Analysis of the danger of electric shock in various networks. Providing first aid for electrical injuries. Danger of electric shock in various electrical networks Schemes for connecting a person to an electrical network
Analysis of electrical hazards in various networks
Electric shock to a person is possible only through direct contact with points of an electrical installation between which there is voltage, or with a point whose potential differs from the ground potential. Analysis of the danger of such a touch, assessed by the magnitude of the current passing through a person or the voltage of the touch, depends on a number of factors: the circuit of connecting a person to the electrical network, its voltage, neutral mode, insulation of live parts, their capacitive component, etc.
When studying the causes of electric shock, it is necessary to distinguish between direct contact with live parts of electrical installations and indirect contact. The first, as a rule, occurs in case of gross violations of the operating rules of electrical installations (PTE and PTB), the second - as a result of emergency situations, for example, in the event of an insulation breakdown.
Schemes for connecting a person to an electrical circuit can be different. However, the most common are two: between two different wires - two-phase connection and between one wire or body of an electrical installation, one phase of which is broken, and the ground - single-phase connection.
Statistics show that the largest number of electrical injuries occur during single-phase switching, and most of them occur in networks with a voltage of 380/220 V. Two-phase switching is more dangerous, since in this case the person is under line voltage, and the current strength passing through the person is will be (in A)
where Ul is the linear voltage, i.e. voltage between phase wires, V; Uph - phase voltage, i.e. voltage between the beginning and end of one winding (or between the phase and neutral wires), V.
As can be seen from Fig. 8.1, the danger of two-phase switching does not depend on the neutral mode. The neutral is the connection point of the windings of a transformer or generator that is not connected to a grounding device or is connected to it through devices with high resistance (a network with an isolated neutral), or directly connected to a grounding device - a network with a solidly grounded neutral.
With a two-phase connection, the current passing through the human body will not decrease when the person is isolated from the ground using dielectric galoshes, boots, rugs, and floors.
When a person is connected single-phase to the network, the current strength is largely determined by the neutral mode. For the case under consideration, the current strength passing through the person will be (in A)
, (8.3)
where w is frequency; C - phase capacitance relative to ground
Rice. 8.1. Connecting a person to a three-phase network with an isolated neutral:
a - two-phase switching; b - single-phase connection; Ra, Rt, Rc - electrical resistance of phase insulation relative to ground. Ohm; Ca, Cb, Cc - wire capacitance relative to the ground, F, Ia, Ib, IС currents flowing to the ground through the phase insulation resistance (leakage currents)
To simplify the formula, it is assumed that Ra = Rb = Rc = Riz, and Ca = Cb = Cc = C.
Under production conditions, phase insulation, made of dielectric materials and having a finite value, changes differently for each phase during the process of aging, humidification, and dust coating. Therefore, the calculation of safe conditions, which is greatly complicated, must be carried out taking into account the real values of resistance R and capacitance C for each phase. If the phase capacitance relative to the ground is small, i.e. Ca = Cb = Cc = 0 (for example, in air networks of short length), then
Ich = Uph/(Rch+Riz/3), (8.4)
If the capacitance is large (Ca = Cb = Cc is not equal to 0) and Riz is large (for example, in cable lines), then the strength of the current flowing through the human body will be determined only by the capacitive component:
, (8.5)
where Xc = 1/wC is capacitive reactance, Ohm.
From the above expressions it is clear that in networks with an isolated neutral, the lower the capacitive and higher the active component of the phase wires relative to the ground, the lower the risk of electric shock to a person. Therefore, in such networks it is very important to constantly monitor Riz to identify and eliminate damage.
Rice. 8.2. Connecting a person to a three-phase network with an isolated neutral during emergency operation. Explanations in the text
If the capacitive component is large, then the high phase insulation resistance does not provide the necessary protection.
