given network is applied to the human body - linear, and the current through a person, being independent of the network scheme, neutral mode and other factors, is of the greatest importance:
where is the line voltage, i.e. voltage between the phase wires of the network, V;
Uph - 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 is the resistance of the human body, Ohm.
Rice. 6.1. Cases of a person touching live parts under voltage: a - two-phase switching: b and c - single-phase switching
Cases of two-phase touch are very rare and cannot serve as a basis for evaluating networks for safety 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 current-carrying parts (open circuit breakers, unprotected terminals of welding transformers, etc.).
Single-phase contact, ceteris paribus, is less dangerous than two-phase, 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 injured by current in networks of any voltage. Therefore, only cases of single-phase contact are analyzed below. In this case, both allowed for use three-phase current networks with voltages up to 1000 V are considered: four-wire with a solidly grounded neutral and three-wire with an isolated neutral.
6.2.4. Three-phase networks with solidly grounded neutral
In a three-phase four-wire network with a dead-earthed neutral, the calculation of the contact voltage Upr, and the current Ih passing through a person, in case of touching one of the phases (Fig. 6.2), is easiest to perform using the symbolic (complex) method.
Let us 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, rн - insulation resistance of phase L and zero (combined) PEN wires, Ohm;
C1, C2, C3, Cn - dispersed capacitances of the phase L and neutral (combined) PEN wires relative to the ground, F.
Then the total conductivities of the phase and neutral wires relative to the ground in complex form will be:
where w is the angular frequency, rad/s;
j is an imaginary unit equal to ().
Rice. 6.2. A person touching 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 conductivities 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, the phase conductor L1, the voltage under which he 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-node method, it can be expressed as follows:
Bearing in mind that for a symmetrical three-phase system
where Uf - phase voltage of the source (module), V;
a is a phase operator that takes into account the phase shift, where
we will have equality
Substituting this value in (6.1), we obtain the desired equation of the touch voltage in complex form, acting on a person who has touched the phase conductor L1 of a three-phase four-wire network with a grounded neutral:
We get the current passing through a person if we multiply this expression by Yh:
In the normal mode of 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 and, with some assumption, can be equated to zero, i.e.
Y1 = Y2 = Y3 = Yn = 0
In this case, equations (6.2) and (6.3) become much simpler. So, the touch voltage will be
or (in real 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. Therefore, without a big error in equations (6.4) and (6.5), we can neglect the value of r0 and assume that when a person touches 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 dividing Uph by Rh.
Another conclusion follows from equation (6.5): the current passing through a person who has touched 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 is maintained that the full the conductivities of the wires relative to the ground are very small compared to the conductivity of the grounding of the network neutral.
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 short to ground in a network with a solidly grounded neutral does little to change the voltage of the phases relative to the ground.
In emergency mode, when one of the phases of the network, for example, the phase conductor L3 (Fig. 6.3, a), is closed to the ground through a relatively small active resistance rzm, and a person touches the phase conductor L1, equation (6.2) will take the following form:
Here we also assume that Y1, Y2 and Yn are small compared to Y0 , i.e. equated to zero.
After making the appropriate transformations and taking into account that
get 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. A person touching a phase wire of a three-phase four-wire network with a grounded neutral in emergency mode: a - network diagram; b - vector voltage diagram.
Let's consider two typical cases.
If the resistance of the wires to ground rzm is considered equal to zero, then equation (6.6) will take the form
Therefore, in this case, a person will be under the influence of the linear voltage of the network.
2. If we take the grounding resistance of the neutral r0 equal to zero, then from equation (6.6) we obtain that Unp = Uf, i.e. the voltage that a person will be under will be equal to the phase voltage.
However, in practical conditions, the resistance rzm and r0 is always greater than zero, therefore, the voltage under which a person touches a working phase wire of a three-phase network with a grounded neutral during the emergency mode is always less than linear, but more than phase, i.e.
> Upr > Uf. (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). Thus, for small values of rw and r0 compared to Rh, the first term in the denominator can be neglected. Then the fraction for any ratios of rg and r0 will always be greater than unity, 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 at least two points of the electrical circuit, between which there is a certain potential difference (voltage).
The danger of such a touch is ambiguous and depends on a number of factors:
schemes for including a person in an electrical circuit;
network voltage;
schemes of the network itself;
network neutral mode;
the degree of isolation of current-carrying parts from the ground;
capacitance of current-carrying parts relative to the ground.
