Direct current check of cable electric parameters

DIRECT CURRENT Check of cable ELECTRIC parameters

PURPOSE OF WORK

The purpose of work is to learn the method of direct current check of the communication cable’s (CC) electric parameters. Such method is used during cable mounting on cable plants, and in a produce of cables.

The main task of the work is acquirement some practical skills in CC measuring, mastering measurement devices used, and acquaintance with the СС electric parameters rate.

Main positions

Electric measurements of cable lines are carried out with the following purposes:

a) taken to exploitation CC electric parameters verification to the standards;

b) exploited cable lines electric parameters verification to the standards and detecting sections which are substandard to prevent line from damage;

c) determination of character and point of damage, if occurred;

d) quality of repair control.

According to it the CC electric measurements are distinguished:

- measurements;

- periodic tests (preventive ones);

- measurements to check quality of construction and repair works;

- measurements to determine character and point of damage.

The following electric parameters can be measured at direct current:

- insulation resistance between threads and resistance of every thread’s insulation to other threads connected with the grounded metallic shell, and in cables with plastic shells to the grounded screen;

- mutual capacitance of circuit and capacitance between a thread and the ground;

- thread's stub resistance;

- ohmic disbalance of circuit|chn|;

- electric strength of insulation.

A direct-current measurement of cable electric|electrical| parameters is the|appear| basic|main| method |definition|of determining transmission characteristics|homology| normality|standa: an insulation resistance|insulant||C.|, a mutual capacitance, a stub resistance, an ohmic disbalance |unsymmetry|.

In this work measurements of cable electric parameters are carried out on the model of transmission line. Model’s scheme, types of cables and their lengths are demonstrated in a fig. 2.1.

 

Figure 2.1 – Brands and lengths of cables used for measuring

 

Expected kilometer| values|value| of mutual capacitance|C.| and thread’s stub resistance are calculated according to the following formulae.

A direct current resistance of a circuit:

ohm/km; , mm²

here c is lay ratio; c = 1,01..1,03;

r - specific resistance of material which conductors are made of; for copper r = 0,0175 ohm·mm²/m;

d₀ - diameter of bare conductor, mm;

S - cross-section area, mm².

A mutual capacitance of a symmetric circuit:

F/km,

here a - distance between the centers of pair of threads, mm;

e _eq - equivalent dielectric permeability of insulation (Table. 2.1);

y - correction factor which characterizes closeness of wires to the earthed shell (Table. 2.2);

d1 - diameter of conductor with insulation, mm.

Table 2.1 (Values of equivalent insulation permittivity for different types of insulation)

Purpose of cable	Insulation type	eeq
City network	Airily-paper Polyethylene	1.5... 1.6 1.9... 2.1
Interurban networks	Styroflex	1.2... 1.3

 

Table 2.2 (Correction factor dependence on diameters’ ratio)

 

d1/d0	Value y for pair twisted threads	Value y for star-twisted threads
1.6	0.608	0.588
1.8	0.627	0.611
2.0	0.644	0.619
2.2	0.655	0.630
2.4	0.665	0.647

 

Calculations of the expected kilometric| values|value| of ohmic resistance|holdout| and capacitance|C.| of circuit|chn| should be recalculated on|lenght| the length of observed section and used for adjusting and working|wrk| with ПКП–4М device during measuring.

A diameter of an insulated conductor|cellular| with a solid airily-paper insulation|insulant| is determined as a sum|amount| of conductor’s diameter||cellular| and a thickness of insulation|insulant|:

D₁ = d₀ + 0.65d₀ = 1.65d₀.

A diameter of an insulated conductor|cellular| with a solid polyethylene insulation|insulant| is determined as a sum|amount| of |conductor’s diameter|cellular| and a doubled thickness of insulation|insulant|:

D₁ = d₀ + 2∆_ins,

here ∆_ins is a thickness of insulation, mm.

A diameter of a conductor|cellular| with an insulated cord||insulant| is determined as

D₁ = d₀ + 2d_c(1 - s) + 2µ_ins,

here d_c - insulated cord’s diameter, mm;

s - bearing ratio of the cord|.

For a styroflex insulation the bearing ratio of cord
s = 0, and for a paper-cord insulation s = 0.1...0.3.

A diameter d₁ of an insulated conductor in cables of urban telephone network (UTN) with airily-paper insulation of threads is determined taking into account calibration (appendix 9.2).

A distance between the centers of the conductors in a pair for cables with pair twisting , for cables with star-type twisting .

Before start of an electric|electrical| parameters verification of the mounted cable sections,| an identification of wires should be done in order|cellular||chn| to check the correctness of the mounting. It|her| enables to determine the breakdown of pairs|couple| and provides|secure| the symmetric connection|inclusion| of telephone pairs|couple| to the line terminal devices.

