Knowledge of the processes occurring in electrical installations allows power engineers to safely operate equipment of any voltage and type of current, to carry out repair work and maintenance of electrical systems.
The information presented in the PTB and PTE - the main documents created by the best specialists based on the analysis of accidents with people affected by hazardous factors accompanying the operation of electrical energy - helps to avoid cases of electric shock to an electrical installation.
Circumstances and reasons for a person's exposure to electric current
Safety guidance documents distinguish three groups of reasons explaining electric shock to workers:
1.intentional, inadvertent approach to live parts with voltage at a distance less than safe or touching them;
2. the occurrence and development of emergency situations;
3. violation of the requirements specified in the guidance documents prescribing the rules of behavior for workers in existing electrical installations.
Assessment of the dangers of injury to a person consists in determining by calculations the magnitudes of the currents that pass through the victim's body. In this case, many situations have to be taken into account when contacts can occur in random places of an electrical installation. In addition, the voltage applied to them varies depending on many reasons, including the conditions and modes of operation of the electrical circuit, its energy characteristics.
Conditions for human injury by electric current
In order for a current to flow through the victim's body, it is necessary to create an electrical circuit by connecting it to at least two points of the circuit, which has a potential difference - voltage. The following conditions may occur on electrical equipment:
1. Simultaneous two-phase or two-pole touching different poles (phases);
2. single-phase or single-pole touching the potential of the circuit, when a person has a direct galvanic connection with the earth potential;
3. Accidental creation of contact with conductive elements of the electrical installation, which turned out to be energized as a result of the development of the accident;
4. falling under the action of step voltage, when a potential difference is created between points on which legs or other parts of the body are located at the same time.
In this case, electrical contact of the victim with the current-carrying part of the electrical installation may occur, which is considered by the PUE as a touch:
1.direct;
2. or indirectly.
In the first case, it is created by direct contact with a live part connected to voltage, and in the second, when touching non-insulated circuit elements, when a dangerous potential has passed through them in the event of an accident.
To determine the conditions for the safe operation of an electrical installation and prepare a workplace for workers inside it, you must:
1. to analyze the cases of possible creation of paths for the passage of electric current through the body of the service personnel;
2. compare its maximum possible value with the current minimum permissible standards;
3. make a decision on the implementation of measures to ensure electrical safety.
Features of the analysis of the conditions of injury to people in electrical installations
To assess the magnitude of the current passing through the victim's body in a DC or AC voltage network, the following types of designations are used for:
1.resistances:
Rh - at the human body;
R0 - for a grounding device;
Rfrom the insulation layer relative to the ground contour;
2.currents:
Ih - through the human body;
Iз - short circuit to the ground contour;
Uc - circuits of direct or single-phase alternating currents;
Ul - linear;
Uf - phase;
Uпр - touching;
Ush - steps.
In this case, the following typical schemes for connecting the victim to the voltage circuits in the networks are possible:
1.dc current at:
single-pole contact of a conductor contact with potential isolated from the ground circuit;
single-pole contact of the potential of the circuit with a grounded pole;
bipolar contact;
2. three-phase networks at;
single-phase contact with one of the potential conductors (generalized case);
two-phase contact.
Circuits of defeat in DC circuits
Single pole human contact with potential isolated from earth
Under the influence of the voltage Uc, a current Ih flows through the doubled insulation resistance of the medium through the sequentially created chain from the potential of the lower conductor, the victim's body (hand-leg) and the ground contour.
Single pole human contact with grounded pole potential
In this scheme, the situation is aggravated by the connection to the ground circuit of one potential wire with resistance R0, close to zero and much lower than that of the victim's body and the insulation layer of the external environment.
The strength of the required current is approximately equal to the ratio of the mains voltage to the resistance of the human body.
Bipolar human contact with network potentials
The mains voltage is directly applied to the victim's body, and the current through his body is limited only by his own insignificant resistance.
