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 solidly grounded 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 organism.
Determination of the current spreading resistance of single grounding conductors and the procedure for calculating the protective ground loop for stationary process equipment (GOST 12.1.030-81. SSBT. Protective grounding, zeroing)
Implementation of grounding devices. A distinction is made between 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 purpose of the calculation of protective grounding is 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 expected 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 amount of 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 rug.
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 one of the phases of the network is touched 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 an increase in 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.
works [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, therefore, 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.
Analysis of the danger of electric shock in various networks
The defeat of a person by electric current is possible only with his direct contact with the points of the electrical installation, between which there is voltage, or with a point whose potential differs from the potential of the earth. The analysis of the danger of such a touch, estimated by the value of the current passing through the person or the voltage of the touch, depends on a number of factors: the scheme for connecting a person to the power grid, its voltage, the neutral mode, the insulation of current-carrying 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 rules for the operation of electrical installations (PTE and PTB), the second - as a result of emergency situations, for example, during insulation breakdown.
Schemes for including a person in an electrical circuit can be different. However, the most common are two: between two different wires - a two-phase connection and between one wire or the body of an electrical installation, one phase of which is broken, and the ground - a 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 a person is under linear voltage, while the current passing through a person will be (in A)
where Ul - linear voltage, i.e. voltage between phase wires, V; Uf - 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. Neutral is the point of connection of the windings of a transformer or generator, not connected to a grounding device or 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, floors.
With a single-phase inclusion of a person in the network, the current strength is largely determined by the neutral mode. For the case under consideration, the current passing through a person will be (in A)
, (8.3)
where w is the frequency; C - phase capacitance relative to earth
Rice. 8.1. Inclusion of a person in a three-phase network with an isolated neutral:
a - two-phase inclusion; b - single-phase inclusion; Ra, Rt, Rc - electrical resistance of phase insulation relative to earth. Ohm; Ca, Cb, Cs - capacitance of the wires relative to the ground, F, Ia, Ib, IC 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, the insulation of phases, made of dielectric materials and having a finite value, changes differently for each phase during aging, moisture, dust coverage. Therefore, the calculation of safe conditions, which is greatly complicated, must be carried out taking into account the actual values of resistance R and capacitances C for each phase. If the capacitance of the phases relative to the ground is small, i.e. Ca \u003d Cb \u003d Cc \u003d 0 (for example, in air networks of small length), then
Ich \u003d Up / (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 \u003d 1 / wС - capacitance, Ohm.
From the above expressions it can be seen that in networks with an isolated neutral, the danger of electric shock to a person is the less, the lower the capacitive and the higher the active component of the phase wires relative to the ground. Therefore, in such networks, it is very important to constantly monitor Riz to identify and eliminate damage.
Rice. 8.2. Inclusion of a person in a three-phase network with an isolated neutral in emergency mode. 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 earth, the current 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 >> Rzm.
A ground fault also 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 current spreading in the ground, which, in turn, leads to a new type of human injury, namely, contact voltage and step voltage. Such closure may be accidental or intentional. In the latter case, the conductor in contact with the ground is called a ground electrode or an electrode.
In the volume of the earth where the current passes, the so-called """field (zone) of current spreading" arises. Theoretically, it extends to infinity, however, in real conditions, already at a distance of 20 m from the ground electrode, the spreading current density and potential are practically equal to zero.
The nature of the potential spreading curve essentially depends on the shape of the ground electrode. So, for a single hemispherical ground electrode, the potential on the earth's surface will change according to the hyperbolic law (Fig. 8.3).
Rice. 8.3. Distribution of potential on the earth's surface around a hemispherical earth electrode (f - change in the potential of the earth electrode on the earth's surface; fz - maximum potential of the earth electrode at earth fault current I3; r - radius of the earth electrode)
Rice. 8.4. Contact 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 as the distance from the ground electrode; 3 - phase breakdown on the body
Depending on the location of a person in the spreading zone and his contact with the electrical installation b, the body of which is grounded and energized, a person can fall under the 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 - phosn (in B)
Upr \u003d f3 - phosn \u003d f3 (1 - phosn / f3), (8.7)
where the expression (1 - phosn/f3) = a1 is the contact voltage coefficient characterizing the shape of the potential curve.
From fig. 8.4 it can be seen that the contact 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 potential of the earth electrode Upr \u003d f3, while a1 \u003d I. If a person stands directly above the earth electrode (electrical installation a), then Unp = 0 and a1 = 0. This is the safest case.
Expression (8.7) allows us to calculate Unp without taking into account additional resistances in the man-ground circuit, i.e., 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, therefore, in real conditions, the value of the contact voltage will be even less.
There are various schemes for including 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 the 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 the 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 the cases of single-phase contact, considered earlier, fully apply to the case of a ground fault.
Rice. 9. Scheme of a person touching a metal
hull under tension
Such diseases that aggravate the outcome of electrical injury include: increased thyroid function, many diseases of the nervous system, angina pectoris. Of particular note is the effect of alcohol intoxication. In addition to the fact that a person in a state of alcoholic intoxication more often makes mistakes and receives an electrical injury, in him, due to alcohol intoxication, the central nervous system loses its regulatory role in controlling breathing and blood circulation, which greatly aggravates the outcome of the lesion.
