Technospheric safety

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Ministry of education and science of the Russian Federation South Ural State University Department of Life Safety 658.382 S57

A.I. Sidorov, O.A Khanzhina

TECHNOSPHERIC SAFETY Training manual for laboratory works

Chelyabinsk Publishing Center of SUSU 2018

UDC 658.382(076.5) S57 Approved by the educational Commission of mechanics and technology faculty Reviewers: A.V. Bogdanov, I.L. Kravchuk

Sidorov, A.I. S57 Technospheric safety: training manual for laboratory works / A.I. Sidorov, O.A. Khanzhina. – Chelyabinsk: Publishing Center of SUSU, 2018. – 41 p. The training manual includes 5 laboratory works, which provide for familiarization of students with the section "Basics of electrical safety" course "Technospheric safety". The training manual considers the following issues: phenomena when the current flows into the ground, the resistance of the human body, the analysis of the danger of human injury by electric current in networks with different neutral conditions, means of ensuring electrical safety (devices of protective shutdown, insulation monitoring, protective earthing, selfearthing). The training manual is intended for students of all forms of training in electrical engineering specialties of various faculties of the university. UDC 658.382(076.5)

© Publishing Center of SUSU, 2018

RESEARCH OF RESISTANCE OF THE BODY OF THE HUMAN Objective To familiarize with a technique of research of electric parameters of a body of the person. To study the dependence of the resistance of the human body on the frequency and area of contact with the leading part. 1. Basic Provisions The first measurements of the electrical parameters of the human body relate to the beginning of the XIX century. So, A.S. Press points out in his monograph "Electrical installations" on the first Volta experiments on measuring the resistance of the human body. The first measurements, which carry the research character, are associated with the name of prof. Weber and refer to 1836. The first great domestic work of this profile should be considered a doctoral thesis. Tishkov (1886), which shows the results of experiments to measure the resistance of the body. D'Arsonval's experience in studying the effect of high-frequency currents on the human body served as the basis for the development of one of the methods of co-temporal electrotherapy-diathermy (treatment with high-frequency currents). Similarly experiments of prof. Ledyuk (1903) on the study of the effect of the duration of the action of currents of different frequencies on neuromuscular excitation were one of the basic research in the field of electrocardiosis. The electrical characteristics of the human body include its resistance, the dependence of this resistance on environmental factors, the type and frequency of the acting current, the duration of the voltage application, the scheme for replacing the human body. The listed parameters are investigated by the ammeter-voltmeter method, and the person (his body) is an element of the experimental setup. The characteristics of the human body are affected by the shape of the electrode, the force of pressing it and a number of other factors. The resistance of the human body, measured at a voltage of 10...12 V in a sinusoidal alternating current of 50 Hz along the hand-arm path, is 3...6 kOhm. It is believed that this resistance consists of three series-connected resistances: two identical resistances of the outer layer of the skin (epidermis), which together constitute the so-called external resistance of the human body, and one, called the internal resistance of the body, which includes resistance inner layer (dermis) and internal tissues of the body (fig. 1).

3

3

2 1

а) 1 2

RH

RH RB

б) CH

CH Rh

в) Ch Fig. 1. Resistance of the human body: a) a measurement of resistance; b) is an equivalent circuit;c) simplified electrical circuit; 1 – electrodes; 2 – epidermis; 3 – internal tissues of a person; RH – active resistance of the outer layer of the skin; СН – capacity of the condenser formed at the place of contact with the electrodes;RB is the internal resistance of the body; Rh – active resistance of the body; Ch is the body capacity

The resistance of the epidermis consists of active RH and capacitive xH resistance, connected in parallel. The capacitive resistance is caused by the fact that at the place where the electrode touches the human body, a condenser is formed, the plates of which are the electrode and well conductive current of the human body tissue lying under the outer layer of the skin, and the dielectric separating the plates – this layer (epidermis) (see fig. 1, a). Usually this is a flat capacitor whose capacitance depends on the area of the electrode S, m2, the thickness of the epidermis dЭ, m, and its electrical permeability ε, which in turn depends on many factors: the frequency of the applied voltage, skin temperature, presence in the skin moisture, etc. At a current f = 50 Hz, the values of ε are in the range 100...200. 4

Capacitor capacitance, F C = εε S⁄d |,

(1)

where ε0 = 8,85·10–12 F/m is the electric constant. The active resistance of the epidermis RH, Ohm, depends on its specific resistance, E, whose values are in the range 104...105 Ohm·m, and also from S and dЭ R = ρ d ⁄S.