In the event of an emergency (Fig. 8.2), when one of the phases is shorted to ground, the current strength passing through the person will be equal (in A)
If we accept that Rzm = 0 or Rzm<< Rч (что бывает в реальных аварийных условиях), то, исходя из приведенного выражения, человек окажется под линейным напряжением, т. е. попадет под двухфазное включение. Однако чаще всего R3M не равно 0, поэтому человек будет находиться под напряжением, меньшим Uл, но большим Uф, при условии, что Rиз/3 >> Rmeas.
A ground fault significantly changes the voltage of the current-carrying parts of the electrical installation relative to the ground and grounded building structures. A ground fault is always accompanied by the spreading of current in the ground, which, in turn, leads to the emergence of a new type of human injury, namely exposure to touch voltage and step voltage. This short circuit may be accidental or intentional. In the latter case, the conductor in contact with the ground is called a ground electrode or electrode.
In the volume of the earth where the current passes, a so-called “field (zone) of current spreading” arises. Theoretically, it extends to infinity, but in real conditions, already at a distance of 20 m from the ground electrode, the spreading current density and potential are practically zero.
The nature of the potential spreading curve significantly depends on the shape of the ground electrode. Thus, for a single hemispherical ground electrode, the potential on the earth’s surface will change according to a hyperbolic law (Fig. 8.3).
Rice. 8.3. Potential distribution on the surface of the earth around a hemispherical ground electrode (f - change in the potential of the ground electrode on the surface of the earth; fz - maximum potential of the ground electrode at the strength of the ground fault current I3; r - radius of the ground electrode)
Rice. 8.4. Touch voltage with a single ground electrode (f3 - total soil resistance to current spreading from the ground electrode):
1 - potential curve; 2 - curve characterizing the change in Upr with distance from the ground electrode; 3 - phase breakdown to the housing
Depending on the location of the person in the spreading zone and his contact with the electrical installation b, the body of which is grounded and energized, the person may come under touch voltage Upr (Fig. 8.4), defined as the potential difference between the point of the electrical installation that the person touches f3, and the point of the ground on which it stands - fosn (in V)
Upr = ph3 - phosn = ph3 (1 - phosn/ph3), (8.7)
where the expression (1 - phosn/f3) = a1 is the touch voltage coefficient characterizing the shape of the potential curve.
From Fig. 8.4 it can be seen that the touch voltage will be maximum when a person is 20 m or more away from the ground electrode (electrical installation c) and is numerically equal to the ground electrode potential Upr = f3, while a1 = I. If a person stands directly above the ground electrode (electrical installation a), then Unp = 0 and a1 =0. This is the safest case.
Expression (8.7) allows you to calculate Unp without taking into account additional resistance in the person-ground circuit, that is, without taking into account the resistance of shoes, the resistance of the supporting surface of the legs and the resistance of the floor. All this is taken into account by the coefficient a2, so in real conditions the magnitude of the touch voltage will be even less.
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,).
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If we take a body resistance of 1 kOhm, and an electrical network with a voltage of 380/220 V, then the current strength 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 RH 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 a dry 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 those that have not been in use for a long time), 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’s second hand comes into contact with electrically conductive objects connected to the ground (the body of a grounded machine tool, a metal or reinforced concrete building structure, a wet wooden wall, a water pipe, a heating battery, etc.). In this case, the current flows along the path of least electrical resistance. These objects are practically short-circuited to the ground, their electrical resistance is very small. Therefore, the resistance of the circuit is equal to the resistance of the body and current will flow through the person
This amount of current is deadly.
When working with electrical devices, do not 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, regardless of the path of current flow, 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 if we do not take into account the resistance of the human circuit (the person is standing on wet ground in damp shoes), the current passing through the person will be safe:
Thus, good phase 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, 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.
Control questions
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?
Cases of electric shock to a person are possible only when an electrical circuit is closed through the human body or, in other words, when a person touches at least two points of the circuit, between which there is some voltage.
The danger of such a touch, assessed by the value of the current passing through the human body, or by the voltage of the touch, depends on a number of factors: the circuit of the person being connected to the circuit, the network voltage, the circuit of the network itself, the mode of its neutral, the quality of insulation of live parts from the ground, as well as capacitance values of live parts relative to ground, etc.