General schemes of defeat in three-phase alternating current circuits
Creation of human contact between phase potential and earth
In general, there is a resistance between each phase of the circuit and the ground potential and capacitance is created. The neutral of the voltage source windings has a generalized resistance Zн, the value of which varies in different grounding systems of the circuit.
Formulas for calculating the conductance of each chain and the total value of the current Ih through the phase voltage Uf are shown in the picture by the formulas.
Formation of human contact between two phases
The greatest value and danger is the current passing through the chain created between the direct contacts of the victim's body with the phase wires. In this case, part of the current can pass along the path through the ground and the insulation resistance of the medium.
Features of two-phase touch
In circuits of direct and three-phase alternating currents, the creation of contacts between two different potentials is most dangerous. With this scheme, a person falls under the influence of the greatest stress.
In a circuit with a constant voltage power supply, the current through the victim is calculated by the formula Ih = Uc / Rh.
In a three-phase AC network, this value is calculated according to the ratio Ih = Ul / Rh = √3 Uph / Rh.
Considering that the average electrical resistance of the human body is 1 kilo-ohm, we will calculate the current that occurs in the network of direct and alternating voltage of 220 volts.
In the first case, it will be: Ih = 220/1000 = 0.22A. This value of 220 mA is sufficient for the victim to undergo convulsive muscle contraction, when, without assistance, he is no longer able to free himself from the effects of accidental touch - the holding current.
In the second case, Ih = (220 1.732) / 1000= 0.38A. A fatal risk of injury arises at this 380 mA value.
We also pay attention to the fact that in an AC three-phase voltage network, the position of the neutral (can be isolated from the ground or vice versa - connected short-circuited) has very little effect on the value of the current Ih. Its main share does not go through the ground chain, but between the phase potentials.
If a person has applied protective equipment that ensures his reliable isolation from the ground contour, then in such a situation they will be useless and will not help.
Features of single phase touch
Three-phase network with a solidly grounded neutral
The victim touches one of the phase wires and falls under the potential difference between it and the ground circuit. This is the most common occurrence.
Although the phase-to-ground voltage is 1.732 times less than the line voltage, such a case remains dangerous. The condition of the victim can worsen:
neutral mode and quality of its connection;
electrical resistance of the dielectric layer of the wires relative to the ground potential;
type of footwear and its dielectric properties;
soil resistance at the location of the victim;
other related factors.
The value of the current Ih in this case can be determined by the ratio:
Ih = Uph / (Rh + Rb + Rp + R0).
Recall that the resistances of the human body Rh, shoes Rb, floor Rp and grounding at neutral R0 are taken in Ohms.
The smaller the denominator, the stronger the current is generated. If an employee wears conductive footwear, for example, he has wet his feet or soles are lined with metal nails, and in addition is on a metal floor or damp ground, then we can assume that Rb = Rp = 0. This ensures the most unfavorable case for the life of the victim.
Ih = Uph / (Rh + R0).
With a phase voltage of 220 volts, we get Ih = 220/1000 = 0.22 A. Or a deadly current of 220 mA.
Now let's calculate the option when the worker uses protective equipment: dielectric shoes (Rp = 45 kOhm) and an insulating base (Rp = 100 kOhm).
Ih = 220 / (1000 + 45000 + 10000) = 0.0015 A.
Received a safe current value of 1.5 mA.
Three-phase network with isolated neutral
There is no direct galvanic connection of the neutral of the current source with the ground potential. The phase voltage is applied to the resistance of the insulation layer Rfrom, which has a very high value, which is controlled during operation and is constantly maintained in good condition.
The chain of current flow through the human body depends on this value in each of the phases. If we take into account all layers of resistance to current, then its value can be calculated by the formula: Ih = Uph / (Rh + Rb + Rp + (Riz / 3)).
During the worst case, when conditions of maximum conductivity through the shoes and the floor are created, the expression will take the form: Ih = Uph / (Rh + (Rf / 3)).
