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 vapor formation from tissue fluid and blood overheated by the current.
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 excitation 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 DC.
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 the insulation of the floor 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;
Turning on under voltage of a step - turning 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 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 on 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- the total insulation resistance of the phase conductor relative to the ground.
If the phase capacitance 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 equivalent 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 defeat of a person by electric current as a result of electric impact, i.e., the passage of current through a person, is a consequence of his touching 2 points of the electric circuit, between which there is some voltage. The danger of such a touch is assessed, as is known, by the current passing through the human body or by the voltage under which it is. It should be noted that the touch voltage depends on a number of factors: the circuit for connecting a person to the electrical circuit, the voltage of the network, the circuit of the network itself, the mode of its neutral, the degree of isolation of the live parts from the ground, as well as the capacitance of the live 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 it and will not be very dangerous, in other cases the currents can reach significant values that can lead to death. This article examines the dependence of the danger of including a person in an electrical circuit, that is, the values of the touch voltage and current flowing through a person, on the listed factors.
This dependence must be known when assessing a particular network according to safety regulations, choosing and calculating appropriate protection measures, in particular grounding, neutralization, protective shutdown, network isolation monitoring devices, etc.
In this case, in all cases, except for those specifically mentioned, 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 typical schemes for connecting a person to an electrical circuit when accidentally touching live conductors are:
1. Connection between two phase conductors of the circuit,
2. Connection between phase and earth.
Of course, in the second option, it is assumed that the considered network is electrically connected to the ground due to, for example, grounding the neutral of the current source or due to poor insulation of the wires relative to the ground, or due to the presence of a large capacitance between them.
Two-phase touch is considered the most dangerous, since in this case a line voltage of 380 volts is applied to the human body, and the current passing through the body does not depend on the network circuit and its neutral mode.
Two-phase touching occurs very rarely and is mainly associated with working under voltage:
On electrical panels, assemblies and overhead lines;
When using defective personal protective equipment;
On equipment with unshielded live parts, etc.
Single-phase contact is usually considered less dangerous, since the current passing through the person in this case is limited by the influence of a number of factors. But it happens in practice much more often than biphasic. Therefore, the topic of this article is to analyze 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 free the victim from the current and immediately begin to provide him with first aid.
Free a person from the action of the current it is necessary as soon as possible, but precautions must be taken. If the victim is at a height, measures should be taken to prevent him from falling.
Touching an energized person, it is dangerous, and when conducting rescue operations, it is necessary to strictly observe certain precautions against possible electric shock to persons carrying out these operations.
The easiest way to release the victim from the current is disconnection of an electrical installation or that part of it that a person touches... When the unit 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 releasing the victim from the current it is necessary to establish the degree of damage and, in accordance with the condition of the victim, provide him with medical assistance. If the victim has not lost consciousness, it is necessary to provide him with rest, and in the presence of injuries or injuries (bruises, fractures, dislocations, burns, etc.), he must be given first aid before the arrival of a doctor or taken to the nearest medical institution.
If the victim has lost consciousness, but breathing is preserved, it is necessary to lay him flat and comfortably on a soft bedding - a blanket, clothes, etc., unfasten the collar, belt, remove embarrassing clothing, cleanse the mouth of blood, mucus, provide fresh air, Allow to smell ammonia, sprinkle with water, grind and warm the body.
In the absence of signs of life (in case of clinical death, there is no breathing and pulse, the pupils of the eyes are dilated due to oxygen starvation of the cerebral cortex) or in case of intermittent breathing, the victim should be quickly released from the clothing that restrains breathing, cleansed the mouth and do artificial respiration and heart massage.
All cases of electric shock to a person as a result of an electric shock are a consequence of touching at least two points of the electrical circuit, between which there is a potential difference. The danger of such a touch largely depends on the characteristics of the electrical network and the scheme for including a person into it. Having determined the amperage / hr passing through a person taking these factors into account, appropriate protective measures can be selected to reduce the risk of injury.
Two-phase inclusion of a person in the current circuit (Fig. 8.1, a). It happens quite rarely, but it is more dangerous compared to single-phase, since the highest voltage in this network is applied to the body - linear, and the current, A, passing through a person does not depend on the network circuit, its neutral mode and other factors, i.e. e.
I = Ul / Rch = √ 3Uph / Rch,
where Ul and Uf are linear and phase voltage, V; Rh is the resistance of the human body, Ohm (according to the Rules for Electrical Installations in the calculations, Rh is taken equal to 1000 Ohm).
Cases of two-phase contact can occur when working with electrical equipment without removing the voltage, for example, when replacing a blown fuse at the entrance to a building, using dielectric gloves with ruptured rubber, connecting a cable to unprotected clamps of a welding transformer, etc.
Single-phase connection. The current passing through a person is influenced by various factors, which reduces the risk of injury compared to two-phase contact.
Rice. 8.1. Schemes of the possible inclusion of a person in a three-phase current network:
a - two-phase touch; b - single-phase contact in a network with a grounded neutral; c - single-phase contact in a network with an isolated neutral
In a single-phase two-wire network, isolated from the ground, the current strength, A, passing through a person, when the insulation resistance of the wires is equal to the ground r1 = r2 = r, is determined by the formula
Ich = U / (2Rh + r),
where U is the mains voltage, V; r - insulation resistance, Ohm.