Inclusion of a person in an electric current circuit
Reasons for inclusion. A person is included in the electric current circuit by direct contact of the body with the current-carrying part of the electrical installation, which is energized. This usually happens due to negligence or as a result of erroneous human actions, as well as due to malfunctioning electrical installations and technical means of protection. Such cases include, for example:
Touching live parts under voltage, assuming that they are de-energized;
Touching during repair, cleaning or inspection to previously de-energized current-carrying parts, but to which an unauthorized person erroneously energized or spontaneous switching on of a faulty starting device occurred;
Touching the metal parts of electrical installations, which are usually not energized, but turned out to be energized relative to the ground due to damage to the electrical insulation or other reasons (short circuit to the case);
The occurrence of step voltage on the surface of a conductive base (floor) on which a person passes; and etc.
Inclusion schemes. A person can join the electric circuit by touching one phase of an electrical installation that is energized, two phases at the same time, or a zero protective conductor and a phase. Contact with the zero protective conductor is safe (Fig. 2, a, I), other cases entail serious consequences.
Rice. Fig. 2. Schemes of the paths for the passage of electric current through the human body: a - touching the wires; b – occurrence of touch voltage; c - The occurrence of step voltage; I-touching the neutral wire; II - touching the phase wire; III - touching the phase and neutral wires; IV - touching the phase wires; 0 - neutral wire; 1, 2, 3 - phase wires; 4 - neutral point; 5- single ground electrode (electrode); A, B, C - electrical installations
Single-phase (single-pole) contact (Fig. 2, a, II and III) occurs most often when replacing lamps and caring for fixtures, changing fuses and servicing electrical installations, etc. In a neutrally grounded system, a person will be under phase voltage Uph (in V), which is less than linear Ul:
Accordingly, the magnitude of the phase current passing through the human body will also be less. If at the same time a person is reliably isolated from the ground (shod in dielectric galoshes, the floor is dry and non-conductive), then a single-phase touch is not dangerous.
Two-phase (two-pole contact) contact is more dangerous, because a person gets under linear voltage (Fig. 2, a, IV). Even at a voltage of 127 V and a calculated human body resistance of 1000 ohms, the current in the circuit will be lethal (127 mA). With a two-phase touch, the danger of injury will not decrease even if the person is reliably isolated from the ground (floor).
Two-phase contact occurs rarely, usually during live work, which is strictly prohibited.
If the insulation of current-carrying parts is damaged and they are shorted to the body of electrical equipment, a significant potential may appear. A person who touches in this case the body of the electrical installation (Fig. 2, b) will be under the contact voltage Up (in V)
where Ich is the value of the current passing through a person along the “arm-leg” path, A; Rh - resistance of the human body, Ohm.
The touch voltage is the potential difference between two points of the electrical circuit, which is simultaneously touched by a person, or the voltage drop in the resistance of the human body.
The touch voltage will increase as the distance between the electrical installation and the ground electrode increases, reaching a maximum at a distance of 20 m or more. When a phase wire falls on the earth's surface, a current spreading zone appears (Fig. 2, c).
A person passing through this zone will be under step voltage (potential difference) between two points of the current circuit, located one from the other at a step distance (0.8 m). The highest step voltage will be near the closing point and, gradually decreasing, will drop to zero at a distance of 20 m.
You should not approach the fallen wire closer than 6-8 m.
Psycho-emotional alertness - "attention factor" when working with electric current
The formation of psycho-emotional alertness among workers, the “attention factor” when working with electric current is the most important condition for personal prevention of electrical injuries. This factor is based on the knowledge of the physiological effect of electric current on the body when the victim enters the electrical circuit.
In particular, the “attention factor” plays a decisive role in many cases of lesions, i.e., in essence, the severity of the outcome of the lesion is determined to a large extent by the state of the human nervous system at the time of the lesion.
It is necessary that a person be "collected", which allows him to expect some event during work that requires attention.
Such a statement is valid mainly in case of electric shock with a voltage of 220-300 V. At high voltages, a severe outcome most often occurs from arc burns. There are already reasons to believe that the risk of burns increases almost linearly depending on the voltage value.
The factor of attention undoubtedly causes the mobilization of the body's defense systems, enhances the blood circulation of the heart muscle, cerebral blood flow through the pituitary-adrenal system and makes them more resistant to external stimuli (electrical injury).
With the factor of attention, it is much more difficult to upset the biosystem of automatic regulation of the most important systems of the body (the central nervous system, blood circulation, respiration).
However, it should be noted that the role of the attention factor is not yet sufficiently reflected in the protective measures for electrical safety.
But there is confidence that new views on the electrical safety of living tissue, further study of the nature of the electrical activity of the human body will reveal the biophysics of the mechanism of human injury, which will be taken into account in the development of measures to protect against the action of electric current.
Measures to ensure the safe operation of electrical equipment
Technical methods and means of protection that ensure electrical safety are indicated taking into account: power supply with electricity of rated voltage, type and frequency of current; neutral mode, type of execution; environmental conditions; the possibility of removing voltage from current-carrying parts; the nature of the possible touch of a person to the elements of the current circuit.