If we consider a 220 volt network with a layer insulation of 90 kΩ, then we get: Ih = 220 / (1000+ (90000/3)) = 0.007 A. Such a current of 7 mA will be felt well, but it will not be able to provide a fatal injury.
Note that in this example we deliberately missed the resistance of the soil and shoes. If we take them into account, then the current will decrease to a safe value, of the order of 0.0012 A or 1.2 mA.
Conclusions:
1.In systems with isolated neutral, the safety of workers is easier to ensure. It directly depends on the quality of the dielectric layer of the wires;
2. Under the same circumstances of touching the potential of one phase, a circuit with a grounded neutral is more dangerous than with an isolated one.
Let us consider the case of touching the metal body of an electrical device, if the insulation of the dielectric layer at the phase potential is broken inside it. When a person touches this body, a current will flow through his body to the ground and then through a neutral to a voltage source.
The equivalent circuit is shown in the picture below. The resistance Rn is possessed by the load created by the device.
Insulation resistance Rfrom together with R0 and Rh limits the phase-to-phase contact current. It is expressed by the ratio: Ih = Uph / (Rh + Rfrom + Ro).
In this case, as a rule, even at the project stage, choosing materials for the case when R0 = 0, they try to comply with the condition: Rfrom> (Uph / Ihg) -Rh.
The value of Ihg is called the threshold of imperceptible current, the value of which a person will not feel.
We conclude: the resistance of the dielectric layer of all live parts relative to the ground contour determines the degree of safety of the electrical installation.
For this reason, all such resistances are normalized and accounted for by the approved tables. For the same purpose, it is not the insulation resistances themselves that are normalized, but the leakage currents that flow through them during tests.
Step voltage
In electrical installations, for various reasons, an accident can occur when the phase potential directly touches the ground contour. If on an overhead power line one of the wires broke off under the influence of various types of mechanical loads, then it is in this case that a similar situation appears.
In this case, a current is formed at the point of contact of the wire with the ground, which creates a spreading zone around the point of contact - an area on the surface of which an electric potential appears. Its value depends on the current Ic and the specific state of the soil r.
A person who finds himself within the boundaries of this zone falls under the influence of the voltage of the step Ush, as shown in the left half of the picture. The area of the spreading zone is limited by the contour where there is no potential.
The value of the step voltage is calculated by the formula: Ush = Uz ∙ β1 ∙ β2.
It takes into account the phase voltage at the place of current spreading - Uz, which is specified by the coefficients of the voltage spreading characteristics β1 and the influence of the resistances of shoes and feet β2. The values β1 and β2 are published in reference books.
The value of the current through the victim's body is calculated by the expression: Ih = (Uz ∙ β1 ∙ β2) / Rh.
On the right side of the figure, in position 2, the victim makes contact with the ground potential of the wire. It is influenced by the potential difference between the point of contact with the hand and the ground contour, which is expressed by the touch voltage Upr.
In this situation, the current is calculated by the expression: Ih = (Uph.z. ∙ α) / Rh
The values of the spreading coefficient α can vary within 0 ÷ 1 and take into account the characteristics that affect Upr.
In the considered situation, the same conclusions apply as when creating a single-phase contact to the victim in the normal operation of the electrical installation.
If a person is located outside the current spreading zone, then he is in a safe zone.
1) Single-phase touching a network wire with an insulated neutral with good insulation (Fig. 1):
Figure 1 - Single-phase connection of a person to an electrical network.
The current passing through a person I h returns to the current source through the insulation of the wires of the network, which, in good condition, has a high insulation resistance R from. Up to 1000V R of is equal to 0.5 MΩ 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 Ohm is taken in the calculations;
R from - the insulation resistance of the phases relative to earth;
U f - phase voltage
Taking into account the resistance of the shoe R about and the floor R p, included 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 the network wire with a grounded neutral (Fig. 2):
Figure 2 - Single-phase contact to the network with earthed neutral
The magnitude of the current through a person is determined only by the resistance of the human body, the values of the insulation resistances of wires 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 40m, then it can be neglected in the calculations. In this case, the resistance of the floor and shoes play an important role in human safety, because included in a chain with a person in series.
