Analysis of electrical safety conditions
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 (10 mA), a conclusion is drawn about the danger of this case. If the magnitude of the current through the human body exceeds the value of the conditionally safe current, the case is considered dangerous. If not, it's 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.
The magnitude of the current through the human body can be determined as follows:
I h = U pr / R h
The difficulty of the calculation lies in finding the touch voltage (U pr). To find this value, they resort to the following 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.
In relation to AC networks, the first circuit is usually called two-phase, and the second single-phase.
9.1.1. Two-phase switching
Two-phase connection, as a rule, is more dangerous, since the highest voltage in a given network is applied to the human body - linear, and therefore a large current will flow through the human body (Figure 9.1.).
Figure 9.1. Two-phase connection of a person to 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 switching.
Single-phase switching occurs much more often, but is less dangerous, because the voltage under which a person finds himself does not exceed phase voltage. In addition, the value of the current through the human body is also influenced by 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.
9.1.2.1. Single-phase network.
Figure 9.3. Connection diagram
Figure 9.4. Substitution scheme
The current through the human body can be found as:
From the expression we can conclude:
1. The greater the insulation resistance relative to ground, the less the danger of single-phase contact with the wire
2. Human contact with a wire with high insulation resistance is more dangerous, because the touch tension will be greater.
9.1 1.2. Three-phase three-wire network with isolated neutral:
Let's consider two network modes:
a) Normal operating mode (insulation resistance has a large (standardized) value.
Figure 9.5. Single-phase connection to a 3-phase network
with isolated neutral
If the insulation resistance is equal R out1 = R out2 = R out3, the amount of current through the human body is determined by the expression
In such networks, the danger to a person who touches the wire, in 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 ensure high insulation resistance and monitor its condition for timely detection and elimination of faults.
According to the PEU, the insulation resistance of wires relative 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 circuit resistance - R zm. (Figure 9.6.)
Figure 9.6 Network emergency mode
Typically Rzm lies in the range from 50 to 200 Ohms.
The current through the human body, as in normal mode, will also flow through the insulation resistance of the wires relative to the ground, but its value will be significantly less than the current flowing through a low 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 solidly 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 Rо is standardized depending on the maximum network voltage.
At U l =660V, R o =2Ohm, at U l =380V, R o =4Ohm, at U l =220V, R o =8Ohm
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 neutral grounding resistance. The magnitude of this current is determined from the expression:
It is clear from the expression that in a network with a solidly grounded neutral during normal operation of the network, touching one of the wires is more dangerous than touching the wire of a normally operating network with an isolated neutral.
b) During emergency operation - when one of the network phases is shorted to ground through a low resistance R ZM (Figure 9.8.).
Figure 9.8.
If we analyze this case, we can draw the following conclusions:
2. If we take R o equal to 0, then the person will be under phase voltage.
In real conditions, R zm and R o are always greater than zero, therefore, a person touching a wire in emergency mode of the network comes under voltage less than linear, but more than phase.
All cases of electric shock to a person are a consequence of touching at least two points of an electrical circuit, between which there is a potential difference. The danger of such contact largely depends on the characteristics of the electrical network and the way a person is connected to it. By determining the current per hour 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 a current circuit (Fig. 8.1, a). It occurs quite rarely, but is more dangerous compared to single-phase, since the highest voltage in a given network is applied to the body - linear, and the strength of the current, A, passing through a person does not depend on the network diagram, the mode of its neutral and other factors, i.e. e.
I = Ul/Rch = √ 3Uph/Rch,
where Uл and Uф are linear and phase voltage, V; Rch is the resistance of the human body, Ohm (according to the Electrical Installation Rules, in calculations Rch is taken equal to 1000 Ohms).
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 rubber breaks, connecting a cable to unprotected terminals of a welding transformer, etc.
Single-phase switching. The current passing through a person is influenced by various factors, which reduces the risk of injury compared to two-phase touch.
Rice. 8.1. Schemes for possible connection of a person to a three-phase current network:
a - two-phase touch; b—single-phase contact in a network with a grounded neutral; c - single-phase touch 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, with equal insulation resistance of the wires relative to the ground r1 = r2 = r, is determined by the formula
Ich = U/(2Rch + r),
where U is the network voltage, V; r — insulation resistance, Ohm.