Measuring of conductor’s insulation resistance, mutual and partial capacitance, loop resistance of circuits and resistance of thread’s ohmic disbalance are carried out with ПКП-4М device. A short explanation of its exploitation is given in the description which is placed next to the device.

The results of measuring are converted to|by| kilometric| values|importance| taking into account temperature coefficient|ratio|, they are compared to the standards|standard| and then there should be made a conclusion concerning operability of |by||wrk|cable section measured|take|. It is also necessary to take into account that an insulation resistance|holdout||insulant| diminishes with the increase of line length|lenght| whereas a|but| loop resistance|holdout| and a capacitance|C.| increase, i.e. an insulation resistance|holdout||insulant| is inversely proportional to line|by| length|lenght|, and a |but| loop resistance|holdout and a capacitance|C.| are directly proportional to it.

 

 

A loop resistance calculation at temperature of t = 20°С is obtained from formula

,

here R_{lr t} is a loop resistance at the temperature of t°С;

a_R is a temperature coefficient of resistance; for copper

A cable insulation resistance calculation at temperature of can be obtained from formula

,

here R_{ins 20} is an insulation resistance at the temperature of 20°С;

R_{ins t} is an insulation resistance at the temperature of t°С;

- temperature coefficient of an insulation resistance.

For cable paper ; for a polyethylene and styroflex temperature coefficient is .

Thus the recalculatation of parameters given above|revalution| as a rule should be done for paper-insulated cables. In cables with styroflex and solid polyethylene insulation|insulant| resistance virtually|holdout||insulant| does not depend on temperature.

Approximate values of soil temperature at a depth of 0.8 meters for Ukraine are given in Table. 2.3.

Extracts from Specification for cables ТГ, ТПП, МКС are shown in Appendix 9.2. All of standard values|importance| are recalculated on
1 km at temperature of 20°С|.

Values of electric parameters of other cables can be taken from reference books.

 

Table 2.3 (Reference values of temperature of soil for Ukraine)

 

Month	Temperature of soil (black earth), °C	Month	Temperature of soil (black earth), °C
January	3.2	July	15.4
February	1.6	August	17.6
March	1.2	September	16.8
April	6.1	October	10.6
May	9.4	November	7.6
June	12.9	December	4.5

KEY QUESTIONS

3.1 Purpose and elements of urban telephone network cables both low frequency (LF) and high frequencies (HF) ranges.

3.2 Purpose, kinds and volume of electric measurements of cables.

3.3 Basic circuits of insulation resistance|holdout|,|insulant| capacitance|C.|, loop resistance|holdout| and treads ohmic disbalance|cellular||electrical||chn| measuring by ПКП-4М device.

3.4 Procedure of mutual capacitance and loop resistance of electric circuits calculation.

3.5 Direct current standards of the cable electric parameters.

HOME TASK

As a result of independent preparation to laboratory work with this workbook and recommended literature it is necessary:

4.1 To learn construction of LF cables for urban telephone network (ТГ and ТПП) and HF cables (МКСГ).

4.2 To learn purpose and instructions for ПКП-4М device.

4.3 To calculate the capacitance and the loop resistance values taking into account the length of the cable.

4.4 To prepare the report from Table 4.1 for noting results of measurements.

4.5 To note down into the Table. 4.1 standards for electric parameters of the corresponding cables.

4.6 To prepare the recitation for key questions.

Table 4.1 (Results of measurements)

The measured parameter	For length of L, m at temperature of t°C	Per 1 km at 20°C	Specification information for 1 km at 20°C	Note
1 pair	2 pair	1 pair	2 pair
Rins ab , Ohm
Rins a , Ohm
Rins b , Ohm
Rins sh , Ohm
С 0, F
Сa, F
Сb, F
Rlr, О
Δ R, О

LABORATORY TASK

5.1 Prepare ПКП-4М device for work.

5.2 Verify the direct current cable electric parameters:

- insulation resistance between threads and each single thread to earth: R_{ins ab}, R_{ins a}, R_{ins b};

- resistance of the shield-earth insulation R_{ins sh}, in case cable is shielded;

- mutual capacitance C₀ of circuits and capacitance of threads C_a, C_b to the earth;

- loop resistance R_lr;

- ohmic disbalance Δ R;

5.3 Convert the results of electric|electrical| parameters measurement to kilometric| values|importance| taking into account the temperature of soil|footwall| for loop resistance|holdout| and insulation resistance|holdout||insulant| (if necessary).

5.4 Compare measurements the data with the standards of the cable electric parameters and make a conclusion concerning their accordance with the cable specification.