(4)
When R p = 0 and R about = 0
I h = = 0,22 A = 220 mA> 100 mA >> 10 mA ,
it is very dangerous!
When a phase is closed to earth, a network with an isolated neutral (Fig. 4) turns out to be more dangerous than with a grounded one (Fig. 5). Since, in a network with an isolated neutral, the voltage that determines the magnitude of the current through the human body is equal to U l, and in a network with a grounded neutral it lies within:
U l> U pr> U f
Figure 4 - Network with isolated neutral
I h= 
, (7)
where R h is the resistance of the human body;
R zm - earth phase closure resistance
In the event of a phase breakdown on the body of the equipment, which under normal conditions should not be energized, the person working with this equipment is in the 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 earth
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 deliberate connection to earth or its equivalent of metal parts of equipment that are not energized under normal conditions, but may be energized due to a violation of the insulation of electrical installations.
Protective grounding works by reducing the voltage between the energized frame of the equipment and earth to a safe value.
Let us explain this using the example of a network with an isolated neutral (Fig. 6). If the body of electrical equipment is not grounded and it is in contact with a phase, then a person's touch to such a body is equivalent to a single-phase connection. If the chassis is grounded, the chassis ground potential drops to a safe low value.
Figure 6 - Protective grounding
It is necessary to ground metal parts of electrical installations, cases of electrical machines, transformers, devices, lamps, drives of electrical devices, secondary windings of measuring transformers, frames of switchboards, control panels, cabinets, etc.
Protective grounding is used in three-phase three-wire networks with a voltage of up to 1000 V with an isolated neutral, and in networks with a voltage of 1000 V and above - with any neutral mode (Fig. 3.18).
The degree of electric shock is influenced by: current strength, voltage, type of current, current path through the human body, individual characteristics of the human body, its psychological state, the presence of alcohol and drugs in the body, microclimate parameters, the time a person is under the influence of an electric current.
Electric current passing through the human body has 4 types of effects:
Thermal action- manifested in burns of individual parts of the body, heating to high temperatures of blood vessels, blood, nerves, heart, brain, which causes a serious disorder of organs.
Electrolytic action- decomposition of organic fluid (lymph and blood) in violation of its composition.
Mechanical action- (dynamic) stratification, rupture of body tissues (muscles of the heart, blood vessels) as a result of the electrodynamic effect; instant explosion-like formation of vapor from overheated tissue fluid and blood.
Biological- manifests itself in the violation of biological processes in the body, accompanied by irritation (destruction) of nervous and other tissues and burns, cessation of the activity of the respiratory and circulatory organs.
Electric shock can cause local injury or general electric shock (electric shock).
TO local include: eclectic burns, skin metallization, mechanical damage, electrophthalmia (inflammation of the outer membranes of the eyes).
TO common: an electric shock that affects (or threatens) the entire body due to disruption of the normal functioning of vital organs. General injuries are accompanied by the excitement of various muscle groups of the human body, which can lead to seizures, paralysis of the respiratory system of the heart, and cardiac arrest.
35. Factors affecting the severity of electric shock
Factors determining the risk of electric shock:
1. Electrical:
Voltage;
Kind of current;
Its frequency;
Human electrical resistance.
2. Non-electrical:
Individual characteristics of a person;
The duration of the current;
His way is through man.
3. State of the environment .
4. Lowest electric current , causing an annoying sensation by a person, is called threshold sensible current... This is approximately 1.1 MA for a 50 Hz current and 6 MA for direct current.
36. Single-phase and two-phase switching on of a person in various electrical networks
The defeat of a person by electric current occurs when an electrical circuit is closed through the human body. This occurs when a person touches at least two points of the electrical circuit, between which there is some voltage. The inclusion of a person in a circuit can occur according to several schemes: between the wire and the ground, called single-phase switching; between two wires - two-phase connection. These circuits are most typical for three-phase AC networks. It is also possible to connect between two wires and ground at the same time; between two points on the earth with different potentials, etc.
