Degree of danger and outcome of the defeat electric shock depend: on the scheme of “connection” of a person in electrical circuit; on the electrical network:
three-phase four-wire with grounded neutral;
three-phase with isolated neutral.
The neutral point of a transformer (generator) is the connection point of the windings of the supply transformer. During normal operation of the electrical network, the voltage at this point is 0. The neutral of the power source can be grounded and isolated from the ground, this determines its operating mode. Neutral grounding is called working grounding R 0 .
The choice of network diagram and neutral mode of the current source is carried out depending on technological requirements and safety conditions.
By technological requirements preference is given to a four-wire network, since this network is characterized by two voltages - linear and phase (380/220 V). A linear voltage of 380 V powers the power load - they turn on the electric motors of production equipment between the phase wires. Phase voltage = 220 V is used for a lighting installation - lamps are connected between the phase and neutral wires. Line voltage is always 1.73 times greater than phase voltage.
By safety conditions It is advisable to use networks with an isolated neutral when it is possible to maintain high level network insulation, providing low wire capacitance relative to ground. These may be sparsely branched networks, not exposed to aggressive environment and under constant supervision of qualified personnel.
A single-phase connection is less dangerous than a two-phase connection, but it occurs much more often and is the main cause of electrical injuries. In this case, the neutral mode of the electrical network has a decisive influence on the outcome of the defeat.
When you touch one of the phases of the network with an isolated neutral (Fig.), in series with the human resistance, the insulation and capacitance resistances relative to the ground of the other two undamaged phases turn on.
Rice. Single-pole contact with isolated neutral during normal operation
During normal operation of the electrical network, the neutral voltage of the power source in relation to ground is zero. The phase voltages relative to ground are identical and equal to the phase voltages of the power source.
The insulation resistance of wires is never infinitely large; leakage currents necessarily occur.
The wires and ground in this case are like the plates of a capacitor, between which an electric field arises. The longer the electrical network, the greater its capacity.
According to technological requirements, preference is given to a four-wire network, since this network is characterized by two voltages - linear and phase (380/220 V). A linear voltage of 380 V powers the power load - they turn on the electric motors of production equipment between the phase wires. Phase voltage = 220 V is used for a lighting installation - lamps are connected between the phase and neutral wires. Line voltage is always 1.73 times greater than phase voltage.
According to safety conditions, networks with an isolated neutral are advisable to use when it is possible to maintain a high level of network insulation, ensuring a low capacitance of the wires relative to the ground. These can be thinly branched networks that are not exposed to aggressive environments and are under the constant supervision of qualified personnel.
Networks with a grounded neutral are used where it is impossible to ensure a high level of insulation of the electrical installation or where damage cannot be quickly found and repaired.
Due to the specifics and insignificant production capacity compared to other food industry enterprises, one- and two-phase networks with a grounded neutral can be used in public catering establishments, and when operating small-scale mechanization equipment during loading and unloading operations, it is recommended electrical network with isolated neutral. The degree of electrical safety in such networks increases due to the high insulation resistance of electrical wires in relation to the ground.
Electric shock to a person can be caused by single-pole (single-phase) or double-pole (two-phase) contact with a live part of the installation.
As the insulation resistance increases, the risk of electric shock decreases.
During emergency operation of the same network, when a solid phase-to-ground fault occurs, the voltage at the neutral point can reach the phase voltage, the voltage of the undamaged phases relative to the ground becomes equal to the line voltage. In this case, if a person touches one phase, he will be under linear voltage, and current will flow through him along the “arm-leg” path. In this situation, the insulation resistance of the wires does not play any role in the outcome of the injury. Such electric shock most often leads to death.
In enterprises where the networks are branched and have a significant length, and therefore a large capacity, a system with an isolated neutral loses its advantage, since the leakage current increases and the resistance of the phase-ground section decreases. From the point of view of electrical safety, in such cases, preference is given to a network with a grounded neutral (Fig.).
Scheme of a person touching one phase of a network with a grounded neutral
Ground resistance, as in the case of an electrical network with an isolated neutral, can be neglected.
Examples indicate that, other things being equal, single-phase connection connecting a person to a network with an isolated neutral is less dangerous than to a network with a grounded neutral.
The most dangerous is the two-phase connection of a person to the electrical network, since he comes under the linear voltage of the network, regardless of the neutral mode and operating conditions of the network.
Cases of two-phase contact occur rarely and mainly in electrical installations up to 1000 V when working on switchboards and assemblies, when operating equipment with non-insulated live parts, etc.
Book table of contents Next page>>§ 3. Danger of electric shock.