Equipment

6.1 Samples of urban telephone network cables

6.2 Model of cable circuits (ТГ-10 2 0.5 - 2 km; ТПП-10 2 0.5 - 3 km; МКСГ-1 4 1.2 - 5 km).

6.3 ПКП-4М.

CONTENTS of a report

7.1 The expected values calculations of loop resistance and mutual capacitance of the corresponding cable.

7.2 Measuring data of the cable electric parameters according to the Table. 4.1.

7.3 Conclusion concerning to serviceability of a given cable section.

LITERATURE

8.1 Гроднев И. И., Курбатов Н. Д. Линии связи. - М.: Связь, 1980. – C. 409–414.

8.2 Ионов А. Д., Попов Б. В. Линии связи. - М.: Радио и связь, 1990. – С. 155–158.

APPENDIX

Direct current measurements

Direct current measurements enable to make a conclusion concerning accordance of the most unstable line characteristics with the set norms. They are:

- electric insulation resistance;

- electric loop resistance;

- ohmic disbalance|unsymmetry|;

- electric capacitance of circuit.

Besides, direct current measurements are widely used for determination of the most widespread damage – an insulation fault.

For a direct current measuring of circuits the purpose-designed portable cable devices have acquired wide use, for instance ПКП-3, ПКП-4 and ПКП-5.

Direct current|electrical| measurements are worth being executed in the following order: an electric|electrical| loop resistance|holdout||c, an ohmic disbalance|unsymmetry|, an|electrical| electric insulation resistance|holdout||insulant|, an electric|electrical| capacitance|C.| of circuit|chn|.

Ohmic disbalance

An ohmic disbalance (a difference of electric resistances of a circuit conductors at direct current Δ R = R_a – R_b) is measured by a dc bridge (Fig. 9.1). The citcuit beginning (the end A) is connected to the terminals 1 and 2, and an earthed shell or a shield are connected to the terminal 3. The opposite end of circuit B becomes short-circuited and earthed. A bridge with fixed ratio of branch resistances equal to 1 is used for measuring. If the bridge is balanced R = R_B = R_a – R_b. The conductor with lower resistance should be connected to the branch, which contains the variable resistor R_B. The bridge won’t be balanced otherwise, in this case the conductors should be swapped using device switch «Line switching».

 

 

 

 

Loop resistance

A loop resistance (an electric resistance of conductors of a two-wire circuit, R_lr = R_a + R_b) is usually measured, using the dc bridge with a fixed ratio of branch resistances (Fig. 9.2). The resistance of the bridge branches is selected to reduce current in the bridge diagonal in which the indicator is connected to zero during measurement. Having balanced the bridge, we can find the value of the loop resistance over the entire length of the circuit.

 

Mutual capacitance

An electric wire-to-wire capacitance or thread capacitance to earthed metallic covering (shield) is measured by ballistic method or charge-discharge method, or method of comparison. All of the noted methods are direct-current ones. Devices ПКП-4М and ПКП-5 are provided with electric capacitance measurement scheme which use voltmeter-ammeter method at alternating current (Fig. 9.4). Before measuring calibration of device is to be executed. The measured value is extended for a capacitance over the entire length of circuit.

 

 

 

Picture 9.4 – Scheme of electric capacitance measurement

 

Laboratory work 2

PURPOSE OF WORK

Learning primary and secondary transmission parameters of circuits, learning methods of parameters calculation and methods of cable transmission primary and secondary parameter measurement.

 

KEY positions

A propagation of an electromagnetic field along a circuit is characterized by parameters which include:

- the primary parameters: an active resistance of a circuit R, Ohm/km; an inductance of a circuit L, H/km; a capacitance of a circuit, F/km; a shunt conductance of a circuit G, S/km;

- the secondary parameters: an attenuation factor a; dB/km; a phase factor b, radian; a surge impedance Z_im, Ohm;a wavepropagation velocity V, m/s.

An attenuation factor (|kilometric attenuation) of a circuit, a, |chn| is a parameter which characterizes|describe| power damping|reduc|capacity| of a signal, propagating along a line. Its|its| value|value| is equal to signal attenuation (in decibels) per circuit|chn| length|lenght| of 1 km.

A phase factor|ratio| is determined by the alteration|variating| of signal phase (current or voltage one) when propagated along a line. The phase factor numerical value|importance||ratio| can be determined as a difference of signal phases (in radians|) for two points|dot-and-dash| of the circuit separated by |1 km distance.

A surge impedance is resistance to a progressive voltage wave. For a uniform line, where indirect waves are absent, a surge impedance is the same in every point and it is equal to the ratio of voltage to current in any point of the circuit. A surge impedance is a complex quantity; its modulus is equal to a ratio of the voltage amplitude to the current one, and its argument is the voltage and the current phase difference in every point of the circuit.