(2)

The resistance of the internal tissues of the body RB is considered to be purely active, although, strictly speaking, it also has a capacitive component. The value of the internal resistance depends on the length and the crosssection of the body section along which the current passes, as well as on the specific resistance of the internal tissues of the organism ρВ, the average value of which at a current up to 1000 Hz is 1.5...2 , 0 Ohm · m. The value of RB is practically independent of the area of the electrodes, the frequency of the current, and is approximately 500...700 ohms. The resistance of the skin, and consequently of the body as a whole, sharply decreases with damage to its stratum corneum, the presence of moisture on its surface, intense perspiration and contamination. Damage to the stratum corneum – cuts, scratches, abrasions and other micro-traumas - can reduce the resistance of the human body to a value close to the value of its internal tissues (500...700 Ohm), which certainly increases the risk of human injury by electric shock. Humidification of the skin reduces its resistance, even if the moisture has a high specific resistance. Thus, the moistening of dry hands with strongly saline water reduces the resistance of the body by 30...50, and with distilled water – by 15...35 %. This is explained by the fact that moisture, which falls on the skin, dissolves on its surface mineral substances and fatty acids, excreted from the body together with sweat and cutaneous fat, and becomes more electrically conductive. When the skin is moistened for a long time, the stratum corneum loosens and becomes saturated with moisture, as a result of which its resistance is almost completely lost. Thus, working with raw hands or in conditions that cause the moistening of any skin areas, creates the prerequisites for a severe outcome in the event of a person falling under tension. Sweating is caused by the activity of the sweat glands located in the lower (inner) layer of the skin (fig. 2). A person has about 500 sweat glands per 1 cm2 of skin. 5

Fig. 2. Structure of the human skin: I – epidermis; II – dermis; III – muscle tissue; 1 – stratum corneum; 2 – the growth layer; 3 – mesh layer; 4 – hair; 5 – sweat gland; 6 – sebaceous gland; 7 – arterial vessel; 8 – venous vessel; 9 – cold receptor; 10 – the receptor of heat; 11 – tactile receptor; 12 – nervous splittings

Sweat well conducts an electric current, because it contains water and dissolved in it mineral salts, as well as some products of metabolism. It is allocated to the surface of the skin along the excretory ducts – thin tubes that penetrate the entire thickness of the skin. Consequently, work in conditions that cause increased sweating exacerbates the danger of human injury by electric current. The contamination of the skin with various substances, especially those that conduct electricity well (metal or coal dust, scale, etc.) is accompanied by a decrease in its resistance, just as it is observed with superficial moistening of the skin. In addition, current-carrying substances, penetrating into the excretory ducts of the sweat and sebaceous glands, create long-lived current-conducting channels in the skin, sharply lowering its resistance. The electrical resistance also depends on the place of application of electrodes to the human body, the values of the current and the applied voltage, the 6

kind and frequency of the current, the area of the electrodes, the duration of the current flow, and some other factors. The place of application of electrodes influences because the resistance of the skin to the same person in different parts of the body is not the same. In addition, different (albeit in a small range) is the internal resistance when the length of the current path changes along the internal tissues of the body. The difference in the values of skin resistance in different parts of the body is explained by a number of factors, including: – different thickness of the stratum corneum; – uneven distribution of sweat glands on the surface of the body; – unequal degree of filling the blood vessels of the skin. The least resistance is to the skin of the face, neck, hands on the area above the palms, especially on the side facing the trunk, sub-muscular cavities, the back of the hands. An increase in the current passing through the human body is accompanied by increased local heating of the skin and an irritating effect on the tissue. This, in turn, causes reflexively, i.e., through the central nervous system, the rapid response of the body in the form of an expansion of the skin co-vessels, and hence the increased supply of blood and increased sweating, which leads to a decrease in the electrical resistance of the skin in this place. An increase in the voltage applied to the human body Uh causes a decrease in its total resistance zh by a factor of tens, which in the limit approaches the lowest value of the resistance of the body's subcutaneous tissues (approximately 300 Ohms). Numerous experiments confirm the nature of this dependence, although the resistivity values obtained from measurements by different authors are usually very different. This is mainly due to the different conditions of the experiments (which were carried out with animals and corpses of people and only within safe currents with living people), as well as individual characteristics of the subjects. For voltages of 5 V and higher at an alternating current of 50 Hz, this dependence can be expressed by the following formula: z =

77 + 0,3 , kOhm U + 10

(3)

where Uh is the applied voltage in volts. The change in zh with increasing applied voltage is mainly due to a decrease in skin resistance and is explained by the influence of a number of factors, including the breakdown of the stratum corneum of the skin under the influence of the applied voltage, which occurs at a voltage of approximately 40 V.