Scheme of a single-phase connection of a person to a three-phase current network with a grounded neutral.
Electric shock occurs when an electrical circuit closes through the human body. This occurs when a person touches at least two points of an electrical circuit, between which there is some voltage. The inclusion of a person in a circuit can occur in several ways: between the wire and the ground, called single-phase connection; between two wires - two-phase connection. These schemes are most typical for three-phase AC networks. It is also possible to switch between two wires and ground at the same time; between two points on the earth having different potentials, etc.
Single-phase connection of a person to the network represents direct contact of a person with parts of an electrical installation or equipment that are normally or accidentally energized. In this case, the degree of danger of injury will vary depending on whether the electrical network has a grounded or insulated neutral, as well as depending on the quality of the insulation of the network wires, its length, operating mode and a number of other parameters.
When connected single-phase to a network with a grounded neutral, a person comes under phase voltage, which is 1.73 times less than linear, and is exposed to current, the magnitude of which is determined by the value of the phase voltage of the installation and the resistance of the human body (Fig. 69). An additional protective effect is provided by the insulation of the floor on which a person stands and shoes.
Rice. 69. Scheme of single-phase connection of a person to the network three-phase current with grounded neutral
Thus, in four wire three-phase network with grounded neutral current circuit, passing through a person, includes the resistance of his body, as well as the resistance of the floor, shoes and grounding of the neutral of the current source (transformer, etc.). In this case, the current value
where U l - linear voltage, V; R t - human body resistance, Ohm; R p - resistance of the floor on which the person is located, Ohm; R rev - resistance of a person’s shoes, Ohm; R 0 - neutral grounding resistance, Ohm.
As an example, consider two cases of single-phase connection of a person to a three-phase four-wire electrical network with a grounded neutral at U l = 380 V.
A case of adverse conditions. A person who touches one phase is on damp ground or a conductive (metal) floor, his shoes are damp or have metal nails. In accordance with this, we accept resistance: human body R t = 1000 Ohm, soil or floor R p = 0; shoes R rev = 0.
The neutral grounding resistance R0 = 4 Ohms is not taken into account due to its insignificant value. A current will pass through the human body
being life-threatening.
A case of favorable conditions. A person is on a dry wooden floor with a resistance of R p = 60,000 Ohm, and has dry, non-conductive (rubber) shoes on his feet with a resistance of R rev = 50,000 Ohm. Then a current will pass through the human body
which is long-term acceptable for humans.
In addition, dry floors and rubber shoes have significantly greater resistance in comparison with the values accepted for the calculation.
These examples show the great importance of the insulating properties of the floor and shoes to ensure the safety of persons working in conditions of possible contact with electric current.
1) Single-phase contact with a network wire with an insulated neutral with good insulation (Fig. 1):
Figure 1 - Single-phase connection of a person to the electrical network.
The current passing through a person I h returns to the current source through the insulation of the network wires, which in good condition has a high insulation resistance R from. Up to 1000V R from is equal to 0.5 MOhm or more. The current flowing through the human body is determined by the expression:
(1)
where R h is the resistance of the human body, 1000 Ohms are taken for calculations;
R from - phase insulation resistance relative to ground;
U f - phase voltage
Taking into account the resistance of the shoes R about and the floor R p, connected in series with the resistance of the human body R h, the current passing through the person will be equal to:
(2)
2) Single-phase contact with a network wire with a grounded neutral (Fig. 2):
Figure 2 - Single-phase contact with a grounded neutral
The magnitude of the current through a person is determined only by the resistance of the human body; the resistance values of the wire insulation do not affect the current passing through the human body.
, (3)
where R 0 is the neutral grounding resistance. When Ul = 380 V R 0 does not exceed 4 0 m, then it can be neglected in calculations. In this case, the resistance of the floor and shoes play a big role in human safety, because connected in series with a person.
(4)
When R p = 0 and R rev = 0
Ih = = 0,22 A = 220 mA> 100 mA >> 10 mA ,
this is very dangerous!
When a phase is shorted to ground, a network with an isolated neutral (Fig. 4) turns out to be more dangerous than one with a grounded neutral (Fig. 5). Since, in a network with an isolated neutral, the voltage that determines the amount of current through the human body is equal to U l, and in a network with a grounded neutral it lies within the limits:
U l >U pr >U f
Figure 4 - Network with isolated neutral
Ih= 
, (7)
where R h is the resistance of the human body;
R zm - earth phase circuit resistance
In the event of a phase breakdown on the body of equipment, which under normal conditions should not be energized, the person working with this equipment finds himself in single-phase contact mode. To protect against electric shock in a network with isolated neutral is used protective grounding (Fig. 6).