A propagation velocity (a phase velocity) V is a velocity of a monochromatic wave front propagation. It does not exceed the speed of light in free space (c = 3·10⁸ mps).

All of parameters of transmission have different values at different frequencies. The frequency dependence diagrams of primary and secondary transmission parameters of bilateral circuits are shown in Figure 2.1.

Among the primary parameters R and G onlycause loss of energy: R characterizesthermal loss in conductors and other metallic parts of cables such as shield, coverings, armor, C and G characterizeloss in conductor insulation.

The values of primary transmission parameters can be obtained by the direct measurement, whereas the secondary parameters can be obtained by the indirect measurements only.

While carrying out the measurements on a line, it is necessary to match an oscillator and load impedances with the impedance of line, therefore, as a rule; each measurement should be started with the line impedance determination.

Using device МПП-300 the input parameters of the line can be measured by short-circuit and free-run methods (SCM, FRM) and therefore the secondary transmission parameters a, b, V, j and | Z _im| are determined. In case high-frequency measurements for a long line its impendance can be measured as an input resistance of this line.

An attenuation factor a can be also obtained by the method of level difference or comparison method (Fig. 2.2).

A phase factor b can be measured by the method of indemnification in accordance with the chart of fig. 2.3.

Voltages on the output of measured line and fading shop differ by a phase shift produced by the measured line, because the fading shop is constructed of active resistances and does not bring in the change of phases in the probed voltage.

A phase difference between voltage on the output of line and the fading shop can equal zero only if length of a line equals zero or integer number of wave lengths measured frequency. Consequently, for every length of line there is a series of frequencies, for which a phase shift produced a line, is 2 p n. These frequencies are named critical. Knowing a length of line and a critical frequency, it is possible to define a phase factor for this frequency as , here L - length of the probed line. A critical frequency is determined by an indicator (IL, indicator of level) which will show minimum at the counter-opposite connection of coils of differential transformer (DT).

 

Figure 2.2 Measuring of fading: by the method of difference of levels and
by the method of comparison

 

 

 

 

Figure 2.3 Measuring phase factor by the method of indemnification

 

The secondary transmission parameter determination for the obtained results of the line input parameters measured in short-circuit and idling modes can be done according to the proper formulae using transmission equations for uniform lines ([1], p. 90):

here is a propagation constant;

x is a distance between the beginning of the line and the point being observed;

U_x, I_x are complex voltage and current at the x point;

U₀, I₀ are complex voltage and current at the line’s beginning.

Solving the equations given above with respect to U0 and I0 at x = l, we get

It follows from this that

here is a load resistance (of receiver).

In case the receiver resistance is equal to zero (SCM, ) the input resistance

(2.1)

In case the receiver resistance is infinite (IM, ),

(2.2)

Thus

(2.3)

An important point is that a short-circuit and an idling mode methods are used for short electric circuits only, where dB. For long lines dB; in this case and Also for lines with attenuation coefficient lying in the range of 4,6…13 dB SCM and IM methods are not used because attenuation coefficient is determinated with low accuracy.

We designate phase shifts between a voltage and a current at the circuit’s beginning as j ₀ and j _¥ (input impedance argument) for rear end short-circuit and idle running respectively. A value of input admittance for SCM and IM can be determined from vector diagrams (Figure 2.3).

 

From Figure 2.3 it is obvious that

(2.4)

(2.5)

It should be noted that usually has a negative value (a sign
is "minus"), and – positive (a sign is "plus").

As j ₀ and j _¥ are the arguments of entrance complex conductivities of Y0, Y _¥, arguments of entrance complex resistances of Z0, Z _¥will differ from them only by signs:

Therefore a modulus and an argument of the impedance in accordance with (2.3) will be

(2.6)

We designate

(2.7)

Where

, (2.8)

With (2.1), (2.2) and (2.7) we get:

(2.9)

Using an identity we can write:

Hence determining real and imaginary parts, we can get the expressions for a and β:

(2.10)

(2.11)

Determination of the secondary transmission parameters using results of measurements of line input admittances G_{вх 0}, G_{вх ∞} and input capacitances C_{вх 0}, C_{вх ∞}inshort-circuit and idling modes is recommended to perform in the sequence given below:

(2.12) (2.14) (2.16) (2.18) (2.20) (2.22)

(2.13) (2.15) (2.17) (2.19) (2.21)

The calculated value of b can fall short of certified one at a rate frequency, because tg 2bL is a periodic function, and not only a single value of b but a range of them satisfies the equation (2.11)

here n is an integer. Therefore the task reduces to determination of p number multiple to be added to the obtained value. The number n is selected to approximate b to a tabular value.

Having worked out the secondary transmission parameter values it is possible to calculate the primary ones.

A propagation coefficient g is a complex number and can be represented as the following expression

A surge impedance

.