Classification of networks with voltage up to 1000 V
Single-phase networks
Single-phase networks are divided into two-wire and single-wire.
Two-wire
Two-wire networks are divided into isolated from the ground and with a grounded wire.
Ground isolated
with earthed wire
These networks are widely used in the 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, metro, etc.).
Three-phase networks
Depending on the neutral mode of the current source and the presence of a neutral or neutral conductor, four schemes can be performed.
The neutral point of the current source is the point at which the voltages at which relative to all phases are the same in absolute value.
The zero point of the current source is a grounded neutral point.
A conductor connected to a neutral point is called a neutral conductor (neutral), and to a zero point - a neutral conductor.
1. Three-wire network with isolated neutral
2. Three-wire sit down with grounded neutral
3. Four-wire network with isolated neutral
4. Four-wire network with grounded neutral
At voltages up to 1000V, circuits "1" and "4" are used in our country.
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 contact - between phase and earth. This assumes the existence of an electrical connection between the network and the ground.
For more information about the schemes for including a person in a chain, see Dolin P.A. Fundamentals of safety in electrical installations.
Single-phase networks
ground isolated
The better the insulation of the wires relative to earth, the less the danger of a single-phase contact with the wire. Touching a person to a wire with a high electrical insulation resistance is more dangerous.
When a wire is shorted to ground, a person who touches a working wire is under voltage equal to almost the full voltage of the line, regardless of the insulation resistance of the wires.
with earthed wire
In this case, the person is almost under the full voltage of the network.
Under normal conditions, touching a grounded wire is practically not dangerous.
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 touch is determined by the total electrical resistance of the wires relative to the ground, with an increase in resistance, the danger of touch decreases.
The touch voltage is almost equal to the line voltage of the network. The most dangerous case.
with earthed neutral
In this case, a person is practically under the phase voltage of the network.
The value of the touch voltage lies between the line and phase voltages, depending on the ratio between the earth fault resistance and the earth resistance.
Measures to ensure electrical safety
Exclusion of human contact with current-carrying parts. It is implemented by locating current-carrying parts in inaccessible places (at 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.
Use of personal protective equipment. Before using PPE, it is necessary to make sure that they are in good condition, 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 it can help when using basic equipment.
- Protective grounding - a deliberate electrical connection of metal non-current-carrying parts that may be energized with the ground or its equivalent (popularly about grounding on geektimes.ru).
In networks up to 1000 V, protective earthing is used in networks with isolated neutral. The principle of operation is to reduce the contact voltage to a safe value.
When grounding is not possible, for protection purposes, the potential of the base on which the person and equipment stands is equalized by raising. 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 in the ground for other purposes, which
jurik-phys.net
Schemes for including a person in an electrical circuit
During the operation of electrical installations, the possibility of a person touching live parts under voltage is not ruled out. In most cases, dangerous contact with current-carrying wattle occurs when a person is standing on the ground and wearing shoes. P has some electrical conductivity.
In the conditions of the tourist complex. The most typical two schemes for including the human body in an electrical circuit are: 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 AC networks are widely used to power various household appliances (vacuum cleaners, refrigerators, irons).
The scheme for including a person in a single-phase two-wire network isolated from the ground is shown in Fig. 41
Fig. 41. A person 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 obtained using isolating transformers. Under normal operating conditions and the wires are well insulated, touching one of the wires 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 can be "arm - leg", and in the second - "arm - arm". current "leg - leg leg".
Three-phase four-wire networks with grounded neutral. With a two-phase (two-pole) touch, a person is under the full operating voltage of the installation. With a 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 the insulation of the network, the floor, and the person's shoes.
Consider the features of various electrical networks. In the tourist complex, there are four wired networks with a tightly grounded neutral with a voltage of 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 (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 a consumer 220. V). In addition, in the event of an insulation failure between the primary and secondary windings when the neutral is earthed 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. Ohm or more for voltages 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 dead ground of the neutral of the power source (transformer or generator) is shown in Figure 42
Figure 42. Single-phase inclusion of a person in a network with a tightly grounded neutral of power sources (transformer)
Due to the low resistance of the spreading current of the working grounding of the neutral 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 section of the "human body" circuit, it is affected by a phase voltage of the network 220. V. If at the same time 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 on the way to "arm - legs" If, under adverse conditions, the resistance of the human body is 1000 ohms, 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 total turn 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 it causes pain, convulsions, and in some cases the inability of the victim to free himself from the action of the current. This proves that a single-phase touch of a person to a network with a tightly grounded neutral is always safe.
