1) Single-phase contact with a network wire with an isolated neutral with good insulation (Fig. 1):
Figure 1 - Single-phase inclusion of a person in an electrical network.
The current passing through the person I h returns to the current source through the insulation of the wires of the network, which in good condition has a large insulation resistance R out. Up to 1000V R out is 0.5 MΩ or more. The current flowing through the human body is determined by the expression:
(1)
where R h is the resistance of the human body, 1000 ohms are taken in the calculations;
R out - insulation resistance of the phases relative to the ground;
U f - phase voltage
Taking into account the resistance of the shoes R about and the floor R p, connected in series with the resistance of the human body R h, the current passing through the person will be equal to:
(2)
2) Single-phase contact with a grounded neutral network wire (Fig. 2):
Figure 2 - Single-phase contact with a grounded neutral network
The magnitude of the current through a person is determined only by the resistance of the human body, the values of the wire insulation resistance do not affect the current passing through the human body.
, (3)
where R 0 is the neutral grounding resistance. At Ul = 380 V R 0 does not exceed 4 0 m, then it can be neglected in the calculations. In this case, the resistance of the floor and shoes play a big role in human safety, because. connected in series with a person in series.
(4)
When R p \u003d 0 and R about \u003d 0
I h = = 0,22 BUT = 220 mA> 100 mA >> 10 mA ,
it is very dangerous!
When a phase is shorted to earth, a network with an isolated neutral (Fig. 4) turns out to be more dangerous than a grounded one (Fig. 5). Since, in a network with an isolated neutral, the voltage that determines the amount of current through the human body is U l, and in a network with a grounded neutral, it lies within:
U l >U pr >U f
Figure 4 - Network with isolated neutral
I h= , (7)
where R h is the resistance of the human body;
R zm - earth phase closing resistance
In the event of a phase breakdown on the body of the equipment, which under normal conditions should not be energized, the person working with this equipment is in the single-phase touch mode. For protection against electric shock in a network with isolated neutral is used protective grounding (Fig. 6).
Figure 5 - Network with grounded neutral
Protective earth
Protective grounding is carried out in order to ensure the safety of people in case of violation of the insulation of live parts. Grounding is also used to protect electrical equipment, buildings and structures from the action of atmospheric electricity.
Protective grounding is the intentional connection to the ground or its equivalent of metal parts of equipment that are not normally energized, but may become energized due to a violation of the insulation of electrical installations.
The effect of protective earthing is that it reduces the voltage between the energized equipment frame and earth to a safe value.
Let us explain this using the example of a network with an isolated neutral (Fig. 6). If the body of the electrical equipment is not grounded and it is in contact with the phase, then a person's touch to such a body is equivalent to a single-phase switch-on. If the chassis is grounded, then the potential of the chassis to ground drops to a safely low value.
Figure 6 - Protective earth
It is necessary to ground metal parts of electrical installations, cases of electrical machines, transformers, devices, lamps, drives of electrical devices, secondary windings of instrument transformers, frames of switchboards, control panels, cabinets, etc.
Protective grounding is used in three-phase three-wire networks with voltage up to 1000 V with an isolated neutral, and in networks with a voltage of 1000 V and above - with any neutral mode (Fig. 3.18).
Schemes for including a person in a current circuit can be different:
Between two wires
Between wire and ground
between two wires and ground at the same time, etc.
However, the first two schemes are the most characteristic. With regard to three-phase 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 (Fig. 11.3.), As a rule, is more dangerous than a single-phase one, since the highest voltage in this network is applied to the human body - linear, and therefore a larger current will flow through the person, the strength of which is determined by the formula:
where I h is the strength of the current passing through the human body, A; U l \u003d 1.73 U f - linear voltage, i.e. voltage between the phase wires of the network, in; U f - phase voltage, V; R h is the resistance of the human body, Ohm.
Rice. 11.3 Two-phase switching circuit
person in the current circuit in a three-phase network
It is easy to see that with a two-phase connection, the current passing through a person is practically independent of the network neutral mode, therefore, a two-phase connection is equally dangerous in a network with both isolated and grounded neutrals.