Multiplying g by Z_sim we get the circuit impedance:

. (2.23)

Dividing g by Z_sim we get the circuit complex admittance:

. (2.24)

Separating, the real and the imaginary parts of and and dividing the imaginary components by w we get the primary parameters of transmission R, L, C, G.

KEY QUESTIONS

3.1 The primary and secondary parameters of transmission, their units of measure, physical interpretation, frequency dependence.

3.2 What does methods of short-circuit and idling modes consist of for measurement of the secondary transmission parameters?

3.3 When is it possible to use methods of SCM and IM?

3.4 Sequence of circuit input admittance and capacitance measurement by МПП-300.

 

HOME work

4.1 Learn the following questions:

– electric processes which take place in the balanced and coaxial cables;

– frequency dependence of the primary and secondary transmission parameters;

– methods of calculation of the primary and second transmission parameters;

– methods of transmission parameter measurement.

4.2 Prepare the laboratory report and plan of work’s implementation according to the sections 5 and 9.

4.3 Prepare to the discussion of key questions.

LABORATORY TASK

5.1 Acquaint yourself with equipment on-site and specify your implementation plan with a lecturer.

5.2 Cut the scheme for parameter measurement by the SCM and IM methods.

5.3 Measure cable input parameters in a short-circuit and an idling modes at the set frequencies. Note down the measured and the calculated values in a Table. 5.1.

Table 5.1

f, Hz	Cin0, F/km	Gin0, S/km	j 0, deg	Cin ¥, F/km	Gin ¥, S/km	j •, deg	a, dB/km	b, radian/km	| Zsim|, Ohm	jim, deg

 

EQIUPMENT

6.1 A model of cable line 0,5 km long, cable ПРППМ-1×2×1,2.

6.2 An input admittance bridge МПП-300.

6.3 A low-frequency generator ГЗ-56/1.

6.4 A selective level indicator ИУУ 5–300 kHz|.

REPORT CONTENTS

7.1 Purpose of work.

7.2 Equipment.

7.3 Schemes and results of measurements, calculation results, frequency dependence diagrams of the primary and secondary transmission parameters, based on the measurement results.

7.4 Conclusions.

LITERATURE

8.1 Гроднев И. И., Курбатов Н. Д. Линии связи. – М.: Связь, 1980. – С. 98–116, 120–133.

8.2 Гроднев И. И., Верник С. М. Линии связи. – М.: Радио и связь, 1988. – С. 120–131, 168–172.

APPENDIX

Laboratory work 3

PURPOSE OF WORK

Familiarization with the method of determination of the coaxial cable secondary transmission parameters by the results of wavelength shorting coefficient measurement.

KEY POSITIONS

Key questions

3.1 Give the definition of the secondary transmission parameters and show their frequency dependences.

3.2 Write down the expression of a wavelength shorting coefficient and give its definition.

3.3 How do you measure a value of shorting coefficient in transmission line?

3.4 What do a, | Z_sim |, b and v _ph depend on in high-frequency range?

3.5 Analyze the expression according to which measured parameter’s values are reduced to for the temperature of 20°С.

3.6 What are temperature coefficients ?

3.7 Principles of transmission line attenuation measurement by the two-voltmeter method.

Home work

For self-reading for the laboratory work it is necessary:

4.1 Learn the recommended literature.

4.2 Learn the methodology of a wavelength shorting coefficient measurement in a transmission line (p. 9.1).

4.3 Learn the principles of attenuation measurement according to a two-voltmeter method (p. 2.2).

4.4 Prepare recitations to the key questions.

4.5 Prepare the report.

Laboratory task

5.1 Familiarize yourself with the laboratory model.

5.2 Turn on instruments make them ready to work.

5.3 Measure according to a task a lector has given to you:

– a wavelength shorting coefficient value in a coaxial cable;

– U ₁and U ₂values at four frequencies in the same cable.

5.4 Calculate:

– the secondary transmission parameters according to x measured at four frequencies;

– an attenuation coefficient of the cable at the same frequencies using U ₁and U ₂ values obtained by two-voltmeter method.

5.5 Chart frequency characteristics of the secondary parameters according to the measurement results.

Equipment

6.1 Models of coaxial cables РК-75, РЖ-58 and РЖ-59 30 m long.

6.2 Р5-10 device or its present-day analogue.

6.3 Generators, voltmeters, resistances.

Report content

7.1 Results of shorting coefficient measurement for three cable models.

7.2 Results of the secondary transmission parameter calculation at four frequencies and frequency dependence diagrams.

7.3 Results of an attenuation coefficient measurement by a two-voltmeter method for the same cable at the same four frequencies and a frequency dependence diagrams.