In practice, the operation of electrical installations, there may be cases of short circuits to the ground of current-carrying parts, for example, through the housing of the electrical receiver or the metal structure of the electrical wiring. If such a short circuit turns out to be deaf, that is, a small transient resistance, then the installation is switched off through a single-phase short circuit by the maximum stream protection (the fuse fuse blows or the automatic switch is turned off). After that, 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 domestic electrical installations in tourist complexes with voltage up to 1000. V and a frequency of 50. Hz should not exceed the value given in Table 41 (Gosstandart 121038-82-82).
tables 41
Maximum allowable levels of touch voltage and current
Normalized value | Current duration, s | Normalized value |
|||
Three-phase networks with neutral isolated from earth
The placement of electrical energy on 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 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, they are switched on to reduce the capacitance of the current components of a single-phase ground fault.
In the event of a single-phase earth fault in a network with a neutral isolated from earth, a current flows at the point of the earth fault, which is caused by the operating voltage of the installation and the conductivity of the phases relative to earth
networks with an isolated neutral are quite effective for their relatively small length. In this case, we can take the capacitance of the wires relative to the ground to be 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
Figure 43. Human touching the wire of a three-phase 3-wire network with an isolated neutral during normal operation:. A. V,. C - wire designation
In networks with an isolated neutral during normal operation, the danger of electric shock to a person touched by 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 to the case). The purpose of this earthing is to intentionally make an electrical connection to earth or. TI is the equivalent of metal non-current-carrying parts that may be energized using grounded ate devices (a combination of ground electrode and ground conductors). One or more metal electrodes (for example, steel rods, pipes) that are in the ground serve as a grounding conductor, providing a sufficiently low transient resistance. The resistance of a grounded device is called the total resistance, consisting of the current spreading resistance of the ground electrode and the resistance of the grounded conductor.
Consider the action 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 u, then a single-phase inclusion of a person in the current circuit will occur.
In the network, when a ground fault occurs, a single-phase ground fault occurs
Due to the relatively small current that flows to the ground, the protection setting 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 will appear between the body 1 and the ground relative to the ground (Fig. 44.4).
Figure 44. Short circuit to the housing of the electric motor connected to a network with isolated neutral
A person exposed to touch voltage can be significant and depends on where the person's feet are, as well as the electrical conductivity (resistance) of the shoes. As always, the touch voltage is less than the ground voltage.
Thus, the magnitude of the voltage value of the grounded case relative to the ground, and hence 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 electrical installation device. Do not ground at a voltage of 42. V and below AC 1110 V and below DC in all rooms and working conditions without increased danger.
Parts of electrical equipment to be grounded. Grounding is subject to: cases of electrical machines, transformers, devices; 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. The following are not subject to grounding: fittings of suspension and support insulators; brackets and lighting fittings when installed on wooden supports and structures; electrical equipment that is installed on metal grounded structures, if in places of contact in connection with them, metal non-current-carrying parts of electrical equipment are provided with reliable electrical contact. Cases of electrical measuring instruments and relays installed on boards, in cabinets are also not subject to grounding. 1. Wall of switchgear chambers; cases of electrical receivers with double or reinforced insulation, for example, electric drills, washing machines, electric shaver
silting in electrical installations and networks with voltages up to 1000. V is the intentional electrical connection of metal non-current-carrying elements of the installation, normally isolated from live parts that are not energized (housings of electrical equipment, cable structures), with a zero protective conductor.
Zero protective conductor in electrical installations with voltages up to 1000. V is a conductor connecting zeroed parts (housings of electrical equipment) with a tightly grounded neutral point of the winding of the current source (generator or transformer) or its equivalent (GOST 121030-811. State Standard 121009-76-76) .
In electrical installations with a tightly grounded neutral wire, in the event of a short circuit to grounded metal structural non-jet conductors, automatic shutdown of equipment from damage should be ensured. Jenny is insulated because this causes a single-phase short circuit.
Zero protective wires are grounded directly in power sources, i.e. in 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 neutral grounding resistance and serves as a backup ground in case of a break in the neutral ground wire (Fig. 45.5).
Fig 45. Schematic diagram of protective silting: 1 - electrical installation, 2 - maximum jet protection
Re-grounding on overhead lines is done every 250 m of their length, at their ends, at branches and branches from the mains of high-voltage lines with a length of branches of 200 m 1 more, as well as at the inputs of overhead lines. Budin.