Single-phase switching occurs much more often, but it is less dangerous than two-phase, since the voltage under which a person finds himself does not exceed the phase voltage, i.e. less than linear by 1.73 times. In addition, the value of this current is also affected by the neutral mode of the current source, the resistance of the floor on which the person stands, the resistance of his shoes, and some other factors.
In a network with a grounded neutral (Fig. 11.4), in series with the resistance of the human body (R h), the resistance of the shoe (R about), the floor resistance (R n) and the grounding resistance of the current source neutral (R o) are included.
Rice. 11.4 Scheme of a single-phase inclusion of a person in a current circuit in a three-phase four-wire network with a grounded neutral
Given these resistances, the current strength (I h) passing through a person will be separated by the formula:
I h = ,
where R h is the resistance of the human body, Ohm; R about - shoe resistance, Ohm; R n - floor resistance, Ohm; R o - neutral grounding resistance, Ohm.
In a network with isolated neutral (Fig.
11.5.), the current passing through a person returns to the current source through the insulation of the wires, which has a high resistance. The value of the current passing through a person is determined for this case by the formula:
I h = ,
where R from is the insulation resistance of one phase of the network relative to the ground, Ohm.
In a network with an isolated neutral, the safety conditions are directly dependent not only on the resistance of the floor and shoes, but also on the resistance of the insulation of wires to earth: the better the insulation, the less current flowing through a person.
Rice. 11.5 Scheme of single-phase inclusion of a person in a current circuit in a three-phase network with an isolated neutral
Thus, ceteris paribus, the single-phase inclusion of a person in a network with an isolated neutral is less dangerous than in a network with a grounded neutral. This conclusion is valid for the day of normal (failure-free) network operation conditions. In the event of an accident, when one of the phases is closed to ground, a network with an isolated neutral may turn out to be more dangerous, since due to aging of the insulation, moisture, and under other adverse conditions, the insulation resistance decreases. As a result, the voltage between any undamaged phase and earth can increase from phase to linear, while in a network with a grounded neutral, the voltage of undamaged phases relative to earth practically does not increase, i.e. stays within phase.
Thus, human safety is ensured by the high quality of insulation, which is controlled during preventive tests. Periodic insulation monitoring is to determine the insulation resistance of each phase to earth and between phases in each section, between two fuses in series, devices or after the last fuse.
The electrical insulation of power or lighting wiring is considered sufficient if its resistance between the wire of each phase and earth, or between different phases in the area limited by fuses connected in series, is at least 0.5 MΩ (according to the rules for electrical installations).
Book title Next page>>§ 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.
Knowledge of the processes occurring in electrical installations allows power engineers to safely operate equipment of any voltage and type of current, perform repair work and maintenance of electrical systems.
The information set out in the PTB and PTE, the main documents created by the best specialists based on an analysis of accidents with people affected by hazardous factors that accompany the operation of electrical energy, helps to avoid cases of electric shock in an electrical installation.
Circumstances and causes of a person falling under the action electric current
Safety guidance documents identify three groups of reasons for electric shock to workers:
1. unintentional, unintentional approach to current-carrying parts with voltage at a distance less than safe or touching them;
2. occurrence and development of emergencies;
3. violation of the requirements specified in the governing documents prescribing the rules of conduct for workers in existing electrical installations.
The assessment of the dangers of human injury consists in determining by calculations the magnitude of the currents that pass through the body of the victim. In this case, it is necessary to take into account many situations when contacts can occur in random places in the electrical installation. In addition, the voltage applied to them varies depending on many reasons, including conditions and operating modes. electrical circuit, its energy characteristics.
Conditions for the defeat of a person by the current of an electrical installation
In order for a current to flow through the body of the victim, it is necessary to create electrical circuit by connecting it to at least two points of the circuit, which has a potential difference - voltage. Electrical equipment may experience the following conditions:
1. simultaneous two-phase or two-pole touch to different poles (phases);
2. single-phase or single-pole contact with the potential of the circuit, when a person has a direct galvanic connection with the potential of the earth;
3. accidental creation of contact with the conductive elements of the electrical installation, which were energized as a result of the development of the accident;
4. falling under the action of the step voltage, when a potential difference is created between the points on which the legs or other parts of the body are simultaneously located.
