The defeat of a person by current as a result of electrical influence, i.e., the passage of current through a person, is a consequence of his touching 2 points of the electrical circuit, between which there is some voltage. The danger of such a touch is estimated, as you know, by the current passing through the human body or the voltage under which it is found. It should be noted that the touch voltage depends on a number of factors: the circuit for connecting a person to an electrical circuit, the network voltage, the circuit of the network itself, the mode of its neutral, the degree of isolation of current-carrying parts from the ground, as well as the capacitance of current-carrying parts relative to the ground, etc.
Consequently, the above danger 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 discusses the dependence of the danger of including a person in an electrical circuit, i.e., the value of the touch voltage and current flowing through a person, on the listed factors.
This dependence must be known when evaluating a particular network according to safety conditions, choosing and calculating appropriate protective measures, in particular grounding, zeroing, protective shutdown, network isolation control devices, etc.
At the same time, in all cases, except for those specifically stipulated, we will assume that the resistance of the foundation on which the person stands (ground, floor, etc.), as well as the resistance of his shoes, are insignificant and therefore they can be taken equal to zero.
So, the most characteristic schemes for including a person in an electrical circuit in case of accidental contact with current-carrying conductors are:
1. Switching between two phase conductors of the circuit,
2. Connection between phase and earth.
Of course, in the second option, it is assumed that the network in question is electrically connected to the ground due, for example, to 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 contact 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 scheme and its neutral mode.
Two-phase touches occur very rarely and are mainly associated with work under voltage:
On electrical panels, assemblies and overhead lines;
When using faulty personal protective equipment;
On equipment with unshielded 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 it happens in practice much more often than two-phase. Therefore, the topic of this article is the analysis of only cases of single-phase contact in the networks under consideration.
In case of electric shock to a person it is necessary to take measures to release the victim from the current and immediately begin to provide him with first aid.
Release a person from the action of the current as soon 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 person who is energized dangerous, and when conducting rescue operations, it is necessary to strictly observe certain precautions against possible defeat current of persons conducting these works.
Most in a simple way release of the victim from the current is shutdown of an electrical installation or that part of it that a person touches. When the installation is turned off, the electric light may go out, therefore, in the absence of daylight, it is necessary to have another light source ready - a lantern, a candle, etc.
After the release of 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 with medical assistance. If the victim has not lost consciousness, it is necessary to provide him with rest, and if there are injuries or injuries (bruises, fractures, dislocations, burns, etc.), he must be given first aid before the doctor arrives or taken to the nearest medical facility.
If the victim has lost consciousness, but breathing is preserved, it is necessary to lay him evenly and comfortably on a soft bedding - a blanket, clothes, etc., unfasten the collar, belt, remove tight clothing, clean the mouth from blood, mucus, provide fresh air, give a sniff of ammonia, sprinkle with water, grind and warm the body.
In the absence of signs of life (with clinical death, there is no breathing and pulse, the pupils of the eyes are dilated due to oxygen starvation of the cerebral cortex) or with intermittent breathing, the victim should be quickly released from clothing that restricts breathing, clean the mouth and do artificial respiration and heart massage.
Knowledge of the processes occurring in electrical installations allows power engineers to safely operate equipment of any voltage and type of current, perform repair work and Maintenance electrical systems.
The information provided in the PTB and PTE - the main documents created by the best specialists based on an analysis of accidents with people affected by dangerous factors accompanying the work of electrical energy.
Circumstances and causes of a person getting under the influence of electric current
Safety guidance documents identify three groups of reasons for electric shock to workers:
1. unintentional, unintentional approach to current-carrying parts with voltage at a distance less than safe or touching them;
2. occurrence and development of emergencies;
3. violation of the requirements specified in the governing documents prescribing the rules of conduct for workers in existing electrical installations.
The assessment of the dangers of human injury consists in determining by calculations the magnitude of the currents that pass through the body of the victim. In this case, it is necessary to take into account many situations when contacts can occur in random places in the 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 the defeat of a person by the current of an electrical installation
In order for current to flow through the body of the victim, it is necessary to create an electrical circuit by connecting it to at least two points of the circuit with a potential difference - voltage. Electrical equipment may experience the following conditions:
1. simultaneous two-phase or two-pole touch to different poles (phases);
2. single-phase or single-pole contact with the potential of the circuit, when a person has a direct galvanic connection with the potential of the earth;
3. accidental creation of contact with the conductive elements of the electrical installation, which were energized as a result of the development of the accident;
4. falling under the action of the step voltage, when a potential difference is created between the points on which the legs or other parts of the body are simultaneously located.