Schemes for connecting a person to an electrical circuit may be different. However, the most typical are two connection schemes: between two wires and between one wire and ground (Figure 13.5). Of course, in the second case, an electrical connection is assumed between the network and the ground.
In relation to alternating current networks, the first circuit is usually called two-phase connection, and the second - single-phase.
Two-phase switching on, i.e. a person touching two phases at the same time, is usually more dangerous, since the highest voltage in a given network is applied to the human body - linear, therefore a larger current (A) will flow through the human body:
I h = 1.73U f /R h = U l /R h, 7)
where U l is the linear voltage, i.e. the voltage between the phase wires of the network, equal to V; U f - phase voltage, i.e. the voltage between the beginning and end of one winding of a current source (transformer, generator) or between the phase and neutral wires, V.
It is not difficult to imagine that two-phase connection is equally dangerous in a network with both isolated and grounded neutrals. With a two-phase switching on, the danger of injury will not decrease even if the person is reliably isolated from the ground, that is, if he has dielectric galoshes or boots on his feet, or stands on an insulating floor or on a dielectric rug.
Single-phase switching occurs much more often, but is less dangerous than two-phase, since the voltage under which a person finds himself does not exceed the phase voltage. Accordingly, the current passing through the human body is less. In addition, the value of this current is also influenced by the neutral mode of the current source, the insulation resistance and capacitance of the wires relative to the ground, the resistance of the floor on which a person stands, the resistance of his shoes and other factors.
IN three-phase three-wire network with isolated neutral the strength of the current (A) passing through the human body when touching one of the phases of the network during its normal operation (Figure 6) is determined by the following expression:
where Z is the complex impedance of one phase relative to the ground, Ohm, Z = r/(l + jwCr), r and C are, respectively, the insulation resistance of the wire (Ohm) and the wire capacitance (F) relative to the ground (for simplicity, taken to be the same for all wires networks).
The current in real form will be, A:
. (9)
If the capacitance of the wires relative to the ground is small, i.e. C » 0, which usually occurs in short-distance overhead networks, then equation (15) will take the form
If the capacitance is large and the conductivity of the insulation is insignificant, i.e. r » ¥, which usually occurs in cable networks, then according to expression (5) the current strength (A) passing through the human body will be equal to
, (11)
where x c is capacitance equal to 1/wС, Ohm; w - angular frequency, rad/s.
From expression (6) it follows that in networks with an isolated neutral, which have insignificant capacitance between the wires and the ground, the danger to a person who touches one of the phases during normal operation of the network depends on the resistance of the wires relative to the ground: with increasing resistance, the danger decreases, Therefore, it is very important in such networks to ensure high insulation resistance and monitor its condition for timely detection and elimination of faults. However, in networks with large capacitance relative to ground, the role of wire insulation in ensuring touch safety is lost, as can be seen from equations (5) and (7).
IN three-phase four-wire network with grounded neutral the conductivity of the insulation and the capacitive conductivity of the wires relative to the ground are small compared to the conductivity of the neutral grounding, therefore, when determining the strength of the current passing through the human body touching the network phase, they can be neglected.
Under normal operating conditions, its r and current strength I h passing through the human body will be (Figure 7) equal to:
I h = U f /(R h + r 0), (12)
where r 0 is the neutral grounding resistance, Ohm.
As a rule, r 0 £ 10 Ohm, but the resistance of the human body R h does not fall below several hundred Ohm×m. Consequently, without a big error in equation (8), we can neglect the value of r 0 and assume that when touching one of the phases of a three-phase four-wire network with a grounded neutral, a person finds himself practically under the phase voltage U f, and the current passing through him is equal to the quotient of dividing U f by R h. It follows that touching a phase of a three-phase network with a grounded neutral during its normal operation is more dangerous than touching a phase of a normally operating network with an isolated neutral (see equations (6) and (8)).