In a three-wire network with an insulated neutral, with r1 = r2 = r3 = r, the current will flow from the point of contact through the human body, shoes, floor and imperfect insulation to other phases (Fig. 8.1, b). Then
Ich = Uph/(Ro + r/3),
where Ro is the total resistance, Ohm; RO = Rch + Rop + Rp; Rob - shoe resistance, cm: for rubber shoes Rob ≥ 50,000 Ohm; Rn - floor resistance, Ohm: for a dry wooden floor, Rп = 60,000 Ohm; g - wire insulation resistance, Ohm (according to the Electrical Regulations, it must be at least 0.5 MOhm per phase of a network section with voltage up to 1000 V).
In three-phase four-wire networks, the current will flow through a person, his shoes, the floor, the grounding of the source neutral and the neutral wire (Fig. 8.1, c). Current strength, A, passing through a person,
Ich=Uph(Ro + Rн),
where RH is the neutral grounding resistance, Ohm. Neglecting resistance RH, we get:
Agricultural enterprises mainly use four-wire electrical networks with a solidly grounded neutral with a voltage of up to 1000 V. Their advantage is that they can be used to obtain two operating voltages: linear Ul = 380 V and phase Uph = 220 V. Such networks do not require high requirements for the quality of wire insulation and are used when the network is highly branched. A three-wire network with an insulated neutral at voltages up to 1000V is used somewhat less frequently; it is safer if the insulation resistance of the wires is maintained at a high level.
Touch tension. It occurs as a result of touching live electrical installations or metal parts of equipment.
If an electric current flows through a grounding rod immersed in the ground so that its upper end is located at ground level, then the touch voltage, V,
where I3 is the ground fault current, A; ρ is the resistivity of the base (soil, floor, etc.) on which the person is located, Ohm*m; l and d—length and diameter of the ground electrode, m; x is the distance from a person to the center of the ground electrode, m; a is the touch voltage coefficient.
α = Rch/(Rch + Rob + Rn) = Rch/Ro.
Neglecting the resistance of the shoes (when it is wet or in the absence of it), we can write for the following cases:
the soles of the feet are removed from one another at a distance of a step
α=1/(1 + 1.5ρ/Rh);
feet are close
α=1/(1 + 2ρ/Rch).
Step voltage. This is the voltage Ush on the human body when the legs are positioned at points in the field of current spreading from the ground electrode or from a wire that has fallen 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, then the other is at a distance x + a, where a is the step length. Usually in calculations we take a = 0.8 m.
The maximum voltage in this case occurs at the point where the current closes to the ground, and as it moves away from it it decreases according to the hyperbola law. It is assumed that at a distance of 20 m from the fault point the earth potential is zero.
Book table of contents Next page>>§ 3. Danger of electric shock.
Scheme of single-phase connection of a person to a three-phase current network with a grounded neutral.
Electric shock occurs when an electrical circuit is closed through the human body. This occurs when a person touches at least two points of an electrical circuit, between which there is some voltage. The inclusion of a person in a circuit can occur in several ways: between the wire and the ground, called single-phase connection; between two wires - two-phase connection. These schemes are most typical for three-phase AC networks. It is also possible to switch between two wires and ground at the same time; between two points on the earth having different potentials, etc.
Single-phase connection of a person to the network represents 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 injury will vary depending on whether the electrical network has a grounded or insulated 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.
When connected single-phase to a network with a grounded neutral, a person comes under phase voltage, which is 1.73 times less than linear, and is exposed to current, the magnitude 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 a person stands and shoes.
Rice. 69. Scheme of single-phase connection of a person to 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 the grounding of the neutral of the current source (transformer, etc.). In this case, the current value
where U l - linear voltage, V; R t - human body resistance, Ohm; R p - resistance of the floor on which the person is located, Ohm; R rev - resistance of a person’s shoes, Ohm; R 0 - neutral grounding resistance, Ohm.
As an example, consider two cases of single-phase connection of a person to a three-phase four-wire electrical network with a grounded neutral at U l = 380 V.