In this case, an 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. straight;
2. or indirect.
In the first case, it is created by direct contact with a live part that is energized, and in the second case, by touching non-isolated 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, it is necessary:
1. to analyze cases of possible creation of paths for the passage of electric current through the body of service personnel;
2. compare its maximum possible value with the current minimum allowable standards;
3. make a decision on the implementation of measures to ensure electrical safety.
Features of the analysis of the conditions of damage to people in electrical installations
To assess the amount of current passing through the body of the victim in a DC or AC voltage network, the following types of designations are used for:
1. resistances:
Rh - in the human body;
R0 - for grounding device;
Riz - insulation layer relative to the ground contour;
2. currents:
Ih - through the human body;
Iz - short circuit to the earth contour;
Uc - circuits of direct or single-phase alternating currents;
Ul - linear;
Uf - phase;
Upr - touch;
Ush - step.
In this case, the following typical schemes for connecting the victim to the voltage circuits in the networks are possible:
1. DC at:
unipolar contact of the conductor contact with the potential isolated from the earth 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.
Damage schemes in DC circuits
Single-pole human contact with potential isolated from earth
Under the action of the voltage Uc, a current Ih flows through a series-created chain of the potential of the lower conductor, the body of the victim (hand-foot) and the earth circuit through the doubled insulation resistance of the medium.
Single-pole human contact with earthed pole potential
In this scheme, the situation is aggravated by the connection to the ground loop of one potential wire with a resistance R0 close to zero and much less than that of the victim's body and the insulation layer of the external environment.
The strength of the desired 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 body of the victim, and the current through his body is limited only by his own insignificant resistance.
General defeat schemes in three-phase alternating current circuits
Creation of human contact between phase potential and earth
In the general case, there is a resistance between each phase of the circuit and the ground potential and a capacitance is created. The neutral of the windings of the voltage source has a generalized resistance Zн, the value of which is in different systems ground circuit is changing.
The formulas for calculating the conductivities of each circuit and the total current Ih through the phase voltage Uf are presented in the picture by formulas.
Formation of human contact between two phases
The greatest magnitude 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 Biphasic Touch
In DC and three-phase AC circuits, making contacts between two different potentials is the most dangerous. With this scheme, a person falls under the influence of the greatest voltage.
In a circuit with a constant voltage power supply, the current through the victim is calculated by the formula Ih \u003d Uc / Rh.
In a three-phase network alternating current this value is calculated by the ratio Ih=Ul/Rh=√3 Uf/Rh.
Considering that the average electrical resistance of the human body is 1 kiloohm, we 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 enough for the victim to undergo convulsive muscle contraction, when, without outside help, he is no longer able to free himself from the effects of an accidental touch - the holding current.
In the second case, Ih=(220 1.732)/1000\u003d 0.38A. At this value of 380 mA, there is a mortal danger of injury.
We also pay attention to the fact that in a three-phase alternating voltage network, the position of the neutral (it can be isolated from the ground or vice versa - short-circuited) has very little effect on the value of the current Ih. Its main share does not go through the earth circuit, but between the phase potentials.
If a person has applied means of protection that ensure his reliable isolation from the earth circuit, then in such a situation they will turn out to be useless and will not help.
Single phase touch features
Three-phase network with deafly grounded neutral
The victim touches one of the phase wires and falls under the potential difference between him and the earth circuit. Such cases occur most often.
Although the phase-to-earth voltage is less than 1.732 times line-to-line, such a case remains dangerous. The condition of the victim can worsen:
neutral mode and the quality of its connection;
electrical resistance of the dielectric layer of 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 relation:
Ih=Uf/(Rh+Rob+Rp+R0).
Recall that the resistances: the human body Rh, shoes Rb, floor Rn and grounding at the neutral R0, are taken in Ohms.
The smaller the denominator, the more current is generated. If an employee wears conductive shoes, for example, his feet are wet or the soles are lined with metal nails, and in addition he 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=Uf/(Rh+R0).
With a phase voltage of 220 volts, we get Ih \u003d 220 / 1000 \u003d 0.22 A. Or a mortal danger current of 220 mA.
Now let's calculate the option when the employee uses protective equipment: dielectric shoes (Rb = 45 kOhm) and an insulating base (Rp = 100 kOhm).
