Analysis of electrical safety conditions
The analysis of electrical safety conditions consists in determining the magnitude of the current through the human body (I h) for a specific case.
Comparing the calculated values of the current through the human body with the value of the conditionally safe current (10mA), a conclusion is made about the danger of this case. If the value of the current through the human body exceeds the value of the conditionally safe current, the case is considered dangerous. If not, not dangerous. Since a person in most cases uses a network up to 1000V, and these networks, as a rule, have a short length, the capacitance of the phase wires relative to the ground can be neglected, considering that the insulation resistance of the wires (R from) relative to the ground is purely active.
You can determine the amount of current through the human body as follows:
I h = U pr / R h
The complexity of the calculation lies in finding the touch voltage (U pr). To find this value, they resort to this technique: they determine the path of the current through the human body, from which they find the source of voltage and resistance through which the current flows.
The most typical are two connection schemes: between two wires and between one wire and ground.
With regard to AC networks, the first circuit is usually called two-phase connection, and the second single-phase.
9.1.1. Two-phase switching
A two-phase connection, as a rule, is more dangerous, since the highest voltage in this network is applied to the human body - linear, and therefore a large current will go through the human body (Figure 9.1.).
Figure 9.1. Two-phase inclusion of a person in the network.
where, I h - current through the human body
U pr - touch voltage
For 380/220 network
Current dangerous to human life
9.1.2. Single-phase connection.
Single-phase switching occurs much more often, but it is less dangerous, because the voltage under which a person finds himself does not exceed the phase voltage. In addition, the neutral mode of the current source, the insulation resistance of the wires relative to the ground, the resistance of the floor on which the person stands, the resistance of the person's shoes and other factors also affect the value of the current through the human body.
9.1.2.1. Single-phase network.
Figure 9.3. Connection diagram
Figure 9.4. Equivalent circuit
The current through the human body can be found as:
From the expression, you can draw conclusions:
1. The greater the insulation resistance to ground, the less the danger of single-phase touching the wire.
2. The touch of a person to a wire with a high insulation resistance is more dangerous, because the tension of the touch will be greater.
9.1 1.2. Three-phase three-wire network with insulated neutral:
Consider two network modes:
a) Normal operation (insulation resistance is of great (normalized) value.
Figure 9.5. Single-phase connection to a 3-phase network
with isolated neutral
If the insulation resistances are equal R of1 = R of2 = R of3, the value of the current through the human body is determined by the expression
In such networks, the danger to a person touching the wire, during the normal state of the network, depends on the insulation resistance. The larger it is, the less danger. Therefore, it is very important in such networks to provide high insulation resistance and monitor its condition for timely detection and elimination of faults that have arisen.
According to PES, the insulation resistance of wires to ground in installations up to 1000V should not be less than 500k.
b) In emergency mode - short circuit of one of the phases to ground through a low short circuit resistance - R zm. (Figure 9.6.)
Figure 9.6 Network Emergency Mode
Typically R gm lies in the range from 50 to 200 Ohm.
The current through the human body, as in the normal mode, will also flow through the insulation resistance of the wires relative to the ground, but its value will be much less than the current flowing through the small circuit resistance. Therefore, the magnitude of the current flowing through the insulation resistance can be neglected and it can be assumed that the current flows only through the circuit resistance and the human body.
It is very dangerous.
9.1.2.3. Three-phase three-wire network with a dead-grounded neutral:
Solidly grounded is the neutral of a transformer or generator connected to a grounding device directly or through a low resistance (for example, a current transformer).
a) Normal operation
Figure 9.7.
The neutral grounding resistance R о is standardized depending on the maximum mains voltage.
With U l = 660V, R about = 2 Ohm, with U l = 380V, R about = 4 Ohm, with U l = 220V, R about = 8 Ohm
The current flowing through the human body and the insulation resistance of the wires can be neglected, compared to the current flowing through the human body and the low resistance of the neutral grounding. The magnitude of this current is determined from the expression:
It can be seen from the expression that in a network with a dead-grounded neutral during the period of normal network operation, touching one of the wires is more dangerous than touching a wire of a normally operating network with an insulated net.
b) In emergency operation - when one of the phases of the network is shorted to ground through a small resistance R zm (Figure 9.8.).