7.4 Make the conclusion which includes the analysis of the obtained results.

RECOMMENDED LITERATURE

8.1 Гроднев И. И., Курбатов Н. Д. Линии связи. - М.: Связь, 1980. – С. 113–115.

8.2 Ефимов И. Е., Останькович Г. А. Радиочастотные линии передачи. - М.: Связь, 1977.

8.3 Измеритель неоднородностей линий Р5-10. Техническое описание и инструкция по эксплуатации. – 1980.

APPENDIX

Cable specification

A radio-frequency exchange coaxial cable РК-75 is used for internal laying in on-air broadcasting systems to transmit signals from a receiving aerial to a TV set in the range of 5…1000 MHz, in cable television systems for a house distribution and an individual connection.

Electrical and structural parameters of cables used in this work to take measurements are given in Table 9.1.

Table 9.1 – Electrical and structural parameters of coaxial cables РК-75, РЖ-58 and РЖ-59

Make of the cable	Surge impedance, Ohm	Attenuation coefficient, dBpkm at frequency of, MHz	Capacitance, nFpkm	Diameter of inner conductor, mm	Inner diameter of outer conductor, mm
РК-75		27.04	85.6	270.7		0.85	4.0
РЖ-58		38.67	122.4	387.0		0.64	3.0
РЖ-59		33.3	105.4	333.3		0.64	3.71

 

 

Laboratory work 4

Measurement and testing transmission lines by
pulse method

PURPOSE OF WORK

Learning of pulse method to determine point and nature of line’s damage as well as learning of methodology of coax attenuation coefficient determination by the results of wavelength shorting coefficient measurement.

KEY POSITIONS

Key questions

3.1 What is the principle of line’s obstacles determination by using a pulse method?

3.2 How do you determine a distance to the obstacle’s location in the transmission line?

3.3 How do you determine a wavelength shorting coefficient’s value?

3.4 What quantity characterizes surge impedance’s obstacle of transmission line?

3.5 Write down the expressions for reflection and shorting coefficients of electromagnetic wave.

3.6 Give expressions for C, , v _ph determination using ε.

Home work

In process of preparation for the laboratory work it is necessary to:

4.1 Study the recommended literature.

4.2 Familiarize yourself with a pulse method of place and nature of line’s fault determination (p. 2.1).

4.3 Learn a methodology of measurement for:

– a distance to an obstacle (a fault) of a transmission line;

– a wavelength shorting coefficient for a line with a known length;

– a surge impedance for transmission line with an unknown length.

4.4 Prepare recitations to key question.

4.5 Prepare the report.

Laboratory task

5.1 Familiarize yourself with the laboratory stand.

5.2 Turn on a Р5-10 device and set it ready for work.

5.3 Connect the device to an artificial extension line and carry out the following actions:

а) run a pulse-response characteristic of an operable line which includes different obstacles;

b) determine the distance to certain points of transmission line (for aerial lines ξ = 1.05);

c) determine locations and nature of line’s faults according to the task assigned by instructor.

5.4 Measure a surge impedance of coax’ sections according to a comparison method and evaluate the shorting coefficient.

5.5 Having evaluated the shorting coefficient, measure each cable section’s length.

EQUIPMENT

Laboratory stands of aerial lines several sections of coaxial cables with different surge impedance values.

A Р5-10 device or its modern analogue.

Resistance set.

Report content

7.1 A drawing of pulse-response characteristics of operable and inoperable aerial lines.

7.2 Results of measurements of a distance to certain TL stations and places of faults determined according to the pulse characteristics.

7.3 Results of measurements of a surge impedance Z_sim and a length L of the cable sections, determination of a shorting coefficient ξ.

RECOMMENDED LITERATURE

8.1 Гроднев И. И., Курбатов Н. Д. Линии связи. - М.: Связь, 1980. – С. 414.

8.2 Ефимов И. Е., Останькович Г. А. Радиочастотные линии передачи. – М.: Связь, 1977.

8.3 Измеритель неоднородностей линий Р5-10. Техническое описание и инструкция по эксплуатации. – 1980.

APPENDIX

Р5-10 work start

9.1.1 Before turning on the device it is necessary to check that the device is switched to corresponding current and voltage.

9.1.2 Set the knobs to the base point:

«Gain» – to the extreme left position;

«Distance» – 0;

«Set of reading» – to the extreme left position;

«Power» – to the lowest position (turned off).

9.1.3 Check if the device is grounded.

9.1.4 Switch on power by the knob «Power». The indicator will light up on the front panel. In 0.5…2 minutes a sweep trace will appear.

9.1.5 Regulate brightness, focus and position of sweep trace on the display by the knobs «☼», «8» and «↕». The sweep trace must be in the center of the screen.