Figure 5 - Network with grounded neutral
Protective grounding
Protective grounding is carried out in order to ensure the safety of people in case of violation of the insulation of live parts. Grounding is also used to protect electrical equipment, buildings and structures from atmospheric electricity.
Protective grounding is the intentional connection to the ground or its equivalent of metal parts of equipment that are not energized under normal conditions, but may become energized due to a violation of the insulation of electrical installations.
The effect of protective grounding is that it reduces the voltage between the energized equipment frame and the ground to a safe value.
Let us explain this using the example of a network with an isolated neutral (Fig. 6). If the housing of electrical equipment is not grounded and it is in contact with a phase, then human touch to such a housing is equivalent to a single-phase switching on. If the case is grounded, then the potential of the case relative to the ground drops to a safely low value.
Figure 6 - Protective grounding
It is necessary to ground metal parts of electrical installations, housings of electrical machines, transformers, devices, lamps, drives of electrical devices, secondary windings of instrument transformers, frames of distribution boards, control panels, cabinets, etc.
Protective grounding is used in three-phase three-wire networks with voltages up to 1000 V with an isolated neutral, and in networks with voltages of 1000 V and above - with any neutral mode (Fig. 3.18).
Electric shock to a person as a result of electrical influence, i.e., the passage of current through a person, is a consequence of his touching 2 points of an electrical circuit, between which there is some voltage. The danger of such a touch is assessed, as is known, by the current passing through the human body or the voltage under which it finds itself. It should be noted that the touch voltage depends on a number of factors: the circuit of connecting a person to the electrical circuit, the network voltage, the circuit of the network itself, the mode of its neutral, the degree of insulation of live parts from the ground, as well as the capacitance of live parts relative to the ground, etc.
Consequently, the danger indicated above is not unambiguous: in one case, the inclusion of a person in an electrical circuit will be accompanied by the passage of small currents through him and will not be very dangerous; in other cases, the currents can reach significant values that can lead to death. This article examines the dependence of the danger of including a person in an electrical circuit, i.e., the value of the touch voltage and the current flowing through a person, on the listed factors.
This dependence must be known when assessing a particular network according to safety conditions, selecting and calculating appropriate protective measures, in particular grounding, zeroing, protective shutdown, network isolation monitoring devices, etc.
In this case, in all cases, except those specifically stated, we will assume that the resistance of the base on which a person stands (ground, floor, etc.), as well as the resistance of his shoes, is insignificant and therefore they can be taken equal to zero.
So, the most typical schemes for connecting a person to an electrical circuit when accidentally touching live conductors are:
1. Connection between two phase conductors of the circuit,
2. Connection between phase and ground.
Of course, in the second option it is assumed that the network in question is electrically connected to the ground due to, for example, grounding the neutral of the current source or due to poor insulation of the wires relative to the ground, or due to the presence of a large capacitance between them.
Two-phase touch is considered the most dangerous, since in this case a linear voltage of 380 volts is applied to the human body, and the current passing through the body does not depend on the network diagram and the mode of its neutral.
Two-phase touches occur very rarely and are mainly associated with working under voltage:
On electrical panels, assemblies and overhead lines;
When using faulty products personal protection;
On equipment with unprotected live parts, etc.
Single-phase touch is usually considered less dangerous, since the current passing through a person in this case is limited by the influence of a number of factors. But in practice it happens much more often than two-phase. Therefore, the topic of this article is to analyze only cases of single-phase touch in the networks under consideration.
If a person is injured by electric shock it is necessary to take measures to free the victim from the current and immediately begin providing him with first aid.
Free a person from the effects of current necessary as quickly as possible, but precautions must be taken. If the victim is at a height, measures must be taken to prevent him from falling.
Touching a energized person, is dangerous, and when carrying out rescue operations, it is necessary to strictly observe certain precautions against possible electric shock to persons carrying out these works.
Most in a simple way releasing the victim from the current is disconnecting an electrical installation or that part of it that a person touches. When the unit is turned off, the electric light may go out, so if there is no daylight you need to have another light source ready - a lantern, a candle, etc.
After releasing the victim from the current it is necessary to establish the degree of damage and, in accordance with the condition of the victim, provide him medical care. If the victim has not lost consciousness, it is necessary to provide him with rest, and if there are injuries or damage (bruises, fractures, dislocations, burns, etc.), he must be given first aid until a doctor arrives or taken to the nearest medical facility.