With power supply via cable lines with a voltage of 380/220. In the re-grounding of the neutral wire, it is carried out in the introduction to the premises in which the device for grounding electrical appliances is provided, among these premises there must be a re-grounding line of the neutral wire, to which the objects appropriate for grounding are connected.
For re-grounding of the neutral wire, natural grounding conductors should be used whenever possible, with the exception of DC networks, where repeated grounding should be using only mechanical grounding conductors. The resistance of the grounding device of each of the repeated groundings should not be more than 10. Vm.
Considering that a current passes through the neutral wire, even with an uneven load, much less than in the phase wires, the cross section of the zero working wire for the four leading lines is chosen to be approximately equal. Half the cross section of the phase wires. In single-phase branches from the mains, the phase - zero crossing of the neutral wire must be the same as the phase wire, since a current equal to the current of the phase wire passes through it.
The resistance of the grounded wires must be so low that, when a phase is shorted to the case, the current of a single-phase short circuit is sufficient for the instantaneous operation of the overcurrent protection according to no. The PUE of the current of the phase-zero circuit when shorting to the case must not be less than 3 times the rated current of the corresponding fuse.
When protecting the electrical installation with an automatic circuit breaker, the neutral wires are selected in such a way that a short-circuit current is provided in the phase-zero loops, which does not exceed the insertion current of the control switch by 1.4 times.
In the 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 there are parts in this vidgal alkalinization that are subject to zeroing. For the purpose of electrical safety, when mounting lamp cartridges, 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 an accident if the lamp base is accidentally touched (for example, when replacing) without disconnecting from the mains. When zeroing to the illuminated fittings, we should attach a separate branch from the neutral wire, and not use a conductive neutral wire for this purpose.
uchebnikirus.com
Presentation on the topic: TYPES OF ELECTRIC NETWORKS
Earth 01 is generally an equipotential conductor.
UAZ UVZ UСЗ - phase voltages 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 with the ground:
insulation resistance,
ground capacity,
grounding.
INSULATION RESISTANCE
R and - an indicator of the ability of insulating structures to pass electric current under the action of a constant voltage applied to these structures
CAPACITY TO GROUND
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. Charge of static electricity.
studfiles.net
Scheme - inclusion - man
Scheme - inclusion - man
Page 1
Schemes for including a person in a current circuit can be different.
Schemes for including a person in a current circuit can be different. However, two of them are most characteristic: 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 switching, and the second - single-phase.
Schemes for including a person in a current circuit can be different. However, two of them are most characteristic: between two wires and between a wire and ground.
On fig. 4.13 shows a diagram of the inclusion of a person in 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 including a person in an electrical circuit, the degree of isolation of current-carrying parts from the ground.
Single-phase (single-pole) contact occurs much more often than two-phase, therefore, this scheme for connecting a person to an electrical network is given the main attention.
In the conditions of technological workshops, the contact voltage depends on the voltage of the network, its circuit, the neutral mode, the circuit for connecting a person to an electrical circuit, and the degree of isolation of current-carrying parts from the ground.
In the conditions of technological shops, the touch voltage depends on the mains voltage, its circuit, the neutral mode, the circuit for connecting a person to the circuit, the degree of isolation of current-carrying 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 ground on which he stands. With any single-phase inclusion of a person in 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 case of damage, can lose electrical resistance, which is very high in the normal state.
Schemes for including a person in an electrical circuit can be bipolar and unipolar.
Electrical installations produce, transform, distribute and consume electricity. During their operation, a person may be in the field of an electromagnetic field or in direct contact with current-carrying 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 defeat depends on the magnitude of the current, the duration of exposure, the type of current (direct or alternating), frequency, current path (schemes 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 the 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, estimated by the magnitude of the current passing through the human body, or by the voltage of the touch, depends on a number of factors: the circuit for connecting a person to the circuit, the network voltage, the circuit of the network itself, the mode of its neutral, the degree of isolation of current-carrying parts from the ground, and also from the value of the capacitance of current-carrying parts relative to the ground, etc.
Schemes for including a person in a chain can be different. However, the most characteristic are two switching 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.
In relation to AC networks, the first circuit is usually called two-phase switching, and the second - single-phase.