One-phase connection of a person to the network represents direct human contact with parts of an electrical installation or equipment that are normally or accidentally energized. In this case, the degree of danger of injury will be different depending on whether the electrical network has a grounded or isolated 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. With a single-phase connection to a network with a grounded neutral, a person falls under a phase voltage that is 1.73 times less than a linear voltage, and is exposed to a current, the magnitude of which is determined by the phase voltage of the installation and the resistance of the human body. An additional protective effect is provided by floor insulation, on which there is a man and shoes.
Two-phase touch is, as a rule, more dangerous, since the highest voltage in this network is applied to the human body (for a three-phase network - linear), and the current // r passing through the human body turns out to be independent of the neutral mode (for a three-phase network) or the presence grounding of one of the wires in a single-phase network and is of greatest importance. Biphasic contact is very rare.
There are various schemes for connecting a person to an electrical current circuit:
Single-phase touch - touching a conductor of one phase of an existing electrical installation;
Two-phase touch - simultaneous touching of conductors of two phases of an operating electrical installation;
Touching non-current-carrying parts of electrical installations that are energized as a result of insulation damage;
Switching on under voltage of a step - switching on between two points of the earth (ground), which are at different potentials.
Let's consider the most typical schemes for connecting a person to an electrical current circuit.
Single-phase contact in a network with a solidly grounded neutral. The current flowing through the human body ( I h) with a single-phase contact (Fig. 6) will close on the circuit: phase L 3 - human body - base (floor) - neutral ground electrode - neutral (zero point).
Rice. 6. Scheme of single-phase contact in the network
with solidly grounded neutral
Ohm's law:,
Where R o - neutral grounding resistance,
R basic is the resistance of the base.
If the base (floor) is conductive, then R main ≈ 0
Given the fact that R O " R h, then
U h = U f
Such touching is extremely dangerous.
Single-phase contact in a network with isolated neutral. The current flowing through the human body (Fig. 7) will close the circuits: phase L 3 - the human body - the 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 the current flowing through the human body, phase isolation turns out to be in series with it. L 2 and L 1 .
Rice. 7. Scheme of single-phase touching in the network
with isolated neutral
Phase insulation resistance Z has an active ( R) and capacitive components ( WITH).
R- characterizes the imperfection of 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 the imaginary capacitor, the "plates" of which are the phases and the ground.
At R 1 = R 2 = R 3 = R f and WITH 1 = WITH 2 = WITH 3 = WITHФ current flowing through the human body:
where Z- total insulation resistance of the phase conductor relative to earth.
If the capacitance of the phases is neglected WITH f = 0 (air networks of short length), then:
whence it follows that the magnitude of the current depends not only on the human resistance, but also on the insulation resistance of the phase wire relative to the ground.
If, for example, R 1 = R 2 = R 3 = 3000 Ohm, then
; U h= 0.0111000 = 110V
Two-phase touch. With a two-phase touch (Fig. 8), regardless of the neutral mode, the person will be under the line voltage of the network U l and according to Ohm's law:
at U l = 380 V: 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, as a rule, the result of working under voltage in electrical installations up to 1000 V, which is a violation of the rules and instructions.
Touching an energized metal housing. Touching the body of the electrical installation (Fig. 9), in which the phase ( L 3) closed on the body, is tantamount to touching the phase itself. Therefore, the analysis and conclusions for single-phase contact cases discussed earlier fully apply to the case of a ground fault.
Rice. 9. Scheme of a person's touch of a metal
enclosure under voltage
The degree of danger and the outcome of electric shock depend on: on the scheme of "connecting" a person to an electrical circuit; on the electrical network:
three-phase four-wire with grounded neutral;
three-phase with insulated neutral.