If the victim has lost consciousness, but is still breathing, it is necessary to lay him flat and comfortably on a soft bedding - blanket, clothes, etc., unfasten the collar, belt, remove restrictive clothing, clear the oral cavity of blood and mucus, ensure an influx of fresh air, give ammonia to sniff, sprinkle with water, rub and warm the body.
In the absence of signs of life (in clinical death, there is no breathing or pulse, the pupils of the eyes are dilated due to oxygen starvation of the cerebral cortex) or intermittent breathing, the victim should quickly free the victim from clothing that restricts breathing, clear the mouth and perform artificial respiration and cardiac massage.
Since from the resistance of the electrical circuit R Since the magnitude of the electric current passing through a person significantly depends, the severity of the injury is largely determined by the circuit of connecting the person to the circuit. The patterns of circuits formed when a person comes into contact with a conductor depend on the type of power supply system used.
The most common electrical networks are those in which the neutral wire is grounded, i.e., short-circuited by a conductor to the ground. Touching the neutral wire poses virtually no danger to humans; only the phase wire is dangerous. However, it is difficult to figure out which of the two wires is neutral - they look the same. You can figure it out using a special device - a phase detector.
Using specific examples, we will consider possible schemes for connecting a person to an electrical circuit when touching conductors.
Two-phase connection to the circuit. The rarest, but also the most dangerous, is a person touching two phase wires or current conductors connected to them (Fig. 2.29).
In this case, the person will be under the influence of line voltage. Current will flow through the person along the “hand-to-hand” path, i.e. the resistance of the circuit will include only the resistance of the body (D,).
 |
If we take the body resistance to be 1 kOhm, and the electrical network to be 380/220 V, then the current passing through a person will be equal to
This is a deadly current. The severity of an electrical injury or even a person’s life will depend primarily on how quickly he frees himself from contact with the current conductor (breaks the electrical circuit), because the time of exposure in this case is decisive.
Much more often there are cases when a person comes into contact with a phase wire or part of a device with one hand, a device that is accidentally or intentionally electrically connected to it. The danger of electric shock in this case depends on the type of electrical network (with grounded or insulated neutral).
Single-phase connection to a circuit in a network with a grounded neutral(Fig. 2.30). In this case, the current passes through the person along the “arm-legs” or “arm-arm” path, and the person will be under phase voltage.
In the first case, the circuit resistance will be determined by the resistance of the human body (I_, shoes (R o 6), grounds (Rzh), on which a person stands, the neutral grounding resistance (RH), and current will flow through the person
Neutral resistance R H is small and can be neglected compared to other circuit resistances. To estimate the magnitude of the current flowing through a person, we will assume a network voltage of 380/220 V. If a person is wearing insulating dry shoes (leather, rubber), he is standing on dry ground. wooden floor, the circuit resistance will be large, and the current strength according to Ohm's law will be small.
For example, floor resistance is 30 kOhm, leather shoes are 100 kOhm, human resistance is 1 kOhm. Current passing through a person
This current is close to the threshold perceptible current. The person will feel the flow of current, stop working, and eliminate the malfunction.
If a person stands on wet ground with damp shoes or bare feet, a current will pass through the body
This current can cause damage to the lungs and heart, and with prolonged exposure, death.
If a person stands on wet soil wearing dry and intact rubber boots, a current passes through the body
A person may not even feel the impact of such a current. However, even a small crack or puncture in the sole of a boot can dramatically reduce the resistance of the rubber sole and make work dangerous.
Before you start working with electrical devices(especially long time not in use), they must be carefully inspected for damage to the insulation. Electrical devices must be wiped free of dust and, if they are wet,- dry. Wet electrical devices must not be used! It is better to store electric tools, instruments, and equipment in plastic bags to prevent dust or moisture from getting into them. You have to wear shoes when working. If the reliability of an electrical device is in doubt, you need to be on the safe side.- place a dry wooden floor or rubber mat under your feet. You can use rubber gloves.
The second path of current flow occurs when a person comes into contact with electrically conductive objects connected to the ground with his second hand (the body of a grounded machine, metal or reinforced concrete structure buildings, damp wooden wall, water pipe, heating battery, etc.). In this case, the current flows along the path of least electrical resistance. These objects are practically short-circuited to the ground, their electrical resistance is very small. Therefore, the circuit resistance is equal to the body resistance and current will flow through the person
This amount of current is deadly.
When working with electrical devices, do not use your other hand to touch objects that may be electrically connected to ground. Working in damp areas, in the presence of highly conductive objects connected to the ground near a person, poses an extremely high danger and requires compliance with increased electrical safety measures.