Ih=220 /(1000 +45000+10000)=0.0015 A.
We got a safe current value of 1.5 mA.
Three-phase network with isolated neutral
There is no direct galvanic connection between the neutral of the current source and the earth potential. The phase voltage is applied to the resistance of the insulation layer Riz, which has a very high value, which is controlled during operation and constantly maintained in good condition.
The circuit of current flow through the human body depends on this value in each of the phases. If we take into account all layers of current resistance, then its value can be calculated by the formula: Ih=Uf/(Rh+Rb+Rp+(Riz/3)).
During the most unfavorable case, when conditions for maximum conductivity through shoes and the floor are created, the expression will take the form: Ih=Uf/(Rh+(Riz/3)).
If we consider a 220 volt network with a layer insulation of 90 kOhm, then we get: Ih \u003d 220 / (1000 + (90000/3)) \u003d 0.007 A. Such a current of 7 mA will be well felt, but it will not be able to provide fatal injury.
Note that in this example we deliberately omitted the resistance of the ground and shoes. If they are taken into account, then the current will decrease to a safe value, on the order of 0.0012 A or 1.2 mA.
Findings:
1. in circuits with an isolated neutral, it is easier to ensure the safety of workers. 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 the most dangerous than with an isolated one.
Consider the case of touching the metal case 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 the neutral to the voltage source.
The replacement circuit is shown in the picture below. The load created by the device has resistance Rn.
The insulation resistance Riz together with R0 and Rh limits the phase-to-phase contact current. It is expressed by the ratio: Ih=Uf/(Rh+Riz+Rо).
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: Riz> (Uf / 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 current-carrying parts relative to the earth contour determines the degree of safety of the electrical installation.
For this reason, all such resistances are normalized and taken into account in 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 testing.
Step Voltage
In electrical installations, for various reasons, an accident can occur when the phase potential directly touches the ground loop. If on an overhead power line one of the wires breaks under the influence of various types of mechanical loads, then just in this case a similar situation manifests itself.
In this case, at the point of contact of the wire with the ground, a current is formed, which creates a spreading zone around the point of contact - a platform on the surface of which an electric potential appears. Its value depends on the short circuit current Iz and the specific state of the soil r.
A person who finds himself within the boundaries of this zone falls under the action of the step voltage Ush, as shown in the left half of the picture. The area of the spreading zone is limited by the contour where the potential is absent.
The step voltage value 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 resistance of shoes and legs β2. The values of 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 potential of the wire shorted to earth. It is influenced by the potential difference between the point of contact with the hand and the earth contour, which is expressed by the contact voltage Upr.
In this situation, the current is calculated by the expression: Ih=(Uph.c.∙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 for the injured 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.
The inclusion of a person in the electrical network can be single-phase and two-phase. Single-phase switching is a connection of a person between one of the phases of the network and the ground. The strength of the striking current in this case depends on the mode of the neutral network, the resistance of a person, shoes, floor, phase insulation relative to earth. Single-phase switching occurs much more often and often causes electrical injuries in networks of any voltage. With two-phase switching, a person touches two phases of the electrical network. With a two-phase connection, the current flowing through the body (damaging current) depends only on the mains voltage and the resistance of the human body and does not depend on the neutral mode of the mains supply transformer. Electrical networks are divided into single-phase and three-phase. The single-phase network can be isolated from earth or have a ground wire. On 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 dead-earthed neutral, then he will be practically under phase voltage (R3≤ RC) and the current passing through a person during normal operation of the network will practically not change with a change in insulation resistance and capacitance wires to ground.
The effect of electric current on the human body
Passing through the body, the electric current has a thermal, electrolytic and biological effect.
Thermal action is manifested in burns of the skin or internal organs.
During the electrolytic action, due to the passage of current, decomposition (electrolysis) of blood and other organic fluid occurs, accompanied by the destruction of erythrocytes 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 injury,
§ electric shocks.
Electric shocks can be roughly divided into four levels:
1. convulsive muscle contractions without loss of consciousness;
2. with loss of consciousness, but with the preservation of breathing and heart function;
3. loss of consciousness and impaired cardiac activity or breathing (or both);
4. clinical death, i.e. lack of respiration and circulation.
Clinical death is a transitional period between life and death, it begins from the moment the activity of the heart and lungs stops. A person who is in a state of clinical death does not show any signs of life: she has no breathing, heartbeat, reactions 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, then 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 body.