Figure 9.8.
If we analyze this case, then we can draw the following conclusions:
2. If we take R about equal to 0, then the person will be under phase voltage.
In real conditions, R zm and R about are always greater than zero, therefore, a person, touching the wire in the emergency mode of the network, gets under voltage less than linear, but more than phase voltage.
Since the resistance of the electrical circuit R the magnitude of the electric current passing through a person depends significantly, the severity of the lesion is largely determined by the scheme for including the person in the circuit. The circuits formed when a person contacts a conductor of circuits depend on the type of power supply system used.
The most common electrical networks in which the neutral wire is grounded, that is, it is short-circuited by a conductor to the ground. Touching the neutral wire practically does not pose a danger to humans, only the phase wire is dangerous. However, it is difficult to figure out which of the two wires is zero - they look the same in appearance. You can figure it out using a special device - a phase detector.
Using specific examples, we will consider possible schemes for connecting a person to an electrical circuit when touching conductors.
Two-phase connection to the circuit. The rarest, but also the most dangerous, is the touch of a person to two phase wires or current conductors connected to them (Fig. 2.29).
In this case, the person will be under the influence of line voltage. A current will flow through the person along the "hand-hand" path, that is, the resistance of the circuit will include only the resistance of the body (D,).
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If we take the body resistance of 1 kOhm, and the electrical network with a voltage of 380/220 V, then the strength of the current passing through a person will be equal to
This is a deadly current. The severity of an electrical injury or even a person's life will depend primarily on how quickly he gets rid of contact with the current conductor (breaks the electrical circuit), because the exposure time in this case is decisive.
Much more often there are cases when a person comes into contact with one hand with a phase wire or part of a device, apparatus that is accidentally or intentionally electrically connected to it. The risk of electric shock in this case depends on the type of electrical network (grounded or isolated neutral).
Single-phase connection to a circuit in a network with a grounded neutral(fig. 2.30). In this case, the current passes through the person along the "hand-to-foot" or "hand-to-hand" path, and the person will be under phase voltage.
In the first case, the resistance of the circuit will be determined by the resistance of the human body (I_, shoe (R o 6), foundations (R g), on which a person stands, with a neutral grounding resistance (R H), and a current will flow through the person
Neutral resistance R H is small and negligible compared to other circuit resistances. To assess the magnitude of the current flowing through a person, we take the mains voltage 380/220 V. If a person is wearing insulating dry shoes (leather, rubber), he stands on a dry wooden floor, the resistance of the circuit will be large, and the current strength according to Ohm's law is small.
For example, floor resistance 30 kOhm, leather shoes 100 kOhm, human resistance 1 kOhm. Current passing through a person
This current is close to the perceptible threshold current. The person will feel the current flow, stop working, and eliminate the malfunction.
If a person stands on wet ground with damp shoes or bare feet, current will flow through the body.
This current can cause disruption in the functioning of the lungs and heart, and with prolonged exposure and death.
If a person stands on wet soil in dry and intact rubber boots, a current flows through the body
A person may not even feel the impact of such a current. However, even a small crack or puncture in the sole of a boot can drastically reduce the resistance of the rubber sole and make work dangerous.
Before you start working with electrical devices (especially those that are not in operation for a long time), they must be carefully inspected for damage to the insulation. Electrical devices must be cleaned of dust and, if wet,- dry. Do not operate wet electrical devices! It is better to store electrical tools, appliances, equipment in plastic bags to prevent dust or moisture from entering them. You have to work in shoes. If the reliability of an electrical device is in doubt, you need to play it safe.- Place a dry wooden floor or rubber mat under your feet. Rubber gloves can be used.