In this case, an electrical contact of the victim with the current-carrying part of the electrical installation may occur, which is considered by the PUE as a touch:
1. straight;
2. or indirect.
In the first case, it is created by direct contact with a live part that is energized, and in the second case, by touching non-isolated circuit elements when a dangerous potential has passed through them in the event of an accident.
To determine the conditions for the safe operation of an electrical installation and to prepare for workers inside it workplace, necessary:
1. to analyze cases of possible creation of paths for the passage of electric current through the body of service personnel;
2. compare its maximum possible value with the current minimum allowable standards;
3. make a decision on the implementation of measures to ensure electrical safety.
Features of the analysis of the conditions of damage to people in electrical installations
To assess the amount of current passing through the body of the victim in a DC or AC voltage network, the following types of designations are used for:
1. resistances:
Rh - in the human body;
R0 - for grounding device;
Riz - insulation layer relative to the ground contour;
2. currents:
Ih - through the human body;
Iz - short circuit to the earth contour;
Uc - circuits of direct or single-phase alternating currents;
Ul - linear;
Uf - phase;
Upr - touch;
Ush - step.
In this case, the following typical schemes for connecting the victim to the voltage circuits in the networks are possible:
1. DC at:
unipolar touch of the conductor contact with the potential isolated from the earth circuit;
single-pole contact of the potential of the circuit with a grounded pole;
bipolar contact;
2. three-phase networks at;
single-phase contact with one of the potential conductors (generalized case);
two-phase contact.
Damage schemes in DC circuits
Single-pole human contact with potential isolated from earth
Under the action of voltage Uc, a current Ih flows through a series-created chain of the potential of the lower conductor, the body of the victim (arm-leg) and the earth circuit through the doubled insulation resistance of the medium.
Single-pole human contact with earthed pole potential
In this scheme, the situation is aggravated by the connection to the ground loop of one potential wire with a resistance R0 close to zero and much less than that of the body of the victim and the insulation layer of the external environment.
The strength of the desired current is approximately equal to the ratio of the mains voltage to the resistance of the human body.
Bipolar human contact with network potentials
The mains voltage is directly applied to the body of the victim, and the current through his body is limited only by his own insignificant resistance.
General defeat schemes in three-phase alternating current circuits
Creation of human contact between phase potential and earth
In the general case, there is a resistance between each phase of the circuit and the ground potential and a capacitance is created. The neutral of the windings of the voltage source has a generalized resistance Zn, the value of which varies in different grounding systems of the circuit.
The formulas for calculating the conductivities of each circuit and the total current Ih through the phase voltage Uf are presented in the picture by formulas.
Formation of human contact between two phases
The greatest magnitude and danger is the current passing through the chain created between the direct contacts of the victim's body with the phase wires. In this case, part of the current can pass along the path through the ground and the insulation resistance of the medium.
Features of Biphasic Touch
In DC and three-phase AC circuits, making contacts between two different potentials is the most dangerous. With this scheme, a person falls under the influence of the greatest voltage.
In a circuit with a constant voltage power supply, the current through the victim is calculated by the formula Ih \u003d Uc / Rh.
In a three-phase AC network, this value is calculated by the ratio Ih=Ul/Rh=√3 Uf/Rh.
Considering that the average electrical resistance of the human body is 1 kiloohm, we calculate the current that occurs in the network of direct and alternating voltage of 220 volts.
In the first case, it will be: Ih=220/1000=0.22A. This value of 220 mA is enough for the victim to undergo convulsive muscle contraction, when, without outside help, he is no longer able to free himself from the impact of an accidental touch - the holding current.
In the second case, Ih=(220 1.732)/1000\u003d 0.38A. At this value of 380 mA, there is a mortal danger of injury.
