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, assessed by the value of the current passing through the human body, or by the voltage of the touch, depends on a number of factors: the circuit for connecting a person to the circuit, the network voltage, the circuit of the network itself, its neutral mode, the quality of insulation of current-carrying parts from the ground, as well as capacitance values of current-carrying parts relative to earth, etc.
Schemes for including a person in an electrical circuit may be different. However, the most characteristic are two connection schemes: between two wires and between one wire and ground (Figure 13.5). 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, i.e. 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, therefore, more current (A) will go through the human body:
I h \u003d 1.73 U f / R h \u003d U l / R h, 7)
where U l - linear voltage, i.e., the voltage between the phase wires of the network, equal to, V; U f - phase voltage, i.e., the voltage between the beginning and end of one winding of the current source (transformer, generator) or between the phase and neutral wires, V.
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 a person is reliably isolated from the ground, i.e. if he has dielectric galoshes or boots on his feet, or stands on an insulating floor or on a dielectric rug.
Single-phase switching occurs much more often, but is less dangerous than two-phase, since the voltage under which a person finds himself does not exceed the phase one. Accordingly, the current passing through the human body 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 other factors.
AT three-phase three-wire network with isolated neutral the current strength (A) passing through the human body, when touching one of the phases of the network during its normal operation (Figure 6), is determined by the following expression:
where Z is the impedance complex of one phase relative to earth, Ohm, Z \u003d r / (l + jwCr), r and C are, respectively, the insulation resistance of the wire (Ohm) and the capacitance of the wire (F) relative to earth (for simplicity, they are taken the same for all wires networks).
The current in real form will be, A:
. (9)
If the capacitance of the wires relative to the ground is small, i.e. C » 0, which usually takes place in air networks of small length, then equation (15) will take the form
If the capacitance is large, and the conductivity of the insulation is insignificant, i.e. r » ¥, which usually takes place in cable networks, then according to expression (5), the current strength (A) passing through the human body will be equal to
, (11)
where x c is the capacitance equal to 1/wС, Ohm; w - angular frequency, rad/s.
It follows from expression (6) 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 for timely detection and elimination of 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 (5) and (7).
AT three-phase four-wire network with grounded neutral the conductivity of the insulation and the capacitive conductivity of the wires relative to the ground are small compared to the conductivity of the neutral ground, therefore, when determining the strength of the current passing through the human body, touching the phase of the network, they can be neglected.
During normal operation, its r and current I hpassing through the human body will be (Figure 7) equal to:
I h \u003d U f / (R h + r 0), (12)
where r 0 is the neutral grounding resistance, Ohm.
As a rule, r 0 £ 10 Ohm, while the resistance of the human body R h does not fall below several hundred Ohm × m. Therefore, without a large error in equation (8), we can neglect the value of r 0 and assume that when a person touches one of the phases of a three-phase four-wire network with a grounded neutral, a person is practically under the phase voltage U f, and the current passing through it is equal to the quotient from dividing U f by R h . It follows that touching a phase of a three-phase network with a grounded neutral during its normal operation is more dangerous than touching a phase of a normally operating network with an isolated neutral (see equations (6) and (8)).
The analysis of the danger of defeat practically comes down to determining the value of the current flowing through the human body in various conditions in which it may be during the operation of electrical installations, or the contact voltage. The danger of defeat depends on a number of factors: the scheme for connecting a person to an electrical circuit, the voltage of the network, the scheme of the network itself, the mode of its neutral, the degree of isolation of current-carrying parts from the ground, the capacitance of current-carrying parts relative to the ground, etc.
What are the schemes for including a person in an electrical circuit?
The most characteristic are two switching schemes: between two phases of the electrical network, between one phase and the ground. In addition, it is possible to touch grounded non-current-carrying parts that are energized, as well as turn on a person under step voltage.
What is called the neutral of a transformer (generator) and what are its modes of operation?
The point of connection 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 grounded.
Grounded is the neutral of the generator (transformer), connected to the grounding device directly or through low resistance (for example, through current transformers).
An isolated neutral is a generator or transformer neutral that is not connected to a grounding device or connected to it through a large resistance (signaling, measuring, protection devices, grounding arc extinguishing reactors).