7

The duration of the current flow significantly affects the resistance of the skin, and consequently, on zh as a whole, due to the increase over time of the blood supply of the skin areas under the electrodes, perspiration, etc. Experiments show that at low voltages (20...30 V) for 1...2 min, the resistance drops usually by 10...40 % (on average by 25 %), and sometimes even more. At higher voltage, and consequently, at a higher current, the resistance of the body decreases more rapidly, which is probably due to a more intense effect on the skin of the current of a larger value. For example, measurements made in the USA during one execution in an electric chair showed that the resistance of a human body equal to 800 ohms at the time of switching on the voltage of 1600 V, in 50 seconds decreased to 517 Ohms, i.e. by 35 %. The value zh other than those considered is affected by other factors, albeit to a much lesser extent. Sex and age. In women, as a rule, the body resistance is less than that of men, and in children – less than in adults, in young people is less than in the elderly. This is explained by the fact that in some people the skin is thinner and more tender, in others it is thicker and coarser. Physical irritation that occurs unexpectedly for a person – bo-left (shots and strokes), sound, light, etc. – can cause for a few minutes a decrease in body resistance by 20...50 %. Reduction or increase in the partial pressure of oxygen in the air compared to the norm, respectively, reduces or increases the resistance of the human body. Therefore, in closed rooms, where the partial pressure of oxygen is usually less, the risk of electric shock, other things being equal, is higher than in the open air. The increased temperature of the ambient air (30...45 °C), or the thermal irradiation of a person, causes a slight decrease in zh, even if the person under these conditions is short-lived (several minutes) and no increase in perspiration is observed. One of the reasons for this may be an increase in the supply of blood vessels of the skin with blood as a result of dilatation, which is the response of the body to the thermal effect. The conventional substitution scheme (see fig. 1, b) does not explain a number of phenomena, in particular, the character of the change in the resistance of the human body in the low-frequency region (fig. 3) and the phenomenon of polarity in the flow through the human body of constant and rectified currents. A joint analysis of the curves zh and Ih shows that a frequency similar to resonance of currents occurs at a frequency of 1 Hz. This means that in the scheme of substitution for a given range f it is necessary to provide inductance. The introduction of an inductive component into a replacement circuit does not mean that there are concentrated inductances in the human body. We can state that in the low frequency region the human body is like a two-terminal network 8

containing inductance. This similarity explains that fact in this frequency range the non-release effect is not observed, since the resulting delay in the change in current provides a faster development of the protective reaction of the organism.

Ih, zh Ih

zh f Fig. 3. Change in the characteristics of the human body in the field low frequencies

The phenomenon of polarity can be explained by the presence of a certain polarity in the current source circuit. Where did the source of the current come from? Effective currents are external stimuli that excite the human nervous system. Transmission of the nerve impulse occurs due to a change in the potential on the nerve fiber membrane. This leads to the formation of a current source when the external stimulus is applied. In fig. 4 shows the scheme of substitution of human resistance. i (t)  RH

RH RB

CH

CH

L

Fig. 4. Generalized scheme of substitution of the human body

9

2. Description of the laboratory stand The stand is made in the form of an independent desktop execution device. The appearance of the front panel is shown in fig. 5.

Fig. 5. Appearance of the stand for the study of resistance of the human body

3. Security measures 3.1. All works are carried out with the permission of the teacher. 3.2. The position of the controls should correspond to the marks indicated on the stand. 4. Preparation of the stand for work 4.1. Make an external examination of the stand and make sure of its integrity, secure attachment of the fixing screws. 4.2. Connect the network cable (from the stand kit) to the stand and the network outlet with a grounding contact. 5. The order of work performance 5.1. Study the content of the work. 5.2. Examine the location of instruments and controls on the stand. 10

5.3. Turn the "Network" switch located at the end of the stand to the "ON" position. The LED indicators on the front panel will light: – frequency of applied voltage, 0,00 kHz; – value of applied voltage, 0,0 V; – current through the human body, 0,0 A. The instrument is ready for operation. 5.4. With the button located under the "Frequency of applied voltage" indicator, set the desired current frequency (tab. 1, 2). 5.5. Using the buttons below the "Applied Voltage" indicator, set the voltage to 3 V. The voltage varies in steps of approximately 0,035 V, therefore the change in the "Voltage" indicator does not occur for each press. 5.6. One of the students covers electrodes of smaller area. Changing the frequency of the applied voltage, remove the dependence zh = ϕ(f). 5.7. Paragraph 5.6. Repeat for another student. 5.8. Paragraph 5.6. repeat for each subject, but for larger electrodes. 5.9. Bring the stand to its original state. ATTENTION! If the measured current exceeds the permissible value during operation, the device will go into emergency mode. The emergency mode is indicated by a steady glow of the "Alarm" LED and an audible signal. Voltage on the test stands will be disabled. To exit the alarm mode, disconnect the device from the network or press and release the "RESET" button located on the side of the device on the right, next to the "NETWORK" toggle switch. Table 1 The magnitude of the current through the human body, mA (S = 1250 mm2) Test subjects 1 2

