The severity of electric shock is largely determined by the scheme of 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 widely used are four-wire networks with a voltage of 380/220 V. What is it? Four wires go from the source of electrical energy to consumers, three of which are called phase, and one is zero. The voltage between two phase wires is 380V (this voltage is called linear), and between the neutral wire and any of the phase wires is 220V (this voltage is called phase).
To power lighting installations, televisions, refrigerators, a single-phase network is used - one phase wire and a neutral wire (that is, 220 V). The most common electrical networks in which the neutral wire is grounded. Touching the neutral wire is practically no danger to humans; only the phase wire is dangerous. However, it is difficult to figure out which of the two wires is zero - they look the same in appearance. This is done using a special device - a phase determiner.
Consider possible schemes for including a person in an electrical circuit when touching the current conductors of a single-phase (two-wire) network. The rarest, but also the most dangerous, is the touch of a person on two wires or current conductors connected to them.
Suppose you decide to repair the wiring - insulate the wires, repair or install a new socket and switch, but forgot to turn off the power supply. When performing installation work, you touched the phase wire with one hand and the neutral wire with the other. A current will flow through you along the hand-to-hand path, that is, the resistance of the circuit will include only the resistance of the body. If we take the resistance of the body to 1 kOhm (this figure is usually taken in calculations), then according to Ohm's law, current will flow through you:
I (current) \u003d 220 V: 1000 Ohm \u003d 0.22 A \u003d 220 mA.
It's deadly dangerous current. The severity of the electrical injury, and even your life, will depend, first of all, on how quickly you get rid of contact with the current conductor (break the electrical circuit), because the exposure time in this case is decisive.
When working with electrical wiring, be sure to turn off the power supply, and hang a warning sign on the switch: “Do not turn on - people are working,” or rather, put an observer.
Electric shock can occur when repairing household electrical appliances (vacuum cleaner, coffee maker, washing machine), television and radio equipment. You know very well that it is impossible to work under voltage, and you turned off the power supply with the switch on the electrical appliance. However, in this case, the voltage will be at the input contacts of the switch. In the process of work, you can forget about it and touch them or accidentally press the switch and turn on the electric current. The voltage on some elements of household equipment can reach very high values. For example, the voltage supplied to the cathode-ray tube of a TV set, a PC monitor reaches 15000-18000 V.
Repair of electrical appliances, television and radio equipment, electrical equipment can only be carried out with the electrical plug of the device removed from the socket.
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 electrically connected to it.
You decide to drill a hole with an electric drill. You have not used the drill for a long time, but it was in good condition. Your work can be completed both successfully and end with an electric shock of varying severity - from a slight blow to death. Why might this happen? Insulation ages over time, and its insulating properties deteriorate (electrical resistance decreases). Insulation deteriorates especially quickly when it is kept in a damp room or in an aggressive environment (for example, in an environment of sulfuric acid vapors) for a long time. Conductive dust, water that got into the drill can close the phase conductor to the body (handle) of the drill. The insulation of the lead wires can be chewed on by a mouse. If the body of the electric drill is metal, you actually come into contact with the phase wire, if it is plastic, electrical contact may occur if the integrity of the body is broken (crack) or the body is wet.
How will current flow through a person, and what electrical circuit is formed? If the second hand also rests on the body of the drill or does not touch any other conductive objects, the current will flow along the arm-to-feet path. The current through a person, shoes, base (floor), reinforced concrete structures of the building will drain into the ground and through it to the neutral wire (after all, the neutral wire is grounded). A closed electrical circuit is formed, the magnitude of the current in which will be determined by its total electrical resistance. If you stand on a dry wooden floor in insulating dry shoes (leather, rubber), the resistance of the circuit will be large, and the current strength, according to Ohm's law, will be small.
For example, the floor resistance is 30 kOhm, leather shoes 100 kOhm, human resistance is 1 kOhm. The current that will flow through a person:
I (current) \u003d 220 V: (30000 + 100000 + 1000) Ohm \u003d 0.00168 A \u003d 1.68 mA.