DIRECT CURRENT Check of cable ELECTRIC parameters

PURPOSE OF WORK

The purpose of work is to learn the method of direct current check of the communication cable’s (CC) electric parameters. Such method is used during cable mounting on cable plants, and in a produce of cables.

The main task of the work is acquirement some practical skills in CC measuring, mastering measurement devices used, and acquaintance with the СС electric parameters rate.

Main positions

Electric measurements of cable lines are carried out with the following purposes:

a) taken to exploitation CC electric parameters verification to the standards;

b) exploited cable lines electric parameters verification to the standards and detecting sections which are substandard to prevent line from damage;

c) determination of character and point of damage, if occurred;

d) quality of repair control.

According to it the CC electric measurements are distinguished:

- measurements;

- periodic tests (preventive ones);

- measurements to check quality of construction and repair works;

- measurements to determine character and point of damage.

The following electric parameters can be measured at direct current:

- insulation resistance between threads and resistance of every thread’s insulation to other threads connected with the grounded metallic shell, and in cables with plastic shells to the grounded screen;

- mutual capacitance of circuit and capacitance between a thread and the ground;

- thread's stub resistance;

- ohmic disbalance of circuit|chn|;

- electric strength of insulation.

A direct-current measurement of cable electric|electrical| parameters is the|appear| basic|main| method |definition|of determining transmission characteristics|homology| normality|standa: an insulation resistance|insulant||C.|, a mutual capacitance, a stub resistance, an ohmic disbalance |unsymmetry|.

In this work measurements of cable electric parameters are carried out on the model of transmission line. Model’s scheme, types of cables and their lengths are demonstrated in a fig. 2.1.

 

Figure 2.1 – Brands and lengths of cables used for measuring

 

Expected kilometer| values|value| of mutual capacitance|C.| and thread’s stub resistance are calculated according to the following formulae.

A direct current resistance of a circuit:

ohm/km; , mm²

here c is lay ratio; c = 1,01..1,03;

r - specific resistance of material which conductors are made of; for copper r = 0,0175 ohm·mm²/m;

d₀ - diameter of bare conductor, mm;

S - cross-section area, mm².

A mutual capacitance of a symmetric circuit:

F/km,

here a - distance between the centers of pair of threads, mm;

e _eq - equivalent dielectric permeability of insulation (Table. 2.1);

y - correction factor which characterizes closeness of wires to the earthed shell (Table. 2.2);

d1 - diameter of conductor with insulation, mm.

Table 2.1 (Values of equivalent insulation permittivity for different types of insulation)

Purpose of cable	Insulation type	eeq
City network	Airily-paper Polyethylene	1.5... 1.6 1.9... 2.1
Interurban networks	Styroflex	1.2... 1.3

 

Table 2.2 (Correction factor dependence on diameters’ ratio)

 

d1/d0	Value y for pair twisted threads	Value y for star-twisted threads
1.6	0.608	0.588
1.8	0.627	0.611
2.0	0.644	0.619
2.2	0.655	0.630
2.4	0.665	0.647

 

Calculations of the expected kilometric| values|value| of ohmic resistance|holdout| and capacitance|C.| of circuit|chn| should be recalculated on|lenght| the length of observed section and used for adjusting and working|wrk| with ПКП–4М device during measuring.

A diameter of an insulated conductor|cellular| with a solid airily-paper insulation|insulant| is determined as a sum|amount| of conductor’s diameter||cellular| and a thickness of insulation|insulant|:

D₁ = d₀ + 0.65d₀ = 1.65d₀.

A diameter of an insulated conductor|cellular| with a solid polyethylene insulation|insulant| is determined as a sum|amount| of |conductor’s diameter|cellular| and a doubled thickness of insulation|insulant|:

D₁ = d₀ + 2∆_ins,

here ∆_ins is a thickness of insulation, mm.

A diameter of a conductor|cellular| with an insulated cord||insulant| is determined as

D₁ = d₀ + 2d_c(1 - s) + 2µ_ins,

here d_c - insulated cord’s diameter, mm;

s - bearing ratio of the cord|.

For a styroflex insulation the bearing ratio of cord
s = 0, and for a paper-cord insulation s = 0.1...0.3.

A diameter d₁ of an insulated conductor in cables of urban telephone network (UTN) with airily-paper insulation of threads is determined taking into account calibration (appendix 9.2).

A distance between the centers of the conductors in a pair for cables with pair twisting , for cables with star-type twisting .

Before start of an electric|electrical| parameters verification of the mounted cable sections,| an identification of wires should be done in order|cellular||chn| to check the correctness of the mounting. It|her| enables to determine the breakdown of pairs|couple| and provides|secure| the symmetric connection|inclusion| of telephone pairs|couple| to the line terminal devices.