Frequency of applied voltage, kHz 0

0,05

0,1

0,2

0,3

0,4

0,5

1

10

25

50

100

Table 2 The magnitude of the current through the human body, mA (S = 2500 mm2) Test subjects 1 2

Frequency of applied voltage, kHz 0

0,05

0,1

0,2

0,3

11

0,4

0,5

1

10

25

50

100

Table 3 Dependence of human body resistance on frequency (S = 1250 mm2) Test subjects 1 2

Frequency of applied voltage, kHz 0

0,05

0,1

0,2

0,3

0,4

0,5

1

10

25

50

100

Table 4 Dependence of the resistance of the human body on frequency (S = 2500 mm2) Test subjects 1 2

Frequency of applied voltage, kHz 0

0,05

0,1

0,2

0,3

0,4

0,5

1

10

25

50

100

6. Contents of the report 6.1. The name, purpose of the work. 6.2. The obtained measurement results (tab. 1–4). 6.3. Graphical dependencies zh = ϕ (f). 6.4. Conclusions. 7. Test questions 7.1. Explain the generalized scheme of substitution of the human body. Justify the presence of each element in the substitution scheme. 7.2. What causes the decrease in resistance of the human body when the applied voltage is increased? 7.3. Why does the resistance of the human body decrease with increasing frequency of applied voltage? 7.4. What method of research of electrotechnical characteristics of a body of the person is applied in the given laboratory work? What is its essence?

12

RESEARCH OF THE HAZARD OF DAMAGE ELECTRIC SHOCK IN THE NETWORK WITH EARTHED NEUTRAL Objective Studying the influence of neutral mode, network parameters and the resistance of the human body to the outcome of electric shock. 1. Basic Provisions Neutral (zero point) is the center of symmetry of the three-phase current network. In the transformers, as well as on generators, electric motors or other devices, zero output is provided. In symmetrical systems of currents and voltages, this terminal has practically the potential of the earth and it does not matter whether the neutral is connected to ground and if so, how this is done. Three-phase electrical networks, depending on their neutral mode, are divided into networks with isolated and earthed neutral. An electrical network with a grounded neutral is considered to be a network in which at least one of the neutrals of the generators or power transformers is grounded directly or through a device with low resistance in comparison with the zero sequence resistance of the network. Under the study of the danger of electric shock, we will understand the definition of possible values of touch and current voltages through the human body in a network with a normal insulation state, and also in case of insulation damage (when one phase is closed to ground). Define the touch voltage. For a network with a grounded neutral (fig. 1), according to Kirchhoff's law, (1)

I =I +I +I +I , or U y = U −U

+ U −U y + U −U y .

y +y

Considering the phase voltage is balanced, we can write: U y = U

−U

y +y

+ U

where a – is a rotational factor. Then 13

∙a −U y + U

∙a−U y ,

U =U

y + a y + ay + y . y +y +y +y +y

(2)

Fig. 1. Network with earthed neutral

As U =U

−U ,

(3)

then substituting expression (2) into equation (3), we obtain U =U

y 1−a +y 1−a +y . y +y +y +y +y

(4)

The conductivity of the neutral y0 >> уA, уB, уC, therefore equation (4) can be represented as the result of the transformation in the following form: y U =U , (5) y +y or, passing to the actual form, U =U

R . R +r

14

(6)

In networks with voltage up to 1000 V r0 ≤ 8 Ohm, and since Rh ≤ 1000 Ohm, we can assume that the maximum value of the touch voltage is the phase voltage. It can be concluded that in a network with a grounded neutral in the normal mode, when touching one of the phases, the touch voltage is close to the phase (Rh >> r0), and the current through the human body is determined by its own resistance. In the emergency mode (phase C is closed to earth – see fig. 1), the contact voltage is determined by the formula y 1−a +y . y +y +y

U =U

I =U

∙y

y 1−a +y . y +y +y

(7)

(8)

We show U for this case on a vector diagram.

Fig. 2. Vector stress diagram for emergency Network operation mode with earthed neutral

From fig. 2 that the value of Uh in the case of the closure of one of the phases to the ground depends on the ratio rcl and r0 and lies in the range: U