This current is close to the threshold perceptible current. You will feel the current flow, stop working, fix the problem.
If you stand barefoot on wet ground, a current will flow through your body:
I (current) \u003d 220 V: (3000 + 1000) Ohm \u003d 0.055 A \u003d 55 mA.
This current can cause damage to the lungs and heart, and with prolonged exposure, death. If you are standing on wet ground with dry and intact rubber boots, a current will flow through your body:
I (current) \u003d 220 V: (500000 + 1000) Ohm \u003d \u003d 0.0004 A \u003d 0.4 mA.
You may not feel the flow of such a current. But a small crack or puncture in the sole of a boot can drastically reduce the resistance of the rubber sole and make the job 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, dried. Wet electrical devices must not be operated! It is better to store electrical tools, appliances, equipment in plastic bags to prevent dust or moisture from entering them. Work must be done in dry shoes. If the reliability of an electrical device is in doubt, you need to play it safe - put dry wooden flooring or a rubber mat under your feet. You can use rubber gloves.
Another current flow pattern occurs when your other hand touches a highly conductive object that is electrically connected to ground. It can be a water pipe, a radiator, a metal garage wall, etc. Current flows along the path of least electrical resistance. These objects are almost short-circuited to the ground, their electrical resistance is very small. The path of current flow through the body in this case is “hand-hand”, that is, it practically coincides with the case of simultaneous touching of hands with two wires - phase and zero. As shown earlier, the current can reach 220 mA, i.e. deadly. In a damp room, even wooden structures become good conductors of electricity.
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. Often in such rooms low voltages are used - 36 and 12 volts.
When working with electrical devices, do not touch objects that may be electrically connected to earth.
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 that carry much higher voltages and are therefore more dangerous. However, the main conclusions and recommendations for ensuring security are almost the same.
Issues of output control.
1. What kind of contact with live conductors is the most dangerous for a person?
2. Why does touching objects connected to earth (for example, a water pipe) when working with electrical devices dramatically increase the risk of electric shock?
3. Why is it necessary to remove the electrical plug from the socket when repairing electrical equipment?
4. Why do I need to wear shoes when working with electrical devices?
5. How can I reduce the risk of electric shock?
6. What electrical safety rules must be observed when operating electrical devices?
7. A man, while in a bathtub filled with water, decided to shave with an electric razor. What can happen and what is the danger of electric shock to a man?
8. The girl took a bath and, standing barefoot on the wet tiled floor, decided to dry her hair with a hairdryer. Assess the danger and possible consequences.
9. Tell about the cases of electric shock that happened to you or other people. What was the cause of the defeat and what electrical safety rules were violated?
10. On the instructions of the teacher, who sets the network parameters and the scheme for a person to touch wires or live objects, assess the risk of electric shock.
I. On cars, a direct electric current with a voltage of 12V is used. The negative pole of the car is connected to the car body, the positive pole is connected to the insulated electrical wiring. Assess the danger of such a current for a person.
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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.
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.
Neutral point of current source- a point, the voltages at which, relative to all phases, are the same in absolute value.
Zero point of current source- 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
Biphasic 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. 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
Normal mode
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.
Emergency mode
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
Touching an ungrounded wire
In this case, the person is almost under the full voltage of the network.
Touching a grounded wire
Under normal conditions, touching a grounded wire is practically not dangerous.
Touching a grounded wire. Emergency operation
In the event of a short circuit, the voltage on the grounded wire can reach dangerous values.
Three-phase networks
With isolated neutral
Normal mode
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.
Emergency mode
The touch voltage is almost equal to the line voltage of the network. The most dangerous case.
with earthed neutral
Normal mode
In this case, a person is practically under the phase voltage of the network.
Emergency mode
The value of the touch voltage lies between the line and phase voltages, depends 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 double insulation.
For example, the execution of an electrical installation housing from a dielectric.
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 provide immediate protection against electric shock.
Additional protective equipment cannot provide security on their own, but can help with the use of fixed assets.