A case of adverse conditions. A person who touches 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 the resistance: human body R t = 1000 Ohm, soil or floor R p = 0; shoes R rev = 0.
The neutral grounding resistance R0 = 4 Ohms is not taken into account due to its insignificant value. A current will pass through the human body
being life-threatening.
A case of favorable conditions. A person is on a dry wooden floor with a resistance of R p = 60,000 Ohm, and has dry non-conductive (rubber) shoes on his feet with a resistance of R rev = 50,000 Ohm. Then a current will pass through the human body
which is long-term acceptable for humans.
In addition, dry floors and rubber shoes have significantly greater resistance in comparison with the values accepted for the calculation.
These examples show the great importance of the insulating properties of the floor and shoes to ensure the safety of persons working in conditions of possible contact with electric current.
There are various schemes for connecting a person to an electrical circuit:
Single-phase touch – touching the conductor of one phase of an active electrical installation;
Two-phase touching – simultaneous touching of 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 insulation damage;
Switching on step voltage is switching on between two points of the ground (soil) that are under 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. Current flowing through the human body ( Ih) with a single-phase touch (Fig. 6) will close in the circuit: phase L 3 – human body – base (floor) – neutral grounding conductor – 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 base - base resistance.
If the base (floor) is conductive, then R base ≈ 0
Given the fact that R O " R h, That
U h = U f
Such touch is extremely dangerous.
Single-phase touch in a network with an isolated neutral. The current flowing through the human body (Fig. 7) is closed in circuits: phase L 3 – human body – floor and then returns to the network through phase insulation 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 current circuit flowing through the human body, phase insulations are connected 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 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 an imaginary capacitor, the “plates” of which are the phases and the grounds.
At R 1 = R 2 = R 3 = R f and WITH 1 = WITH 2 = WITH 3 = WITH F current flowing through the human body:
Where Z- total insulation resistance of the phase wire relative to the ground.
If the phase capacitance is neglected WITH f = 0 (short-length air networks), then:
from which it follows that the magnitude of the current depends not only on 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 = 110 V
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 touch 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 a metal body that is energized. Touching the body of the electrical installation (Fig. 9), in which the phase ( L 3) closed to the body, equivalent to touching the phase itself. Therefore, the analysis and conclusions for single-phase contact cases discussed earlier are fully applicable to the case of a ground fault.
Rice. 9. Scheme of a person touching metal
body under voltage
Since from the resistance of the electrical circuit R Since the magnitude of the electric current passing through a person significantly depends, the severity of the injury is largely determined by the circuit of connecting the person to the circuit. The patterns of circuits formed when a person comes into contact with a conductor depend on the type of power supply system used.
The most common electrical networks are those in which the neutral wire is grounded, i.e., short-circuited by a conductor to the ground. Touching the neutral wire poses virtually no danger to humans; only the phase wire is dangerous. However, it is difficult to figure out which of the two wires is neutral - they look the same. 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 a person touching 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. Current will flow through the person along the “hand-to-hand” path, i.e. the resistance of the circuit will include only the resistance of the body (D,).
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If we take a body resistance of 1 kOhm, and an electrical network with a voltage of 380/220 V, then the current strength 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 frees himself from contact with the current conductor (breaks the electrical circuit), because the time of exposure in this case is decisive.
Much more often there are cases when a person comes into contact with a phase wire or part of a device with one hand, a device that is accidentally or intentionally electrically connected to it. The danger of electric shock in this case depends on the type of electrical network (with grounded or insulated 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 “arm-legs” or “arm-arm” path, and the person will be under phase voltage.
In the first case, the circuit resistance will be determined by the resistance of the human body (I_, shoes (R o 6), grounds (Rzh), on which a person stands, the neutral grounding resistance (RH), and current will flow through the person
Neutral resistance RH is small and can be neglected compared to other circuit resistances. To estimate the magnitude of the current flowing through a person, we will assume a network voltage of 380/220 V. If a person is wearing insulating dry shoes (leather, rubber), he is standing on a dry wooden floor, the circuit resistance will be large, and the current strength, according to Ohm’s law, will be small.