We also pay attention to the fact that in a three-phase alternating voltage network, the position of the neutral (it can be isolated from the ground or vice versa - short-circuited) has very little effect on the value of the current Ih. Its main share does not go through the earth circuit, but between the phase potentials.
If a person has applied means of protection that ensure his reliable isolation from the earth circuit, then in such a situation they will turn out to be useless and will not help.
Single phase touch features
Three-phase network with deafly grounded neutral
The victim touches one of the phase wires and falls under the potential difference between him and the earth circuit. Such cases occur most often.
Although the phase-to-earth voltage is less than 1.732 times line-to-line, such a case remains dangerous. The condition of the victim can worsen:
neutral mode and the quality of its connection;
electrical resistance of the dielectric layer of wires relative to the ground potential;
type of footwear and its dielectric properties;
soil resistance at the location of the victim;
other related factors.
The value of the current Ih in this case can be determined by the relation:
Ih=Uf/(Rh+Rob+Rp+R0).
Recall that the resistances: the human body Rh, shoes Rb, floor Rn and grounding at the neutral R0, are taken in Ohms.
The smaller the denominator, the more current is generated. If the worker wears conductive shoes, for example, his feet are wet or the soles are lined with metal nails, and in addition he is on a metal floor or damp ground, then we can assume that Rb = Rp = 0. This ensures the most unfavorable case for the life of the victim.
Ih=Uf/(Rh+R0).
With a phase voltage of 220 volts, we get Ih \u003d 220 / 1000 \u003d 0.22 A. Or a mortal danger current of 220 mA.
Now let's calculate the option when the employee uses protective equipment: dielectric shoes (Rb = 45 kOhm) and an insulating base (Rp = 100 kOhm).
Ih=220 /(1000 +45000+10000)=0.0015 A.
We got a safe current value of 1.5 mA.
Three-phase network with isolated neutral
There is no direct galvanic connection between the neutral of the current source and the earth potential. The phase voltage is applied to the resistance of the insulation layer Riz, which has a very high value, which is controlled during operation and constantly maintained in good condition.
The circuit of current flow through the human body depends on this value in each of the phases. If we take into account all layers of current resistance, then its value can be calculated by the formula: Ih=Uf/(Rh+Rb+Rp+(Riz/3)).
During the most unfavorable case, when conditions for maximum conductivity through shoes and the floor are created, the expression will take the form: Ih=Uf/(Rh+(Riz/3)).
If we consider a 220 volt network with a layer insulation of 90 kOhm, then we get: Ih \u003d 220 / (1000 + (90000/3)) \u003d 0.007 A. Such a current of 7 mA will be well felt, but it will not be able to provide fatal injury.
Note that in this example we deliberately omitted the resistance of the ground and shoes. If they are taken into account, then the current will decrease to a safe value, on the order of 0.0012 A or 1.2 mA.
Conclusions:
1. in circuits with an isolated neutral, it is easier to ensure the safety of workers. It directly depends on the quality of the dielectric layer of the wires;
2. under the same circumstances of touching the potential of one phase, a circuit with a grounded neutral is the most dangerous than with an isolated one.
Consider the case of touching the metal case of an electrical device, if the insulation of the dielectric layer at the phase potential is broken inside it. When a person touches this body, a current will flow through his body to the ground and then through the neutral to the voltage source.
The replacement circuit is shown in the picture below. The load created by the device has resistance Rn.
The insulation resistance Riz together with R0 and Rh limits the phase-to-phase contact current. It is expressed by the ratio: Ih=Uf/(Rh+Riz+Rо).
In this case, as a rule, even at the project stage, choosing materials for the case when R0=0, they try to comply with the condition: Riz> (Uf / Ihg) -Rh.
The value of Ihg is called the threshold of imperceptible current, the value of which a person will not feel.
We conclude: the resistance of the dielectric layer of all current-carrying parts relative to the earth contour determines the degree of safety of the electrical installation.
For this reason, all such resistances are normalized and taken into account in approved tables. For the same purpose, it is not the insulation resistances themselves that are normalized, but the leakage currents that flow through them during testing.