Two-phase switching, 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 - linear voltage, i.e. voltage between the phase wires of the network, V; Uf - 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 inclusion; b, c - single-phase inclusions
It is easy to imagine that two-phase switching is equally dangerous in a network with both isolated and grounded neutrals.
With a two-phase connection, the danger of injury will not decrease even if the person is reliably isolated from the ground, i.e. 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 one, that is, 1.73 times less than the linear one. Accordingly, the current passing through the person is less.
In addition, the value of this current is also affected 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 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 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 the same for all wires of the network).
Rice. 69. Touching a person to the wire of a three-phase three-wire network with an isolated 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 earth is small, i.e. C = 0, which usually takes place in overhead networks of small length, then equation (35) will take the form
, (36)If the capacitance is large, and the conductivity of the insulation is insignificant, i.e. r ≈ ∞, which usually takes place in cable networks, then according to expression (35), the current through a person (A) will be:
, (37)where xc \u003d 1 / wC - capacitance, Ohm.
It follows from expression (36) that in networks with an isolated 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 relative to the ground: with increasing resistance, the danger decreases.
Therefore, it is very important to ensure high insulation resistance in such networks and monitor its condition in order to timely identify and eliminate faults.
However, in networks with a large capacity 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, i.e. when one of the phases was shorted to the ground through a small resistance gzm, the current through a person who touched a healthy phase (Fig. 69, b) will be (A):
, (38) and contact voltage (V): , (39)If we assume that rzm = 0 or at least assume that rzm< Rh (так обычно бывает на практике), то согласно выражению (39)
, (40)i.e., a person will be under linear voltage.
Under actual conditions, gzm > 0, therefore, the voltage under which a person who touches a healthy phase of a three-phase network with an isolated neutral during an emergency period will be significantly greater than the phase 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 during normal operation.
work [see equations (36) and (39), bearing in mind that r/3>rzm].
In a three-phase four-wire network with a grounded neutral, the conductivity of the insulation and the capacitance of the wires relative to earth are small compared to the conductivity of the neutral ground, so when determining the current through a person touching the phase of the network, they can be neglected.
In the normal mode of operation of the network, the current through a person will be (Fig. 70, a):
, (41)where r0 is the neutral grounding resistance, Ohm.
Rice. 70. Touching a person to a 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. equations (38) and (41)], since in some cases rzm can differ little from r0.
Helpful information:ohrana-bgd.narod.ru
Lecture summary
APPROVED
Head cafe OP KHNURE
prof. Dziundzyuk B.V.
"____" _______2014
from the discipline "Fundamentals of the protection of practice"
Topic 2.2: "Wash the injured people with an electric jet"
Lecturer - Art. wickl cafe. OP
Mamontov O.V.
2.2.1 Schemes for including a person in an electric circuit
According to the PUE, the risk of electric shock is possible with direct and indirect contact of a person or animals with parts of electrical installations that are energized.
Direct contact is the electrical contact of people or animals with current-carrying parts that are energized, or approaching them at a dangerous distance.
Indirect touch is the electrical contact of people or animals with an open conductive part that has become 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 thus a closed circuit is formed, a current passes through his body. The value of this current depends on the touch circuit, i.e., which parts of the electrical installation the person touches, as well as the parameters of the electrical network. Without touching the network parameters, we will consider the schemes for switching on a person.
Two-phase (two-pole) contact with live parts
On fig. 1 a and 1 b shows direct contact with two poles of a single-phase network. In this case, the current through the human body is equal to
Operating (phase) mains voltage, 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) contact with live parts
If a person, standing on the ground, touches one of the poles or one of the phases, the current closes through him to the ground and then through the insulation resistance and phase capacitance 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 magnitude of the current depends on the insulation resistance and the capacitance of the phases relative to the ground (to 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 isolated neutral (a) and in a three-phase network with grounded neutral (b)
Shortcut http://bibt.ru
9.2. Schemes of the possible inclusion of a person in an electric current circuit.
During the operation of electrical installations, the possibility of a person accidentally touching live parts that are energized is not ruled out. The touch will be most dangerous if a person is standing on the ground or on a conductive base (floor, platform) and his shoes have some electrical conductivity.
Human contact with live parts can be single-phase (single-pole in DC circuits) or two-phase (two-pole). In both cases, an electrical circuit is formed, one of the sections of which will be the human body. The current path through a person in the first case can be "arm - legs". In the second case - "hand - hand". Other schemes for including a person in an electrical circuit are also possible, for example, when touching current-carrying parts with the face, neck, back, etc., or switching on "leg - leg".