The connection of a person to the electrical network can be single-phase or two-phase. Single-phase connection is a human connection between one of the network phases and the ground. The strength of the damaging current in this case depends on the neutral mode of the network, human resistance, shoes, floor, and phase insulation relative to the ground. Single-phase switching occurs much more often and often causes electrical injuries in networks of any voltage. With a two-phase connection, a person touches two phases of the electrical network. With a two-phase switching on, the strength of the current flowing through the body (striking current) depends only on the network voltage and the resistance of the human body and does not depend on the neutral mode of the network supply transformer. Electrical networks are divided into single-phase and three-phase. A single-phase network can be isolated from the ground or have a grounded wire. In Fig. 1 shows possible options for connecting a person to single-phase networks.
Thus, if a person touches one of the phases of a three-phase four-wire network with a solidly grounded neutral, then he will be practically under phase voltage (R3≤ RF) and the current passing through the person during normal operation of the network will practically not change with changes in insulation resistance and capacitance wires relative to ground.
The effect of electric current on the human body
Passing through the body, electric current has thermal, electrolytic and biological effects.
The thermal effect manifests itself in burns of the skin or internal organs.
During electrolytic action, due to the passage of current, decomposition (electrolysis) of blood and other organic liquid occurs, accompanied by the destruction of red blood cells and metabolic disorders.
The biological effect is expressed in irritation and excitation of living tissues of the body, which is accompanied by spontaneous convulsive contraction of muscles, including the heart and lungs.
There are two main types of electric shock:
§ electrical injuries,
§ electric shocks.
Electric shocks can be divided into four degrees:
1. convulsive muscle contractions without loss of consciousness;
2. with loss of consciousness, but with preservation of breathing and heart function;
3. loss of consciousness and disturbance of cardiac activity or breathing (or both);
4. clinical death, i.e. lack of breathing and blood circulation.
Clinical death is a transition period between life and death, begins from the moment the activity of the heart and lungs stops. A person in a state of clinical death does not show any signs of life: she has no breathing, no heartbeat, no reaction to pain; The pupils of the eyes are dilated and do not react to light. However, it should be remembered that in this case the body can still be revived if help is given to it correctly and in a timely manner. The duration of clinical death can be 5-8 minutes. If help is not provided in a timely manner, biological (true) death occurs.
The result of electric shock to a person depends on many factors. The most important of them are the magnitude and duration of the current, the type and frequency of the current and the individual properties of the organism.
Determination of the current spreading resistance of single grounding conductors and the procedure for calculating the protective grounding loop for stationary process equipment (GOST 12.1.030-81. CCBT. Protective grounding, grounding)
Implementation of grounding devices. A distinction is made between artificial grounding devices, intended exclusively for grounding purposes, and natural ones - third-party conductive parts that are in electrical contact with the ground directly or through an intermediate conducting medium, used for grounding purposes.
For artificial grounding electrodes, vertical and horizontal electrodes are usually used.
The following can be used as natural grounding conductors: water supply and other metal pipes laid in the ground (with the exception of pipelines of flammable liquids, flammable or explosive gases); casing pipes of artesian wells, wells, pits, etc.; metal and reinforced concrete structures of buildings and structures that have connections to the ground; lead sheaths of cables laid in the ground; metal sheet piles for hydraulic structures, etc.
The calculation of protective grounding aims to determine the basic parameters of grounding - the number, dimensions and order of placement of single grounding conductors and grounding conductors, at which the touch and step voltages during the phase closure to the grounded body do not exceed permissible values.
To calculate grounding, the following information is required:
1) characteristics of the electrical installation - type of installation, types of main equipment, operating voltages, methods of grounding neutrals of transformers and generators, etc.;
2) electrical installation plan indicating the main dimensions and placement of equipment;
3) the shapes and sizes of the electrodes from which it is planned to construct the designed group grounding system, as well as the expected depth of their immersion into the ground;
4) data from measurements of soil resistivity in the area where the ground electrode is to be constructed, and information about the weather (climatic) conditions under which these measurements were made, as well as characteristics of the climatic zone. If the earth is assumed to be two-layer, then it is necessary to have measurement data on the resistivity of both layers of the earth and the thickness of the top layer;
5) data on natural grounding conductors: what structures can be used for this purpose and their resistance to current spreading, obtained by direct measurement. If for some reason it is impossible to measure the resistance of the natural ground electrode, then information must be provided that allows this resistance to be determined by calculation;
6) calculated ground fault current. If the current is unknown, then it is calculated using the usual methods;
7) calculated values of permissible touch (and step) voltages and protection duration, if the calculation is made based on touch (and step) voltages.