Step Voltage
In electrical installations, for various reasons, an accident can occur when the phase potential directly touches the ground loop. If on an overhead power line one of the wires breaks under the influence of various types of mechanical loads, then just in this case a similar situation manifests itself.
In this case, at the point of contact of the wire with the ground, a current is formed, which creates a spreading zone around the point of contact - a platform on the surface of which an electric potential appears. Its value depends on the short circuit current Iz and the specific state of the soil r.
A person who finds himself within the boundaries of this zone falls under the action of the step voltage Ush, as shown in the left half of the picture. The area of the spreading zone is limited by the contour where the potential is absent.
The step voltage value is calculated by the formula: Ush=Uz∙β1∙β2.
It takes into account the phase voltage at the place of current spreading - Uz, which is specified by the coefficients of the voltage spreading characteristics β1 and the influence of the resistance of shoes and legs β2. The values of β1 and β2 are published in reference books.
The value of the current through the victim's body is calculated by the expression: Ih=(Uz∙β1∙β2)/ Rh.
On the right side of the figure, in position 2, the victim makes contact with the potential of the wire shorted to earth. It is influenced by the potential difference between the point of contact with the hand and the earth contour, which is expressed by the contact voltage Upr.
In this situation, the current is calculated by the expression: Ih=(Uph.c.∙α )/ Rh
The values of the spreading coefficient α can vary within 0÷1 and take into account the characteristics that affect Upr.
In the considered situation, the same conclusions apply as when creating a single-phase contact for the victims in the normal operation of the electrical installation.
If a person is located outside the current spreading zone, then he is in a safe zone.
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 particular case.
Comparing the values of the current through the human body obtained by calculation with the value of the conditionally safe current (10mA), a conclusion is made about the danger of this case. If the magnitude of the current through the human body exceeds the magnitude 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 earth can be neglected, assuming that the insulation resistance of the wires (R out) relative to earth is purely active.
You can determine the amount of current through the human body as follows:
I h \u003d U pr / R h
The complexity 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 characteristic 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 switching, and the second single-phase.
9.1.1. Two-phase switching
Two-phase switching, 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 flow 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 network 380/220
Current dangerous to human life
9.1.2. Single phase switch.
Single-phase switching occurs much more often, but is less dangerous, because. the voltage under which a person finds himself does not exceed the phase voltage. In addition, the value of the current through the human body is also affected by the neutral mode of the current source, the resistance of the insulation 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. Switching scheme
Figure 9.4. equivalent circuit
The current through the human body can be found as:
From the expression, we can conclude:
1. The greater the insulation resistance relative to earth, the less the danger of a single-phase touch to the wire
2. A person touching a wire with a high insulation resistance is more dangerous, because. the touch voltage will be greater.
9.1 1.2. Three-phase three-wire network with isolated neutral:
Consider two network modes:
a) Normal mode of operation (insulation resistance has a large (normalized) value.
Figure 9.5. Single-phase connection to a 3-phase network
with isolated neutral
If the insulation resistances are equal, R out of 1 = R out of 2 = R out of 3, 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 provide high insulation resistance and monitor its condition for timely detection and elimination of faults.
According to the PES, the insulation resistance of wires relative to earth in installations up to 1000V should not be less than 500k.
b) In emergency mode - the short circuit of one of the phases to the ground through a small circuit resistance - R zm. (Figure 9.6.)
Figure 9.6 Network emergency mode
Usually R zm lies in the range from 50 to 200 ohms.
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 a 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 solidly grounded neutral:
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 normalized depending on the maximum mains voltage.
At U l \u003d 660V, R o \u003d 2 Ohm, at U l \u003d 380V, R o \u003d 4 Ohm, at U l \u003d 220V, R o \u003d 8 Ohm
The current flowing through the human body and the insulation resistance of the wires can be neglected, compared with the current flowing through the human body and the low neutral ground resistance. The value of this current is determined from the expression:
It can be seen from the expression that in a network with a solidly grounded neutral during the period of 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) In emergency operation - when one of the phases of the network is closed to the ground through a small 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 about 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 the wire in the emergency mode of the network, gets under a voltage less than linear, but more than phase.