For example, floor resistance is 30 kOhm, leather shoes are 100 kOhm, human resistance is 1 kOhm. Current passing through a person
This current is close to the threshold perceptible current. The person will feel the flow of current, stop working, and eliminate the malfunction.
If a person stands on wet ground with damp shoes or bare feet, a current will pass through the body
This current can cause damage to the lungs and heart, and with prolonged exposure, death.
If a person stands on wet soil wearing dry and intact rubber boots, a current passes 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 dramatically reduce the resistance of the rubber sole and make work dangerous.
Before you start working with electrical devices (especially those that have not been in use for a long time), they must be carefully inspected for damage to the insulation. Electrical devices must be wiped free of dust and, if they are wet,- dry. Wet electrical devices must not be used! It is better to store electric tools, instruments, and equipment in plastic bags to prevent dust or moisture from getting into them. You have to wear shoes when working. If the reliability of an electrical device is in doubt, you need to be on the safe side.- place a dry wooden floor or rubber mat under your feet. You can use rubber gloves.
The second path of current flow occurs when a person’s second hand comes into contact with electrically conductive objects connected to the ground (the body of a grounded machine tool, a metal or reinforced concrete building structure, a wet 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 small. Therefore, the resistance of the circuit is equal to the resistance of the body and current will flow through the person
This amount of current is deadly.
When working with electrical devices, do not use your other hand to touch objects that may be electrically connected to ground. Working in damp areas, in the presence of highly 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 touched by a person) is shorted to ground, voltage redistribution occurs, and the voltage of the healthy phases differs from the phase voltage of the network. When touching a working phase, a person comes under voltage, which is greater than the phase voltage, but less than the linear one. Therefore, regardless of the 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 an insulated neutral are used to supply power to power 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 resistance r A, r V, r With insulation of wires of each phase and capacitance S A, S v, S s each phase relative____________________
being under significantly higher voltages, and therefore more dangerous. However, the main conclusions and recommendations for ensuring safety are almost the same.
Even if we do not take into account the resistance of the human circuit (the person is standing on wet ground in damp shoes), the current passing through the person will be safe:
Thus, good phase insulation is the key to safety. However, with extensive electrical networks, this is not easy to achieve. In long 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, phase capacitance cannot be neglected (CV0). Even with very good phase insulation (r = oo), the current will flow through a person through the capacitance 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 insulation.
If the insulation of any phase is broken, touching a network with an isolated neutral becomes more dangerous than touching a network with a grounded neutral wire. In emergency mode (Fig. 2.31, b) the current passing through a person who has touched the serviceable phase will flow through the ground fault circuit to the emergency phase, and its value will be determined by the formula:
Since the closure resistance D, the emergency phase on earth, is usually small, the person will be under linear voltage, and the resistance of the resulting 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 have become most widespread.
We have not considered all possible electrical network diagrams and touch options. In production, you may be dealing with more complex power supply circuits, especially ground circuits.
To simplify the analysis, let us assume g A - g c= g c = g, A S A= L B= C c = C
If a person touches one of the wires or any object electrically connected to it, current will flow through the person, the shoe, the base, and through the insulation and capacitance of the wires to the other two wires. Thus, a closed electrical circuit is formed, in which, unlike the previously considered cases, the phase insulation resistance 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 greater than the resistance of the circuit formed in a 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,
co = 2nd - circular frequency of the current, rad/s (for industrial frequency current = 50 Hz, therefore co = YuOl).
If the phase capacitance is small (this is the case for short air networks), we can take C « 0. Then the expression for the amount of 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 phase insulation resistance is 300 kOhm, the current that passes through the 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 current thresholds.
7. Which path of electric current flow through the human body is most dangerous?
8. Indicate the sources of the greatest electrical danger in production related to your future profession.
9. Do a hazard analysis of electrical networks with a grounded neutral.
10. Give an analysis of the dangers of electrical networks with an isolated neutral.
11.Which touching of live conductors is most dangerous for a person?
12. Why does touching objects electrically connected to the ground (for example, a water pipe) with your hand when working with electrical devices sharply increase the risk of electric shock?
13.Why do you need to remove the electrical plug from the socket when repairing electrical equipment?
14.Why do you need to wear shoes when working with electrical devices?
15.How can you reduce the risk of electric shock?