Determination of the current spreading resistance of single grounding conductors and the procedure for calculating the protective ground loop for stationary technological equipment (GOST 12.1.030-81. SSBT. Protective grounding, zeroing)
Implementation of grounding devices. There are artificial ground electrodes, intended exclusively for grounding purposes, and natural - third-party conductive parts that are in electrical contact with the ground directly or through an intermediate conductive medium used for grounding purposes.
For artificial ground electrodes, vertical and horizontal electrodes are usually used.
The following can be used as natural grounding conductors: water 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 of hydraulic structures, etc.
The calculation of protective grounding aims to determine the main grounding parameters - the number, dimensions and order of placement of single grounding conductors and grounding conductors, at which the touch and step voltages during the phase closing to the grounded case do not exceed the allowable values.
To calculate the grounding, the following information is required:
1) characteristics of the electrical installation - type of installation, types of main equipment, operating voltages, methods of grounding the 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 build the designed group ground electrode system, as well as the estimated depth of their immersion in the ground;
4) measurement data of the soil resistivity in the area where the ground electrode system is to be built, and information about the weather (climatic) conditions under which these measurements were made, as well as the characteristics of the climatic zone. If the earth is assumed to be two-layer, then it is necessary to have measurements of the resistivity of both layers of the earth and the thickness of the upper layer;
5) data on natural grounding conductors: what structures can be used for this purpose and the resistance to their current spreading, obtained by direct measurement. If for some reason it is impossible to measure the resistance of a natural grounding conductor, then information must be provided to determine this resistance by calculation;
6) Rated earth fault current. If the current is unknown, then it is calculated by the usual methods;
7) calculated values of admissible contact (and step) voltages and the duration of the protection, if the calculation is made on the basis of contact (and step) voltages.
The calculation of grounding is usually carried out for cases where the ground electrode is placed in a homogeneous ground. In recent years, engineering methods for calculating grounding conductors in multilayer soil have been developed and began to be applied.
When calculating grounding conductors in homogeneous soil, the resistance of the upper layer of the earth (layer of seasonal changes) due to freezing or drying of the soil is taken into account. The calculation is carried out by a method based on the use of ground electrode conductivity utilization factors and is therefore called the utilization factor method. It is performed both with simple and complex designs of group ground electrodes.
When calculating grounding conductors in a multilayer earth, a two-layer earth model is usually taken with the specific resistances of the upper and lower layers r1 and r2, respectively, and the thickness (power) 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 ground electrode, and therefore called the method of induced potentials. The calculation of grounding conductors in multilayer earth is more laborious. However, it gives more accurate results. It is advisable to use it for complex designs of group grounding, which usually take place in electrical installations with an effectively grounded neutral, i.e. in installations with a voltage of 110 kV and above.
When calculating a grounding device in any way, it is necessary to determine the required resistance for it.
The determination of the required resistance of the grounding device is carried out in accordance with the PUE.
For installations with voltage up to 1 kV, the resistance of the grounding device used for protective grounding of exposed conductive parts in an IT type system must comply with the condition:
where Rz is the resistance of the grounding device, ohm; Upr.adm - touch voltage, the value of which is assumed to be 50 V; Iz is the total earth fault current, A.
As a rule, it is not required to accept the resistance value of the grounding device as less than 4 ohms. Grounding device resistance up to 10 Ohm 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 high earth fault currents);
250 / Iz, but not more than 10 ohms with an isolated neutral (i.e., at low earth fault currents) and provided that the earthing switch is used only for electrical installations with voltages above 1000 V.
In these expressions, Iz is the rated earth fault current.
During operation, an increase in the resistance to the spreading of the current of the grounding conductor in excess of the calculated value may occur, therefore, it is necessary to periodically monitor the value of the resistance of the grounding conductor.
Ground loop
The ground loop is classically a group of vertical electrodes of small depth connected by a horizontal conductor, mounted near the object at a relatively small mutual distance from each other.
As grounding electrodes in such a grounding device, a steel angle or reinforcement 3 meters long was traditionally used, which were driven into the ground with a sledgehammer.
A 4x40 mm steel strip was used as a connecting conductor, which was placed in a previously prepared ditch 0.5–0.7 meters deep. The conductor was connected to the mounted ground electrodes by electric or gas welding.
To save space, the ground loop is usually “folded” around the building along the walls (along the perimeter). If you look at this earth electrode from above, you can say that the electrodes are mounted along the contour of the building (hence the name).