In a three-wire network with an isolated neutral, at r1 = r2 = r3 = r, the current will go from the point of contact through the human body, shoes, floor and imperfect insulation to other phases (Figure 8.1, b). Then
Ich = Uph / (Ro + r / 3),
where Ro is the total resistance, Ohm; RO = Rh + Rop + Rp; Rob - shoe resistance, cm: for rubber footwear Rab ≥ 50 000 Ohm; Rn - floor resistance, Ohm: for a dry wooden floor, Rp = 60,000 Ohm; d - insulation resistance of wires, Ohm (according to the PUE, it should be at least 0.5 megohm per phase of the network section with voltage up to 1000 V).
In three-phase four-wire networks, the current will go through the person, his shoes, the floor, the neutral ground of the source and the neutral wire (Figure 8.1, c). The strength of the current, A, passing through a person,
Ich = Uph (Rо + Rн),
where RH is the neutral grounding resistance, Ohm. Neglecting the resistance RH, we get:
At agricultural enterprises, four-wire electrical networks with a dead-grounded neutral voltage up to 1000 V are mainly used. Their advantage is that through them two operating voltages can be obtained: linear Ul = 380 V and phase Uph = 220 V. high requirements for the quality of insulation of wires and they are used with a large branching network. Somewhat less often, a three-wire network with an isolated neutral is used at voltages up to 1000V - it is safer if the insulation resistance of the wires is maintained at a high level.
Touch voltage. It occurs as a result of touching live electrical installations or metal parts of equipment.
Step voltage. This is the voltage Ush on the human body when the legs are positioned at the points of the current spreading field from the ground electrode or from the wire that fell to the ground, where the feet are located, when a person walks in the direction of the ground electrode (wire) or away from it (Fig. 8.2).
If one leg is at a distance x from the center of the ground electrode system, then the other is at a distance x + a, where a is the step length. Usually, a = 0.8 m is taken in the calculations.
The maximum voltage in this case arises at the point of the current short circuit to the ground, and with distance from it it decreases according to the law of hyperbola. It is considered that at a distance of 20 m from the point of short circuit, the earth potential is zero.
Step voltage, V,
Rice. 8.2. Step voltage generation circuit
Even with a small step voltage (50 ... 80 V), an involuntary convulsive contraction of the leg muscles can occur and, as a consequence, a person falls to the ground. At the same time, he simultaneously touches the ground with his hands and feet, the distance between which is greater than the length of the stride, so the acting stress increases. In addition, in this position of a person, a new pathway for the passage of current is formed, affecting the vital organs. This creates a real threat of fatal defeat. As the stride length decreases, the step voltage decreases. Therefore, in order to get out of the zone of action of the step voltage, one should move by jumping on one leg or on two closed legs, or in as short steps as possible (in the latter case, a voltage of no more than 40 V is considered permissible).
The connection point of the windings of the supply transformer (generator) is called the neutral point or neutral. The neutral of the power supply can be isolated and earthed. Grounded is called the neutral of the generator (transformer), connected to the grounding device directly or through a low resistance (for example, through current transformers). Isolated is called the neutral of a generator or transformer, not connected to the grounding device or connected to it through a large resistance (signaling devices, measurements, protection, grounding arc suppression reactors).
An electric shock to a person 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 .
Single-phase switching represents direct human contact with parts of an electrical installation or equipment that are normally or accidentally energized. With a single-phase connection to a network with an isolated and 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 that depends on the phase voltage of the installation, the resistance of the person's body, shoes, floor, neutral grounding, insulation.
At single-phase connection to a three-phase four-wire network with earthed 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. V; R h is the resistance of the human body, Ohm; r p is the resistance of the floor on which the person is. Ohm; r o is the resistance of the shoe. Ohm; r n - grounding resistance neutr. Ohm; U pr - touch voltage, V.
As an example, two cases of a single-phase connection of a person into 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 with unfavorable conditions. A person who touches one phase is on damp ground or a conductive (metal) floor, his shoes are damp or has metal nails. In accordance with this, the following resistances are accepted: human body = 1000 ohms; soil or floor r p = 0; shoes r o = 0. Resistance of neutral grounding r n = 4 Ohm (in the calculation, due to the insignificant value, can be neglected).
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 with favorable conditions. A person is on a dry wooden floor with a resistance of r p = 100000 Ohm, on his feet there are dry non-conductive (rubber) shoes with a resistance of r about = = 45000 Ohm. Then a threshold current that is 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 flooring and footwear for the safety of persons working in conditions of possible electrical shock.
Two-phase switching is the simultaneous touch of a person to two different phases of the same energized network. In this case, the person turns 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 . With a two-phase connection, the insulation resistance of the wires has no protective effect:
I h = 1.73 U f / R h = 380/1000 = 0.38A = 380mA U pr = I h R h = 380 V
This value of current (voltage) is fatal to human life. In this case, the neutral mode for a two-phase connection is practically irrelevant. Cases of two-phase switching are relatively rare: they are most likely when working under voltage, when the live 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 - line and phase. So, from a four-wire 380 network, you can supply both a power load - three-phase, including it between the phase wires for a line voltage of 380 V, and lighting, including it between the phase and neutral wires, that is, for a phase voltage of 220 V. At the same time, an electrical installation is much cheaper due to the use of a smaller number of transformers, a smaller cross-section of wires, 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 high values that are dangerous to life person. Such networks include networks of large industrial enterprises, urban distribution networks, etc. The existing opinion about a higher degree of reliability of networks with isolated neutral is not sufficiently substantiated. Statistical data indicate that in terms of reliability of operation, both networks are practically the same.
At voltages above 1000V up to 35 kV, networks for technological reasons have an insulated neutral, and above 35 kV - grounded.
The premises according to the degree of danger can be attributed: to the 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.; to the 3rd class - all workshops of machine-building plants: galvanic, storage batteries, etc. These also include areas of work outdoors.