Equipment and network isolation control.
- Output control.
- Planned.
- Extraordinary, etc.
Protective separation of networks.
Allows you to reduce the capacitance of lines near consumers of electrical energy.
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 that can be used as grounding conductors. Exceptions according to the criterion of fire and explosion hazard (gas pipelines, etc.).
Grounding resistance should be no more than a few ohms. At the same time, as a result of corrosion, the resistance of the ground electrode increases over time. Therefore, its value must be periodically monitored (winter/summer).
Protective grounding - a deliberate connection of metal non-current-carrying parts that may be energized with a repeatedly grounded zero protective conductor.
Scope - electrical installations with grounded neutral with voltage up to 1000V.
The principle of operation is the transformation of a short circuit to the equipment case into a single-phase short circuit, with the subsequent shutdown of the equipment when the maximum allowable current strength is exceeded.
Current protection is implemented either with circuit breakers or fuses. Special attention it is necessary to pay attention to the choice of the thickness of the neutral protective conductor sufficient to conduct the short-circuit current.
The use of RCDs (residual current devices).
This type of protection is triggered when the incoming and outgoing currents in the monitored circuit do not match in magnitude, i.e., when there is a current leakage. For example, when a person touches a phase wire, part of the current goes past the main circuit into the ground, which causes the equipment to be turned off in the controlled circuit. More details.
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 networks alternating current The first circuit is called - two-phase inclusion, 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 housing 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 single-phase switching person in the circuit.
In the network, when a ground fault occurs, a single-phase ground fault occurs.
Due to the relatively small current flowing to ground, established by the defense 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. circuit diagram 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.
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 touch voltage (U etc).
Two-phase contact, ceteris paribus, is more dangerous, since the greatest voltage in this 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 
-
line voltage, i.e. voltage between the 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.
Rice. 6.1. Cases of a person touching live parts under voltage: a - two-phase inclusion: b and c - single-phase inclusions
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 solidly grounded neutral, the calculation of the touch voltage U etc , and current I h passing through a person, in case of touching one of the phases (Fig. 6.2), it is easiest to perform 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.
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 the ground, F.
Then the total conductivities 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 ( 
).
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 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, the phase conductor L1, the voltage under which he will be determined by the expression
, (6.1)
The current is found by the formula
where 
- complex voltage of phase 1 (phase voltage), V;
-
complex voltage between the neutral of the current source and earth (between the points 00"
on the equivalent circuit).
Using the well-known two-node 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;
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:
. (6.2)
The current passing through a person, we get if we multiply this expression by Y h :
. (6.3)
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.
Y 1 = Y 2 = Y 3 = Y n = 0
In this case, equations (6.2) and (6.3) become much simpler. So, the touch voltage will be
,
or (in real 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 assume 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 .
From equation (6.5) one more conclusion follows: 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 total conductivities of the wires relative to the ground are very small compared to the conductivity network neutral grounding.
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 r gp, and a person touches the phase conductor L1, equation (6.2) will take the following form:
.
Here we also accept that Y 1 , Y 2 and Y n small compared to Y 0 , i.e. equated to zero.
After making the appropriate transformations and taking into account that
,
and 
,
get the touch voltage in real form
.
To simplify this expression, let us assume that
.
As a result, we finally obtain that the voltage U etc equals
. (6.6)
The current passing through a person is determined by the formula
.
(6.7)
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 r gp be considered equal to zero, then equation (6.6) takes the form
.
Therefore, in this case, a person will be under the influence of the linear voltage of the network.
2. If we take equal to zero the neutral grounding resistance r 0 , then from equation (6.6) we obtain that U np = U f , those. the voltage that a person will be under will be equal to the phase voltage.
However, in practical conditions of resistance r gp and r 0 always greater than zero, so 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.
> U etc > 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 gp
and r 0
compared with R h ,
the first term in the denominator can be neglected. Then the fraction for any ratio r gp
and r 0
will always be greater than one, but less 
, i.e. we obtain expression (6.8).