What is the basis for choosing the neutral mode?
The choice of the network scheme, and, consequently, the neutral mode of the current source is made on the basis of technological requirements and safety conditions.
At voltages up to 1000 V, both schemes of three-phase networks are widely used: three-wire with an isolated neutral and four-wire with a grounded neutral.
According to technological requirements, preference is often given to a four-wire network; it uses two operating voltages - linear and phase. So, from a four-wire network of 380 V, it is possible to supply both a power load - a three-phase one, including it between the phase wires for a linear voltage of 380 V, and a lighting one, including it between the phase and neutral wires, i.e., for a phase voltage of 220 V. At the same time the electrical installation becomes much cheaper due to the use of a smaller number of transformers, a smaller cross-section of wires, etc.
According to the safety conditions, one of the two networks is selected based on the situation: according to the conditions of touching the phase wire during the normal operation of the network, the network with isolated neutral is safer, and during the emergency period, the network with grounded neutral. Therefore, it is advisable to use isolated neutral networks when it is possible to maintain a high level of network isolation and when the network capacitance relative to earth is negligible. These can be small branched networks that are not exposed to aggressive environments and are under the constant supervision of qualified personnel. An example is the network of small enterprises, mobile installations.
Networks with grounded neutral are used where it is impossible to provide 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 life-threatening 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 an isolated neutral is not sufficiently substantiated.
Statistical data indicate that both networks are almost identical in terms of reliability.
At voltages above 1000 V up to 35 kV, the networks for technological reasons have an isolated neutral, and above 35 kV they have a grounded one.
Since such networks have a large capacity of wires relative to the ground, it is equally dangerous for a person to touch the wire of the network with both isolated and grounded neutrals. Therefore, the network neutral mode above 1000 V is not selected for safety reasons.
What is the danger of two-phase touch?
Two-phase contact means simultaneous contact with two phases of an electrical installation that is energized (Fig. 1).
Rice. 1. Scheme of a two-phase touch of a person to an alternating current network
Biphasic touch is more dangerous. With a two-phase touch, the current passing through the human body along one of the most dangerous paths for the body (hand-hand) will depend on the voltage applied to the human body, equal to the linear voltage of the network, as well as on the resistance of the human body:
- U l - linear voltage, i.e., the voltage between the phase wires of the network;
- R people - the resistance of the human body.
In a network with a linear voltage U l \u003d 380 V with a resistance of the human body R people \u003d 1000 Ohm, the current passing through the human body will be equal to:
This current is deadly for a person. With a two-phase touch, the current passing through the human body is practically independent of the network neutral mode. Therefore, two-phase contact is equally dangerous both in a network with isolated and grounded neutral (provided that the line voltages of these networks are equal).
Cases of a person touching two phases are relatively rare.
What characterizes single-phase touch?
A single-phase touch is a touch to one phase of an electrical installation that is energized.
It occurs many times more often than a two-phase touch, but is less dangerous, since the voltage under which a person finds himself does not exceed the phase voltage. Accordingly, the current passing through the human body is also less. In addition, this current is greatly influenced by the neutral mode of the current source, the insulation resistance of the network wires relative to the ground, the resistance of the floor (or base) on which the person stands, the resistance of his shoes, and some other factors.
What is the danger of a single-phase touch in a network with a grounded neutral?
Rice. 2. Scheme of a person touching one phase of a three-phase network with a grounded neutral
In a network with a grounded neutral (Fig. 2), the current circuit passing through the human body includes the resistance of the human body, his shoes, the floor (or base) on which the person stands, as well as the grounding resistance of the current source neutral. Given the indicated resistances, the current passing through the human body is determined from the following expression:
- U f - phase voltage of the network, V;
- R people - the resistance of the human body, Ohm;
- R about - the resistance of a person's shoes, Ohm;
- R p - resistance of the floor (base) on which the person stands, Ohm;
- R o - grounding resistance of the current source neutral, Ohm.