The second path of current flow occurs when a person with his second hand touches electrically conductive objects connected to the ground (the body of a grounded machine tool, a metal or reinforced concrete structure of a building, a damp wooden wall, a water pipe, a heating battery, etc.). In this case, the current flows along the path of least electrical resistance. These objects are practically short-circuited to the ground, their electrical resistance is very low. Therefore, the resistance of the circuit is equal to the resistance of the body and a current will flow through the person
This current is lethal.
When working with electrical devices, do not touch objects with your other hand that may be electrically connected to the ground. Working in damp rooms, in the presence of well-conductive objects connected to the ground near a person, poses an extremely high danger and requires compliance with increased electrical safety measures.
In emergency mode (Fig. 2.30, b), when one of the phases of the network (another phase of the network, different from the phase that the person touched) turned out to be closed to the ground, a redistribution of voltage occurs, and the voltage of the serviceable phases differs from the phase voltage of the network. Touching a working phase, a person gets under a voltage that is more than the phase voltage, but less than the linear one. Therefore, for any path of current flow, this case is more dangerous.
Single-phase connection to a circuit in a network with an isolated neutral(fig. 2.31). In production, three-wire electrical networks with insulated neutral are used to supply power to electrical installations. In such networks, there is no fourth grounded neutral wire, and there are only three phase wires. In this diagram, rectangles conventionally show electrical resistances r A, r v, r with insulation of the wire of each phase and capacity C A, C c, C c refer to each phase __________________________
under much higher voltages, and therefore more dangerous. However, the main conclusions and recommendations for ensuring safety are practically the same.
Even if you do not take into account the resistance of a person's circuit (a person is standing on wet ground in damp shoes), the current passing through a person will be safe:
Thus, good phase isolation is the key to ensuring safety. However, with extensive electrical networks, this is not easy to achieve. In extended and branched networks with a large number of consumers, the insulation resistance is low, and the danger increases.
For long electrical networks, especially cable lines, the phase capacitance cannot be neglected (CV0). Even with very good phase isolation (r = oo), the current will flow through the person through the capacitive resistance of the phases, and its value will be determined by the formula:
Thus, long electrical circuits of industrial enterprises with high capacitance are highly dangerous, even with good phase isolation.
If the insulation of any phase is violated, touching a network with an isolated neutral becomes more dangerous than a network with a grounded neutral wire. In emergency operating mode (Fig. 2.31, b) the current passing through a person who touches the good phase will flow through the earth fault circuit to the emergency phase, and its value will be determined by the formula:
Since the resistance of the short circuit D, the emergency phase on the ground is usually small, the person will be under line voltage, and the resistance of the formed circuit will be equal to the resistance of the person's circuit ____, which is very dangerous.
For these reasons, as well as because of ease of use (the ability to obtain voltages of 220 and 380 V), four-wire networks with a grounded neutral wire for a voltage of 380/220 V are most widespread.
We have considered far from all possible electrical circuits and touch options. In production, you can deal with more complex power supply circuits, especially earth.
To simplify the analysis, we take g A - g c= r c = r, a C A= L B= C c = C
If a person touches one of the wires or any object electrically connected to it, the current will flow through the person, shoes, base and through the insulation and capacitance of the wires will flow to the other two wires. Thus, a closed electrical circuit is formed, in which, in contrast to the previously considered cases, the insulation resistance of the phases is included. Since the electrical resistance of good insulation is tens and hundreds of kilo-ohms, the total electrical resistance of the circuit is much higher than the resistance of the circuit formed in the network with a grounded neutral wire. That is, the current through a person in such a network will be less, and touching one of the phases of the network with an isolated neutral is safer.
The current through a person in this case is determined by the following formula:
where is the electrical resistance of the human circuit,
ω = 2π - the circular frequency of the current, rad / s (for a current of industrial frequency = 50 Hz, therefore ω = 10Ol).
If the capacitance of the phases is small (this is the case for non-extended overhead networks), we can take C «0. Then the expression for the magnitude of the current through a person will take the form:
For example, if the floor resistance is 30 kOhm, leather shoes are 100 kOhm, the human resistance is 1 kOhm, and the insulation resistance of the phases is 300 kOhm, the current that passes through a person (for a 380/220 V network) will be equal to
A person may not even feel such a current.