Thus, the ground loop is a ground electrode, consisting of several electrodes (a group of electrodes) connected to each other and mounted around the building along its contour.
Cases of electric shock to a person are possible only when the electrical circuit is closed through the human body or, in other words, when a person touches at least two points of the circuit, between which there is some voltage.
The danger of such a touch, estimated by the magnitude of the current passing through the human body, or by the voltage of the touch, depends on a number of factors: the circuit for connecting a person to the circuit, the network voltage, the circuit of the network itself, the mode of its neutral, the degree of isolation of current-carrying parts from the ground, and also from the value of the capacitance of current-carrying parts relative to the ground, etc.
Schemes for including a person in a chain can be different. However, the most characteristic are two switching schemes: between two wires and between one wire and ground (Fig. 68). Of course, in the second case, it is assumed that there is an electrical connection between the network and the ground.
In relation to AC networks, the first circuit is usually called two-phase switching, and the second - single-phase.
Two-phase switching, that is, a person touching two phases at the same time, as a rule, is more dangerous, since the highest voltage in this network is applied to the human body - linear, and therefore more current will flow through the person:
where Ih is the current passing through the human body, A; UL \u003d √3 Uf - linear voltage, i.e. voltage between the phase wires of the network, V; Uf - phase voltage, i.e., the voltage between the beginning and end of one winding (or between the phase and neutral wires), V.
Rice. 68. Cases of including a person in a current circuit:
a - two-phase inclusion; b, c - single-phase inclusions
It is easy to imagine that two-phase switching is equally dangerous in a network with both isolated and grounded neutrals.
With a two-phase connection, the danger of injury will not decrease even if the person is reliably isolated from the ground, i.e. if he has rubber galoshes or boots on his feet or stands on an insulating (wooden) floor, or on a dielectric mat.
Single-phase switching occurs much more often, but is less dangerous than two-phase switching, since the voltage under which a person finds himself does not exceed the phase one, that is, 1.73 times less than the linear one. Accordingly, the current passing through the person is less.
In addition, the value of this current is also affected by the neutral mode of the current source, the insulation resistance and capacitance of the wires relative to the ground, the resistance of the floor on which the person stands, the resistance of his shoes, and some other factors.
In a three-phase three-wire network with an isolated neutral, the current passing through a person, when touching one of the phases of the network during its normal operation (Fig. 69, a), is determined by the following expression in complex form (A):
where Z is the complex impedance of one phase relative to earth (Ohm):
here r and C are, respectively, the insulation resistance of the wire (Ohm) and the capacitance of the wire (F) relative to the ground (for simplicity, they are taken the same for all wires of the network).
Rice. 69. Touching a person to the wire of a three-phase three-wire network with an isolated neutral: a - in normal mode; b - in emergency mode
The current in real form is (A):
, (35)
If the capacitance of the wires relative to earth is small, i.e. C = 0, which usually takes place in overhead networks of small length, then equation (35) will take the form
, (36)
If the capacitance is large, and the conductivity of the insulation is insignificant, i.e. r ≈ ∞, which usually takes place in cable networks, then according to expression (35), the current through a person (A) will be:
, (37)
where xc \u003d 1 / wC - capacitance, Ohm.
It follows from expression (36) that in networks with an isolated neutral, which have an insignificant capacitance between the wires and the ground, the danger to a person who touches one of the phases during the normal operation of the network depends on the resistance of the wires relative to the ground: with increasing resistance, the danger decreases.
Therefore, it is very important to ensure high insulation resistance in such networks and monitor its condition in order to timely identify and eliminate faults.
However, in networks with a large capacity relative to earth, the role of wire insulation in ensuring touch safety is lost, as can be seen from equations (35) and (37).
In the emergency mode of operation of the network, i.e. when one of the phases was shorted to the ground through a small resistance gzm, the current through a person who touched a healthy phase (Fig. 69, b) will be (A):
, (38)
and the touch voltage (V):
, (39)
If we assume that rzm = 0 or at least assume that rzm< Rh (так обычно бывает на практике), то согласно выражению (39)
, (40)
i.e., a person will be under linear voltage.
Under actual conditions, gzm > 0, therefore, the voltage under which a person who touches a healthy phase of a three-phase network with an isolated neutral during an emergency period will be significantly greater than the phase and somewhat less than the linear voltage of the network. Thus, this case of touching is many times more dangerous than touching the same phase of the network during normal operation.
work [see equations (36) and (39), bearing in mind that r/3>rzm].