The neutral point of the transformer (generator) is the connection point of the windings of the supply transformer. During normal operation of the electrical network at this point, the voltage is 0. The neutral of the power source can be grounded and isolated from the ground, this determines the mode of its operation. The neutral ground is called the working ground R 0.
The choice of the network diagram and the neutral mode of the current source is carried out depending on the 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). Linear voltage of 380 V is used to power the power load - the electric motors of the production equipment are switched on between the phase conductors. Phase voltage = 220 V is used for a lighting installation - lamps are connected between the phase and neutral wires. The line voltage is always 1.73 times higher than the phase voltage.
By safety conditions networks with an insulated neutral are advisable to use when it is possible to maintain a high level of network isolation, providing an insignificant capacitance of the wires relative to the ground. These can be sparsely branched networks that are not exposed to an aggressive environment and are under constant supervision of qualified personnel.
A single-phase connection is less dangerous than a two-phase one, however, it occurs much more often and is the main cause of electrical injuries. The outcome of the defeat in this case is decisively influenced by the neutral mode of the power grid.
When you touch one of the phases of the network with an isolated neutral (Fig.), In series with the human resistance, the insulation resistance and capacitance relative to the ground of the other two undamaged phases turn on in series.
Rice. Single-pole contact to the mains with isolated neutral during normal operation
During normal mains operation, the neutral voltage of the power supply with respect to earth is zero. The phase voltages relative to earth are the same and equal to the phase voltages of the power supply.
The insulation resistance of wires is never equal to an infinitely large value; leakage currents are bound to occur.
In this case, the wires and the ground are like plates of a capacitor, between which an electric field arises. The more extended 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). Linear voltage of 380 V is used to power the power load - the electric motors of the production equipment are switched on between the phase conductors. Phase voltage = 220 V is used for a lighting installation - lamps are connected between the phase and neutral wires. The line voltage is always 1.73 times higher than the phase voltage.
According to the safety conditions of a network with an insulated neutral, it is advisable to use it when it is possible to maintain a high level of network isolation, providing an insignificant capacitance of the wires relative to the ground. These can be sparsely branched networks that are not exposed to an aggressive environment and are under constant supervision of qualified personnel.
Networks with a grounded neutral are used where it is impossible to ensure a high level of insulation of an electrical installation or where it is impossible to quickly find and repair its damage.
Due to the specifics and insignificant production capacity in comparison with other food industry enterprises, catering enterprises can use one- and two-phase networks with a grounded neutral, and when operating small-scale mechanization equipment during loading and unloading operations, an electrical network with an isolated neutral is recommended. The degree of electrical safety in such networks increases due to the high insulation resistance of electrical wires in relation to the ground.
An electric shock to a person can be caused by a single-pole (single-phase) or two-pole (two-phase) touching the live part of the installation.
As the insulation resistance increases, the risk of electric shock decreases.
In the emergency mode of operation of the same network, when a dead phase-to-ground short circuit 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, a current will flow through him along the "hand - foot" path. In this situation, the insulation resistance of the wires does not play any role in the outcome of the defeat. Such an electric shock is most often fatal.
At enterprises where the networks are branched and have a considerable length, and therefore a large capacity, a system with an isolated neutral loses its advantage, since the leakage current increases, the resistance of the phase-to-earth section decreases. From the point of view of electrical safety, in such cases, preference is given to a network with a grounded neutral (Fig.).
Diagram of a person's touch to one phase of a network with a grounded neutral
The earth resistance, as in the case of an electrical network with an isolated neutral, can be neglected.
Examples indicate that, all other things being equal, a single-phase connection of a person to a network with an isolated neutral is less dangerous than to a network with a grounded neutral.
The most dangerous is a two-phase connection of a person to the electrical network, since he falls under the line voltage of the network, regardless of the neutral mode and the operating conditions of the network.
Cases of two-phase contact occur rarely and mainly in electrical installations up to 1000 V when working on panels and assemblies, when operating equipment with non-insulated live parts, etc.