Control questions
1. What types of electrical networks are most common in production?
2. Name the sources of electrical hazards at work.
3. What is touch voltage and step voltage? How do their values depend on the distance from the point where the current flows into the ground?
4. How are premises classified according to the degree of electrical hazard?
5. How does electric current affect a person? List and describe the types of electrical injuries.
6. What parameters of electric current determine the severity of electric shock? Specify the current thresholds.
7. What path of electric current flow through the human body is the most dangerous?
8. Indicate the sources of the greatest electrical hazards in the workplace associated with your future profession.
9. Conduct a hazard analysis for grounded neutral electrical networks.
10. Give a hazard analysis of electrical networks with isolated neutral.
11. What touching live conductors is the most dangerous for a person?
12.Why touching an object that is electrically connected to the ground (such as a water pipe) with your hand while working with electrical devices dramatically increases the risk of electric shock?
13.Why do I need to unplug the power plug when repairing electrical equipment?
14.Why wear shoes when working with electrical devices?
15.How can you reduce the risk of electric shock?
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
II . ELECTRICAL SAFETY
3. Analysis of electrical safety of various electrical networks
The outcome of an electric shock to a person, determined by the current flowing through the human body I h and the tension of touch U h , significantly depends on the type of network supplying electricity consumers and its parameters, including:
- mains voltage and frequency;
- network neutral mode;
- circuits for connecting a person to an electrical circuit;
- insulation resistance of phase conductors of the network relative to the ground;
- the capacitance of the phase conductors of the network relative to the ground;
- network operating mode.
Typical schemes for connecting a person to an electrical circuit
There are various “connection schemes” of a person in an electric current circuit (typical “connection schemes” are shown in Fig. 3.5, using the example of an IT network):
Rice. 3.5. Typical schemes for connecting a person to an electrical circuit
(3.1.)
From expressions (3.1.) And (
3.2. ) follows that with two-phase a person touches the line voltage of the network regardless of the type of network, neutral mode, network operating mode, conductivity of phase wiresY L1 , Y L2 , Y L3relative to the ground. Such a scheme for connecting a person to an electrical circuit is a great danger.Cases of two-phase contact occur relatively rarely and are, as a rule, the result of work 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 diagram for the analysis of three-phase networks
(3.3)
(3.4)
Since the value of the electric current passing through a person significantly depends on the resistance of the electric circuit R, the severity of the lesion is largely determined by the scheme for including the person into the circuit. The circuits formed when a person contacts a conductor of circuits depend on the type of power supply system used.
The most common electrical networks in which the neutral wire is grounded, that is, it is short-circuited by a conductor to the ground. Touching the neutral wire practically does not pose a danger to humans, only the phase wire is dangerous. However, it is difficult to figure out which of the two wires is zero - they look the same in appearance. You can figure it out using a special device - a phase detector.
Using specific examples, we will consider possible schemes for connecting a person to an electrical circuit when touching conductors.
Two-phase connection to the electric circuit
The rarest, but also the most dangerous, is the touch of a person to two phase wires or current conductors connected to them (Fig. 1).
In this case, the person will be under the influence of line voltage. A current will flow through a person along the "hand-hand" path, i.e. the resistance of the circuit will include only the resistance of the body ()
If we take the body resistance of 1 kOhm, and the electric network with a voltage of 380-220 V, then the strength of the current passing through a person will be equal to
This is a deadly current The severity of an electrical injury or even a person's life will depend primarily on how quickly he gets rid of contact with the current conductor (breaks the electrical circuit), because the exposure time in this case is decisive.
Much more often there are cases when a person with one hand comes into contact with a phase wire or part of a device: an apparatus that is accidentally or intentionally electrically connected to it. The danger of electric shock in this case depends on the type of electrical network (grounded or isolated neutral).