With a two-phase (two-pole) connection, a person is under the full operating voltage of the electrical installation and the current passing through it will be equal to
I people \u003d U l / R people, (9.1)
where U l - linear voltage; R people - the resistance of the human body.
With a single-pole (single-phase) touch, which is more common, the current flowing through a person will depend not only on the voltage of the electrical installation and the resistance of the human body, but also on other factors: the neutral mode of the power source, the state of network insulation, the state (electrical conductivity) of the floor, human shoes, air humidity, etc.
During the operation of electrical installations, the possibility of a person touching live parts under voltage is not ruled out. In most cases, it is dangerous to touch live parts when a person is standing on the ground, and P shoes 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 a wire and ground. In three-phase AC networks, the first circuit is called two-phase switching, and the second - single-phase. In the hotel industry, in addition to three-phase AC networks, single-phase AC networks are widely used to power various household appliances (vacuum cleaners, refrigerators, irons).
The scheme for including a person in a single-phase two-wire network isolated from the ground is shown in fig. 4.1.
Rice. 4.1. A person 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 isolating transformers. Under normal operation and good 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, the 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 including a person in an electrical circuit are also possible, for example, touching current-carrying parts with the face, head, neck, or switching on the leg-to-foot current path.
Three-phase four-wire networks with grounded neutral. With a two-phase (two-pole) contact, a person is under the full operating voltage of the installation. With unipolar 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 network insulation, floors, and human shoes.
Consider the features of various electrical networks. In the tourist complex, there are four leading networks with a tightly grounded neutral voltage 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". 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 the phase voltage, that is, for a transformer with a secondary voltage of 400 V, it will be no more than 230 V (to the consumer 220 V). In addition, in the event of an insulation failure between the primary and secondary windings when the neutral is grounded, the highest voltage goes to the secondary network in relation to the ground, is significantly reduced due to the small neutral grounding resistance (2.4.8 ohms or more for a voltage of 660, 380 and 220 V three-phase network (Gosstandart 12.1.030-81)).
A simplified diagram that explains the single-pole touch of a person to a four-wire network with dead earthing of the neutral of the power source (transformer or generator) is shown in fig. 4.2.
Rice. 4.2. Single-phase inclusion of a person in a network with a tightly grounded neutral of power sources (transformer).
Due to the low resistance of the spreading current of the working grounding of the neutral compared to the resistance of the human body, it is equal to zero. The touch of a person who is standing on the ground (or on a grounded structure, floor) causes a closed electrical circuit: power supply winding - line wire - human body - earth - wire - working grounding - source winding. In the “human body” section of the circuit, it is affected by a phase voltage of the network 220 V. If at the same time the person’s shoes are electrically conductive, then the floor or structure on which it stands will also be electrically conductive, and almost all the voltage will be applied to the person along the path “hand - legs ". If, under adverse conditions, the resistance of the human body is 1000 ohms, 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 total are 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 a 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 a single-phase human contact with a network with a tightly grounded neutral is always dangerous.
In practice, the operation of electrical installations, there may be cases of short circuits to the ground of current-carrying parts, for example, through the body of the power receiver or the metal structure of the electrical wiring. If such a circuit turns out to be deaf, that is, a small transient resistance, then the installation is switched off through a single-phase short circuit by the maximum brook protection (the fuse blows or the circuit breaker is turned off). After that, 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 domestic electrical installations in tourist complexes with voltage up to 1000 V and a frequency of 50 Hz should not exceed the values \u200b\u200bspecified in Table. 4.1 (Gosstandart 12.1.038-82).
Table 4.1.
Maximum allowable levels of touch voltage and current
Normalized value |
Current duration, s |
Normalized value |
|||
Three-phase networks with the neutral isolated from the earth.
The placement of electrical energy on 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 power 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, they are switched on to reduce the capacitance of the component current of a single-phase earth fault.
In the event of a single-phase earth fault in a network with an isolated neutral from earth, a current will flow at the point of the earth fault, caused by the operating voltage of the installation and the conductivity of the phases relative to earth.
Networks with an isolated neutral are quite effective with a relatively small length. In this case, we can take the capacitance of the wires relative to the ground to be zero, and the resistance of the wires is large enough.
On fig. 4.3 shows the inclusion of a person in three-phase networks with an isolated neutral.
Rice. 4.3. A person touching a wire of a three-phase 3-wire network with an isolated neutral during normal operation A. B, C - designation of wires.