In a three-phase four-wire network with a grounded neutral, the conductivity of the insulation and the capacitance of the wires relative to earth are small compared to the conductivity of the neutral ground, so when determining the current through a person touching the phase of the network, they can be neglected.
In the normal mode of operation of the network, the current through a person will be (Fig. 70, a):
, (41)
where r0 is the neutral grounding resistance, Ohm.
Rice. 70. A person touching a phase wire of a three-phase four-wire network with a grounded neutral:
a - in normal mode; b - in emergency mode
In ordinary networks r0< 10 Ом, сопротивление тела человека Rh не опускается ниже нескольких сотен Ом. Следовательно, без большой ошибки в уравнении (41) можно пренебречь значением г0 и считать, что при прикосновении к одной из фаз трехфазной четырехпроводной сети с заземленной нейтралью человек оказывается практически под фазным напряжением Uф, а ток, проходящий через него, равен частному от деления Uф на Rh
It follows that touching a phase of a three-phase network with a grounded neutral during its normal operation is more dangerous than touching a phase of a normally operating network with an isolated neutral [cf. equations (36) and (41)], but it is less dangerous to touch the intact phase of the network with isolated neutral during the emergency period [cf. equations (38) and (41)], since in some cases rzm can differ little from r0.
Exist various schemes inclusion of a person in an electric current circuit:
Single-phase contact - touching the conductor of one phase of an existing electrical installation;
Two-phase contact - simultaneous contact with the conductors of two phases of an existing electrical installation;
Touching non-current-carrying parts of electrical installations that are energized as a result of damage to the insulation;
Switching on step voltage - switching between two points of the earth (soil) that are under different potentials.
Consider the most characteristic schemes for including a person in an electric current circuit.
Single-phase touch in a network with a solidly grounded neutral. The current flowing through the human body ( I h) with a single-phase touch (Fig. 6) closes in the circuit: phase L 3 - human body - base (floor) - neutral grounding - neutral (zero point).
Rice. 6. Scheme of single-phase touch in the network
with solidly grounded neutral
According to Ohm's law:
Where R o - neutral grounding resistance,
R osn - base resistance.
If the base (floor) is conductive, then R base ≈ 0
Given the fact that R about " R h, then
U h = U f
Such contact is extremely dangerous.
Single-phase contact in a network with isolated neutral. The current flowing through the human body (Fig. 7) will close in circuits: phase L 3 - human body - floor and then returns to the network through phase isolation L 2 and L 1 , i.e. then the current follows the circuits: phase isolation L 2 - phase L 2 - neutral (zero point) and phase isolation L 1 - phase L 1 - neutral (zero point). Thus, in the circuit of current flowing through the human body, phase isolations are switched on in series with it. L 2 and L 1 .
Rice. 7. Scheme of single-phase touch in the network
with isolated neutral
Phase insulation resistance Z has active ( R) and capacitive components ( FROM).
R- characterizes the imperfection of the insulation, i.e. the ability of insulation to conduct current, although much worse than metals;
FROM- the capacitance of the phase relative to the ground is determined by the geometric dimensions of an imaginary capacitor, the "plates" of which are phases and grounds.
At R 1 = R 2 = R 3 = R f and FROM 1 = FROM 2 = FROM 3 = FROM F current flowing through the human body:
where Z- impedance of the insulation of the phase wire relative to the ground.
If the capacitance of the phases is neglected FROM f = 0 (aerial networks of small extent), then:
whence it follows that the magnitude of the current depends not only on the resistance of a person, but also on the resistance of the insulation of the phase conductor to earth.
If, for example, R 1 = R 2 = R 3 = 3000 Ohm, then
; U h= 0.0111000 = 110 V
Biphasic touch. With a two-phase touch (Fig. 8), regardless of the neutral mode, a person will be under the line voltage of the network U l and according to Ohm's law:
at U l=380V: I= 380/1000 = 0.38 A = 380 mA.
Rice. 8. Scheme of two-phase human touch
Two-phase contact is extremely dangerous, such cases are relatively rare and are usually the result of working under voltage in electrical installations up to 1000 V, which is a violation of the rules and regulations.
Touching a metal case that is energized. Touching the body of the electrical installation (Fig. 9), in which the phase ( L 3) closed on the case, tantamount to touching the phase itself. Therefore, the analysis and conclusions for single-phase touch cases discussed earlier fully apply to the ground fault case.
Rice. 9. Scheme of a person touching a metal
hull under tension