Grounding calculations are usually made for cases where the ground electrode is placed in homogeneous ground. In recent years, engineering methods for calculating grounding systems in multilayer soil have been developed and began to be used.
When calculating grounding conductors in homogeneous soil, the resistance of the upper layer of the earth (layer of seasonal changes), caused by freezing or drying out of the soil, is taken into account. The calculation is made using a method based on the use of grounding conductivity utilization factors and is therefore called the utilization factor method. It is performed with both simple and complex designs of group grounding conductors.
When calculating grounding systems in a multilayer earth, a two-layer earth model is usually adopted with the resistivities of the upper and lower layers r1 and r2, respectively, and the thickness (thickness) of the upper layer h1. The calculation is made by a method based on taking into account the potentials induced on the electrodes that are part of the group grounding system, and is therefore called the method of induced potentials. Calculation of grounding conductors in multi-layer earth is more labor-intensive. At the same time, it gives more accurate results. It is advisable to use it in complex designs of group grounding systems, which usually take place in electrical installations with an effectively grounded neutral, i.e. in installations with voltages of 110 kV and above.
When calculating a grounding device by any method, it is necessary to determine the required resistance for it.
The required resistance of the grounding device is determined in accordance with the PUE.
For installations with voltages up to 1 kV, the resistance of the grounding device used for protective grounding of exposed conductive parts in an IT type system must meet the following conditions:
where Rз is the resistance of the grounding device, ohm; Upred.add – touch voltage, the value of which is assumed to be 50 V; Iз – total ground fault current, A.
As a rule, it is not necessary to accept a grounding device resistance value of less than 4 ohms. A grounding device resistance of up to 10 Ohms is allowed if the above condition is met, and the power of transformers and generators supplying the network does not exceed 100 kVA, including the total power of transformers and (or) generators operating in parallel.
For installations with voltages above 1 kV above 1 kV, the resistance of the grounding device must correspond to:
0.5 Ohm with an effectively grounded neutral (i.e. with large earth fault currents);
250/Iz, but not more than 10 Ohms with an isolated neutral (i.e. with low ground fault currents) and the condition that the ground electrode is used only for electrical installations with voltages above 1000 V.
In these expressions, Iз is the calculated ground fault current.
During operation, there may be an increase in the resistance to the spreading of the ground electrode current above the calculated value, therefore it is necessary to periodically monitor the value of the ground electrode resistance.
Ground loop
The ground loop is classically a group of vertical electrodes of small depth connected by a horizontal conductor, mounted near an object at a relatively small mutual distance from each other.
As grounding electrodes in such a grounding device, a steel corner or reinforcement 3 meters long was traditionally used, which was driven into the ground using a sledgehammer.
A 4x40 mm steel strip was used as a connecting conductor, which was laid in a pre-prepared ditch 0.5 - 0.7 meters deep. The conductor was connected to the mounted grounding conductors by electric or gas welding.
To save space, the ground loop is usually “rolled” around the building along the walls (perimeter). If you look at this ground electrode from above, you can say that the electrodes are mounted along the contour of the building (hence the name).
Thus, a ground loop is a ground electrode consisting of several electrodes (groups of electrodes) connected to each other and mounted around the building along its contour.
The point of connection of the windings of the supply transformer (generator) is called the neutral point or neutral. The neutral of the power source can be isolated and grounded. Grounded is called the neutral of the generator (transformer), connected to the grounding device directly or through low resistance (for example, through current transformers). Isolated called the neutral of a generator or transformer, not connected to a grounding device or connected to it through a high resistance (signaling, measuring, protection devices, grounding arc suppression reactors).