Measuring of conductor’s insulation resistance, mutual and partial capacitance, loop resistance of circuits and resistance of thread’s ohmic disbalance are carried out with ПКП-4М device. A short explanation of its exploitation is given in the description which is placed next to the device.

The results of measuring are converted to|by| kilometric| values|importance| taking into account temperature coefficient|ratio|, they are compared to the standards|standard| and then there should be made a conclusion concerning operability of |by||wrk|cable section measured|take|. It is also necessary to take into account that an insulation resistance|holdout||insulant| diminishes with the increase of line length|lenght| whereas a|but| loop resistance|holdout| and a capacitance|C.| increase, i.e. an insulation resistance|holdout||insulant| is inversely proportional to line|by| length|lenght|, and a |but| loop resistance|holdout and a capacitance|C.| are directly proportional to it.

 

 

A loop resistance calculation at temperature of t = 20°С is obtained from formula

,

here R_{lr t} is a loop resistance at the temperature of t°С;

a_R is a temperature coefficient of resistance; for copper

A cable insulation resistance calculation at temperature of can be obtained from formula

,

here R_{ins 20} is an insulation resistance at the temperature of 20°С;

R_{ins t} is an insulation resistance at the temperature of t°С;

- temperature coefficient of an insulation resistance.

For cable paper ; for a polyethylene and styroflex temperature coefficient is .

Thus the recalculatation of parameters given above|revalution| as a rule should be done for paper-insulated cables. In cables with styroflex and solid polyethylene insulation|insulant| resistance virtually|holdout||insulant| does not depend on temperature.

Approximate values of soil temperature at a depth of 0.8 meters for Ukraine are given in Table. 2.3.

Extracts from Specification for cables ТГ, ТПП, МКС are shown in Appendix 9.2. All of standard values|importance| are recalculated on
1 km at temperature of 20°С|.

Values of electric parameters of other cables can be taken from reference books.

 

Table 2.3 (Reference values of temperature of soil for Ukraine)

 

Month	Temperature of soil (black earth), °C	Month	Temperature of soil (black earth), °C
January	3.2	July	15.4
February	1.6	August	17.6
March	1.2	September	16.8
April	6.1	October	10.6
May	9.4	November	7.6
June	12.9	December	4.5

KEY QUESTIONS

3.1 Purpose and elements of urban telephone network cables both low frequency (LF) and high frequencies (HF) ranges.

3.2 Purpose, kinds and volume of electric measurements of cables.

3.3 Basic circuits of insulation resistance|holdout|,|insulant| capacitance|C.|, loop resistance|holdout| and treads ohmic disbalance|cellular||electrical||chn| measuring by ПКП-4М device.

3.4 Procedure of mutual capacitance and loop resistance of electric circuits calculation.

3.5 Direct current standards of the cable electric parameters.

HOME TASK

As a result of independent preparation to laboratory work with this workbook and recommended literature it is necessary:

4.1 To learn construction of LF cables for urban telephone network (ТГ and ТПП) and HF cables (МКСГ).

4.2 To learn purpose and instructions for ПКП-4М device.

4.3 To calculate the capacitance and the loop resistance values taking into account the length of the cable.

4.4 To prepare the report from Table 4.1 for noting results of measurements.

4.5 To note down into the Table. 4.1 standards for electric parameters of the corresponding cables.

4.6 To prepare the recitation for key questions.

Table 4.1 (Results of measurements)

The measured parameter	For length of L, m at temperature of t°C	Per 1 km at 20°C	Specification information for 1 km at 20°C	Note
1 pair	2 pair	1 pair	2 pair
Rins ab , Ohm
Rins a , Ohm
Rins b , Ohm
Rins sh , Ohm
С 0, F
Сa, F
Сb, F
Rlr, О
Δ R, О

LABORATORY TASK

5.1 Prepare ПКП-4М device for work.

5.2 Verify the direct current cable electric parameters:

- insulation resistance between threads and each single thread to earth: R_{ins ab}, R_{ins a}, R_{ins b};

- resistance of the shield-earth insulation R_{ins sh}, in case cable is shielded;

- mutual capacitance C₀ of circuits and capacitance of threads C_a, C_b to the earth;

- loop resistance R_lr;

- ohmic disbalance Δ R;

5.3 Convert the results of electric|electrical| parameters measurement to kilometric| values|importance| taking into account the temperature of soil|footwall| for loop resistance|holdout| and insulation resistance|holdout||insulant| (if necessary).

5.4 Compare measurements the data with the standards of the cable electric parameters and make a conclusion concerning their accordance with the cable specification.

Equipment

6.1 Samples of urban telephone network cables

6.2 Model of cable circuits (ТГ-10 2 0.5 - 2 km; ТПП-10 2 0.5 - 3 km; МКСГ-1 4 1.2 - 5 km).

6.3 ПКП-4М.

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