In emergency mode (Fig. 2.30, b), when one of the phases of the network (another phase of the network, different from the phase touched by a person) is shorted to ground, voltage redistribution occurs, and the voltage of the healthy phases differs from the phase voltage of the network. When touching a working phase, a person comes under voltage, which is greater than the phase voltage, but less than the linear one. Therefore, no matter how the current flows, this case is more dangerous.
Single-phase connection to a circuit in a network with an isolated neutral(Fig. 2.31). In production, three-wire electrical networks with an insulated neutral are used to supply power to power electrical installations. In such networks there is no fourth grounded neutral wire, and there are only three phase wires. In this diagram, rectangles conventionally show electrical resistance r A, r V, r With insulation of wires of each phase and capacitance S A, S v, S s each phase relative____________________
being under significantly higher voltages, and therefore more dangerous. However, the main conclusions and recommendations for ensuring safety are almost the same.
Even without taking into account resistance human chains(a person stands on wet ground in damp shoes), the current passing through the person will be safe:
Thus, good phase insulation is the key to safety. However, with extensive electrical networks, this is not easy to achieve. In long and branched networks with a large number of consumers, the insulation resistance is low, and the danger increases.
For long electrical networks, especially cable lines, the phase capacitance cannot be neglected (CV0). Even with very good phase insulation (r = oo), the current will flow through a person through the capacitance of the phases, and its value will be determined by the formula:
Thus, long electrical circuits of industrial enterprises with high capacitance are highly dangerous, even with good phase insulation.
If the insulation of any phase is broken, touching a network with an isolated neutral becomes more dangerous than touching a network with a grounded neutral wire. In emergency mode (Fig. 2.31, b) the current passing through a person who has touched the serviceable phase will flow through the ground fault circuit to the emergency phase, and its value will be determined by the formula:
Since the closure resistance D, the emergency phase on earth, is usually small, the person will be under linear voltage, and the resistance of the resulting circuit will be equal to the resistance of the person’s circuit ____, which is very dangerous.
For these reasons, as well as because of ease of use (the ability to obtain voltages of 220 and 380 V), four-wire networks with a grounded neutral wire for a voltage of 380/220 V have become most widespread.
We have not considered all possible electrical network diagrams and touch options. In production, you may be dealing with more complex power supply circuits, especially ground circuits.
To simplify the analysis, let us assume g A - g c= g c = g, A S A= L B= C c = C
If a person touches one of the wires or any object electrically connected to it, current will flow through the person, the shoe, the base, and through the insulation and capacitance of the wires to the other two wires. Thus, a closed electrical circuit is formed, in which, unlike the previously considered cases, the phase insulation resistance is included. Since the electrical resistance of good insulation is tens and hundreds of kilo-ohms, the total electrical resistance of the circuit is much greater than the resistance of the circuit formed in a network with a grounded neutral wire. That is, the current through a person in such a network will be less, and touching one of the phases of the network with an isolated neutral is safer.
The current through a person in this case is determined by the following formula:
where is the electrical resistance of the human circuit,
co = 2nd - circular frequency of the current, rad/s (for industrial frequency current = 50 Hz, therefore co = YuOl).
If the phase capacitance is small (this is the case for short air networks), we can take C « 0. Then the expression for the amount of current through a person will take the form:
For example, if the floor resistance is 30 kOhm, leather shoes are 100 kOhm, the human resistance is 1 kOhm, and the phase insulation resistance is 300 kOhm, the current that passes through the person (for a 380/220 V network) will be equal to
A person may not even feel such a current.
1. What types of electrical networks are most common in production?
2. Name the sources of electrical hazards at work.
3. What is touch voltage and step voltage? How do their values depend on the distance from the point where the current flows into the ground?
4. How are premises classified according to the degree of electrical hazard?
5. How does electric current affect a person? List and describe the types of electrical injuries.
6. What parameters of electric current determine the severity of electric shock? Specify current thresholds.
7. Which path of electric current flow through the human body is most dangerous?
8. Indicate the sources of the greatest electrical danger in production related to your future profession.
9. Do a hazard analysis of electrical networks with a grounded neutral.
10. Give an analysis of the dangers of electrical networks with an isolated neutral.
11.Which touching of live conductors is most dangerous for a person?
12. Why does touching objects electrically connected to the ground (for example, a water pipe) with your hand when working with electrical devices sharply increase the risk of electric shock?
13.Why do you need to remove the electrical plug from the socket when repairing electrical equipment?
14.Why do you need to wear shoes when working with electrical devices?
15.How can you reduce the risk of electric shock?