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 .
Single-phase connection represents direct contact of a person with parts of an electrical installation or equipment that are normally or accidentally energized. When connected single-phase to a network with an insulated and grounded neutral, a person is exposed to phase voltage, which is 1.73 times less than linear, and is exposed to current, which depends on the phase voltage of the installation, the resistance of the human body, shoes, floor, neutral grounding, and insulation.
At single-phase connection in a three-phase four-wire network with a grounded neutral The strength of the current passing through the human body can be expressed as:
I h =U f /(R h +r p +r o +r n) => I h R h = U f R h /(R h +r p +r o +r n)
where U f is the phase voltage. IN; R h - human body resistance, Ohm; r p is the resistance of the floor on which the person is located. Ohm; r o - shoe resistance. Ohm; r n - neutral grounding resistance. Ohm; U pr - touch voltage, V.
As an example, two cases of single-phase connection of a person to a three-phase four-wire electrical circuit with a grounded neutral at line voltage are considered
U f = 380V; U l = 220 V = U f = 1.73 U f
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, the following resistances are accepted: human body = 1000 Ohm; soil or floor r p = 0; shoes r o = 0. Neutral grounding resistance r n = 4 Ohms (can be neglected in the calculation due to its insignificant value).
A deadly current will pass through the human body:
I h =U f /R h = U l /(1.73 R h)= 220/1000 = 0.22 A = 220 mA;
U pr = U f = 220 V.
A case of favorable conditions. A person is on a dry wooden floor with a resistance r p = 100,000 Ohm, on his feet are dry non-conductive (rubber) shoes with a resistance r o = = 45000 Ohm. Then a threshold current, long-term permissible for a person, will pass through the human body:
I h =220/(1000+100000+45000)=220/146000=0.0015A=1.5mA
U pr =220*1000/146000=1.5V
These examples illustrate the importance of the insulating properties of floors and shoes to ensure the safety of persons working in conditions of possible contact with electric current.
Two-phase switching is the simultaneous contact of a person with two different phases of the same energized network. In this case, the person is turned on to the full line voltage of the installation. The strength of the current acting on a person depends on the line voltage And human body resistance R h . When switching on two-phase, the insulation resistance of the wires does not have a protective effect:
I h =1.73 U f /R h =380/1000=0.38A=380mA U pr =I h R h =380 V
This current (voltage) value is deadly to human life. In this case, the neutral mode for two-phase switching is practically unimportant. Cases of two-phase switching are relatively rare: they are most likely when working under voltage, when the current-carrying parts of different phases are located at a small distance from each other.
According to technological requirements, preference is often given to a four-wire network; it uses two operating voltages - linear and phase. Thus, from a four-wire network 380 it is possible to supply both a power load - three-phase, including it between phase wires at a linear voltage of 380 V, and a lighting load, including it between the phase and neutral wires, that is, at a phase voltage of 220 V. At the same time, the electrical installation is much cheaper due to the use of fewer transformers, smaller wire cross-sections, etc.
Networks with a grounded neutral are used where it is impossible to ensure good insulation of electrical installations (due to high humidity, aggressive environment, etc.) or it is impossible to quickly find and eliminate insulation damage when the capacitive currents of the network, due to its significant branching, reach large values that are life-threatening person. Such networks include networks of large industrial enterprises, city distribution networks, etc. The existing opinion about a higher degree of reliability of networks with an isolated neutral is not sufficiently substantiated. Statistical data indicate that in terms of operational reliability, both networks are almost identical.
At voltages above 1,000V up to 35 kV, the networks, for technological reasons, have an insulated neutral, and above 35 kV, a grounded neutral.
Premises according to the degree of danger can be classified as: 1st class - office premises and laboratories with precision instruments, assembly shops of instrument factories, watch factories, etc.; to the 2nd class - unheated warehouse premises, staircases with conductive floors, etc.; Class 3 includes all workshops of machine-building plants: galvanic, battery, etc. These also include areas of outdoor work.