Under the most unfavorable conditions (a person who has touched the phase has conductive shoes on his feet - damp or lined with metal nails, stands on damp ground or on a conductive base - a metal floor, on a grounded metal structure), i.e. when R rev \u003d 0 and R p \u003d 0, the equation takes the form:
Since the resistance of the neutral R o is usually many times less than the resistance of the human body, it can be neglected. Then
However, under these conditions, even single-phase contact, despite the lower current, is very dangerous. So, in a network with phase voltage U f \u003d 220 V at R people \u003d 1000 Ohm, the current passing through the human body will have the value:
This current is deadly to humans.
If a person wears non-conductive footwear (e.g. rubber overshoes) and is standing on an insulating base (e.g. a wooden floor), then
- 45 000 - resistance of human shoes, Ohm;
- 100 000 - floor resistance, Ohm.
A current of such strength is not dangerous to humans.
From the above data it can be seen that insulating floors and non-conductive shoes are of great importance for the safety of workers in electrical installations.
What are the features of a single-phase touch in a network with an isolated neutral?
In a network with an isolated neutral (Fig. 3), the current passing through the human body to the ground returns to the current source through the insulation of the wires of the network, which in good condition has high resistance.
Taking into account the resistance of the shoe R about and the floor or base R p on which the person stands, connected in series with the resistance of the human body R people, the current passing through the human body is determined by the equation:
where R from is the insulation resistance of one phase of the network relative to the ground, Ohm.
Rice. 3. Scheme of a person touching one phase of a three-phase network with an isolated neutral
In the most unfavorable case, when a person has conductive shoes and is standing on a conductive floor, i.e. when R vol \u003d 0 and R n \u003d 0, the equation will be greatly simplified:
For this case, in a network with a phase voltage U f \u003d 220 V and a phase insulation resistance R of \u003d 90,000 Ohms at R people \u003d 1000 Ohms, the current passing through a person will be equal to:
This current is much less than the current (220 mA) calculated by us for the case of a single-phase contact under similar conditions, but in a network with a grounded neutral. It is determined mainly by the insulation resistance of the wires relative to ground.
Which network is safer - with isolated or grounded neutral?
Ceteris paribus, a person touching one phase of a network with an isolated neutral is less dangerous than in a network with a grounded neutral. However, this conclusion is valid only for normal (accident-free) network operation conditions, in the presence of an insignificant capacitance relative to the ground.
In the event of an accident, when one of the phases is shorted to ground, a network with an isolated neutral may be more dangerous. This is explained by the fact that during such an accident in a network with an isolated neutral, the voltage of an undamaged phase relative to earth can increase from phase to linear, while in a network with a grounded neutral, the voltage increase will be insignificant.
However, modern electrical networks, due to their branching and considerable length, create a large capacitive conductivity between the phase and the ground. In this case, the danger of a person touching one and two phases is almost the same. Each of these touches is very dangerous, since the current passing through the human body reaches very large values.
What is step voltage?
Step voltage is understood as the voltage between two points of the current circuit, located one from the other at a distance of a step, on which a person is simultaneously standing. The step size is usually taken equal to 0.8 m.
For some animals (horses, cows), the step voltage is greater than for humans, and the current path captures the chest. For these reasons, they are more susceptible to damage by step voltage.
Step voltage occurs around the point where current flows from a damaged electrical installation to earth. The largest value will be near the transition point, and the smallest - at a distance of more than 20 m, i.e. beyond the limits limiting the current spreading field in the soil.
At a distance of 1 m from the ground electrode, the voltage drop is 68% of the total voltage, at a distance of 10 m - 92%, at a distance of 20 m the potentials of the points are so small that they can practically be equal to zero.
Such points of the soil surface are considered to be outside the current spreading zone and are called "ground".
The danger of step stress is increased if the person subjected to it falls. And then the tension of the step increases, since the current path no longer passes through the legs, but through the whole body.
Cases of injury to people due to the impact of step stress are relatively rare. They can occur, for example, near a wire that has fallen to the ground (at such moments, before the line is disconnected, people and animals should not be allowed to come close to the place where the wire fell). The most dangerous step voltage during a lightning strike.
Once in the zone of step voltage, you should leave it in small steps in the direction opposite to the place of the alleged ground fault, and in particular the wire lying on the ground.