In networks with isolated neutral, during normal operation, the danger of electric shock to a person touched one of the phases. depends on the resistance of the conductor relative to earth, that is, with increasing resistance, the danger decreases.
Protective grounding is one of the protective measures against electric shock to a person when touching non-conductive metal parts with damaged insulation (for example, a short to the case). The purpose of this earthing is to intentionally electrically connect to earth or TE equivalent non-conductive metal parts that may be energized by means of earthed devices (combination of earth electrode and earth conductors). One or more metal electrodes (for example, steel rods, pipes) that are in the ground serve as a grounding conductor, providing a sufficiently low transient resistance. The resistance of a grounded device is called the total resistance, consisting of the resistance of the spreading of the ground current and the resistance of the grounded conductors.
Consider the action 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 inclusion of a person in the current circuit will occur.
In the network, when a ground fault occurs, 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 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 body 1 (Fig. 4.4).
Rice. 4.4. Short circuit on the case of the electric motor connected to a network with isolated neutral.
A person who will be exposed to touch voltage, which can be significant and depends on where the person's feet are, as well as the electrical conductivity (resistance) of the shoes. As always, the touch voltage is less than the ground voltage.
Thus, the magnitude of the voltage value of the grounded case relative to the ground, and hence 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 AC 1 110 V and below DC in all rooms and working conditions without increased danger.
Parts of electrical equipment to be grounded. Grounding is subject to: cases of electrical machines, transformers, devices; drives of electrical 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. The following are not subject to grounding: fittings of suspension and support insulators; brackets and lighting fittings when installed on wooden supports and structures; electrical equipment, installed on metal grounded structures, if reliable electrical contact is provided at the points of contact with them of metal non-current-carrying parts of electrical equipment. Cases of electrical measuring instruments and relays installed on boards, in cabinets 1, walls of switchgear chambers are also not subject to grounding; cases of electrical receivers with double or reinforced insulation, for example, electric drills, washing machines, electric shavers.
Silting in electrical installations and networks with voltage up to 1000 V is a deliberate electrical connection of metal non-current-carrying elements of the installation, normally isolated from live parts that are not energized (electrical equipment cases, cable structures), with a zero protective conductor.
Zero protective conductor in electrical installations with voltage up to 1000 V is a conductor connecting grounded parts (housings of electrical equipment) with a tightly grounded neutral point of the current source winding (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 closing to grounded metal structural non-stream-conductive parts, automatic shutdown of equipment with damaged insulation should be ensured, since this causes a single-phase short circuit.
Zero protective earth wires directly in power sources, that is, 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 total neutral grounding resistance and serves as a backup ground in case of a break in the neutral ground wire (Fig. 4.5).
Rice. 4.5. Schematic diagram of protective silting: 1 - electrical installation; 2 - maximum inkjet protection
Re-grounding on overhead lines is done every 250 m of their length, at their ends, at branchings and branches from the mains of high-voltage lines with a length of branches of 200 m 1 more, as well as at the inputs of air lines into the house.
When power is supplied via cable lines with a voltage of 380/220 V, 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. Inside these rooms there should be a line for re-grounding the neutral wire, to which objects appropriate for grounding are connected.
To re-ground the neutral wire, if possible, use natural ground electrodes, excluding DC networks, where re-grounding should be using only artificial ground electrodes. The resistance of the grounding device of each of the repeated groundings should not exceed 10 ohms.
Considering that a current passes through the neutral wire, even with an uneven load, much less than in the phase wires, the cross section of the zero working wire for the four leading lines is chosen to be approximately Half the intersection of the phase wires. In single-phase branches from the mains, the phase - zero crossing of the neutral wire must be the same as the phase wire, since a current passes through it, which 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 case, the current of a single-phase short circuit is sufficient for the instantaneous operation of the overcurrent protection. According to PUE. circuit current phase - zero when shorting to the body must be at least 3 times the rated current of the corresponding fuse.
When protecting the electrical installation with an automatic switch, the neutral wires are selected so that in the phase-zero loop to provide a short-circuit current 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 there are parts in this branch that are subject to zeroing. For the purpose of electrical safety, when mounting lamp cartridges, 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 an accident if the lamp base is accidentally touched (for example, during P replacement) without disconnecting from the mains. When zeroing, separate branches from the neutral wire should be connected to the illuminated fittings, and not using a conductive neutral wire for this purpose.