II . ELECTRICAL SAFETY
3. Analysis of the electrical safety of various electrical networks
Outcome of electric shock to a person, determined by the current flowing through the human body I h and touch voltage U h , significantly depends on the type of network supplying electricity consumers and its parameters, including:
- network voltage and frequency;
- network neutral mode;
- schemes for including a person in an electrical circuit;
- insulation resistance of the phase wires of the network relative to the ground;
- capacitance of the phase wires of the network relative to the ground;
- network mode.
Typical schemes for including a person in an electrical circuit
There are various “connection schemes” for a person in an electrical current circuit (typical “connection schemes” are shown in Fig. 3.5 using the IT network as an example):
Rice. 3.5. Typical schemes for including a person in an electrical circuit
(3.1.)
From expressions (3.1.) and (
3.2. ) follows that with two-phase when touched, a person falls under the line voltage of the network regardless of the type of network, neutral mode, network operation mode, conductivity of phase wiresY L1 , Y L2 , Y L3relative to the ground. Such a scheme for including a person in an electrical circuit is a great danger.Cases of two-phase contact are relatively rare and are usually the result of working under voltage in electrical installations up to 1 kV, which is a violation of the rules and instructions for performing work.
Rice. 3.6. Generalized scheme for the analysis of three-phase networks
(3.3)
(3.4)
§ 3. Danger of electric shock to a person.
Scheme of a single-phase inclusion of a person in a three-phase current network with a grounded neutral.
Electric shock occurs when an electrical circuit is closed through the human body. This happens when a person touches at least two points of the electrical circuit, between which there is some voltage. The inclusion of a person in the circuit can occur according to several schemes: between the wire and the ground, called single-phase inclusion; between two wires - two-phase switching. These schemes 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 of the earth having different potentials, etc.
Single-phase inclusion of a person in the network is the direct contact of a person with parts of an electrical installation or equipment that are normally or accidentally energized. In this case, the degree of danger of damage will be different depending on whether the electrical network has a grounded or insulated neutral, as well as depending on the quality of the insulation of the wires of the network, its length, mode of operation 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, which is 1.73 times less than the linear one, and is exposed to a current, the value of which is determined by the value of the phase voltage of the installation and the resistance of the human body (Fig. 69). An additional protective effect is provided by the insulation of the floor on which the person stands, and shoes.
Rice. 69. Scheme of a single-phase inclusion of a person in a three-phase current network with a grounded neutral
Thus, in a four-wire three-phase network with a grounded neutral, the current circuit passing through a person includes the resistance of his body, as well as the resistance of the floor, shoes and grounding of the neutral of the current source (transformer, etc.). In this case, the magnitude of the current
where U l - linear voltage, V; R t is the resistance of the human body, Ohm; R p - resistance of the floor on which the person is located, Ohm; R about - the resistance of a person's shoes, Ohm; R 0 - neutral grounding resistance, Ohm.
As an example, consider two cases of single-phase inclusion of a person in a three-phase four-wire electrical network with a grounded neutral at U l \u003d 380 V.
Case with adverse conditions. A person who has touched one phase is on damp ground or a conductive (metal) floor, his shoes are damp or have metal nails. In accordance with this, we accept resistance: the human body R t \u003d 1000 Ohm, soil or floor R p \u003d 0; shoes R about \u003d 0.
Neutral grounding resistance R 0 = 4 ohms is not taken into account due to its insignificant value. A current passes through the human body
being life-threatening.
Favorable case. A person is on a dry wooden floor with a resistance of R n = 60,000 ohms, has dry non-conductive (rubber) shoes on his feet with a resistance of R vol \u003d 50,000 ohms. Then a current will pass through the human body
which is long-term acceptable for a person.
In addition, dry floors and rubber shoes have a significantly higher resistance in comparison with the values accepted for the calculation.
These examples show the great importance of the insulating properties of the floor and footwear to ensure the safety of persons working in conditions of possible contact with electric current.
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 a person or the voltage of contact, depends on a number of factors: the scheme for connecting a person to the mains, 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; Uph - phase voltage, i.e. voltage between the beginning and end of one winding (or between the phase and neutral wires), V.
As can be seen from fig. 8.1, the danger of two-phase switching does not depend on the neutral mode. 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 to (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 the strength of the 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 contact 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.