Crossings With High Voltage Lines, CategoryII
8.1 For the crossing span, the telecommunication alignment shall be taken close to the power pole to obtain increased vertical clearance between wires.
8.2 A light cantilever framework shall be fixed to the power support as indicated in Figure 3 (a) and 3 (b). The framework will be designed for each case depending upon the type of support used for the power line and the type of brackets on the telecommunication line.
8.3 In deciding the structural details for these crossing, provisions should be made for the possible erection of additional wires on the telecommunication line to meet future requirements. The clearances both horizontal and vertical, and between wires and supports shall conform to the standards shown in Figure 3 (a) and 3 (b).
8.4 In those cases where the poles of either of the lines cannot be or are not located near the crossing location cradle guards on the power line shall be provided, the arrangement being similar to that in Para 6.2, but with clearances indicated in Figures 3 (a) and 3 (b).
8.5 Power contact protectors shall be installed on all the exposed wires generally carried on the topmost bracket by the telecommunication supports.
Note: The technical details of power contact protectors are described in the Appendix I to Chapter VI.

Crossing With Extra High Voltage Lines

No guarding arrangements are considered necessary in such cases.
9.2 The telecommunication line shall cross the power line as close to the power line supports as practicable.
9.3 The minimum clearances between the power wires and telecommunication wires shall be:
For lines of voltage above 36 KV
up to and including 72.5 KV: 2440 mm (8′0″)
For lines of voltage above 72.5 KV
up to and including 145 KV: 2740 mm (9′0″)
For lines of voltage above 145 KV
up to and including 245 KV: 3050 mm (10′0″)
For lines of voltage above 245 KV: 3050 mm (10 feet)
Plus 305 mm
(1 foot) for every additional
33 KV or part thereof

Crossing With Power Lines Having Telecommunication Wires On The Line Supports Below The Power Line
10.1 Crossing should be so arranged that the telecommunication line passes close to a power pole to obtain increased vertical clearance between the wires of the two alignments.
10.2 A guard shall be provided both on the power supports as well as on telecommunication line, as per arrangements indicated in Figures 4 (a) and 4 (b).
Appendix I to Chapter VI
(Refer Paras 4.4. and 8.5)
Power Contact Protectors

Operation
Power contact protectors (see Figure 5) are air gap arrestors connected between overhead telecommunication wires and ground. The arrester gaps are designed to breakdown at a voltage of 3,000 volts to possess a high current carrying capacity.
When wires of a telecommunication line on which these protectors are installed come in contact with a power wire of voltage to ground exceeding 3,000 volts, the protector gaps breakdown and provide a low impedance path to the power system fault current. Rapid deenergization of the power supply circuit, in the event of a power contact with the wires of the telecommunication line is thus rendered possible.

Installation
Power contact protectors shall be provided on exposed telecommunication lines involved in crossing with HT power lines referred to in Paras 4.4. and 8.5 of the Code. These protectors should be installed at the pole nearest to the crossing point on the exposed wires (i.e. all the wires carried on the top most bracket) of the telecommunication line.

Earthing of Power Contact Protectors
To ensure maximum possible protection to the telecommunication line, the protector earth resistance shall be as possible a figure of ten ohms or lower, as recommended for this purpose and it is very essential that the protector earth resistance should be periodically checked and maintained within this value, as far as possible.

CHAPTER VII
Low Frequency Induction Test

Introduction
The induced voltage on a telecommunication line during an earth fault on the power line is given by the expression:
V = M x I
Where V is induced voltage in Volts,
I is Earth Fault Current in Amps and
M is Mutual Coupling in Ohms between Power line and telecom line
The value of Mutual Coupling (MC) depends upon the distance between the power and telecom lines, the length of parallelism, the fundamental frequency of the power supply and the soil resistivity of that area. The mutual coupling between paralleling power and telecom lines is theoretically calculated or computed with the help of Carson’s Curves. There is always an amount of uncertainly in the theoretically computed values of MC on account of unreliability in soil resistivity data collected and unawareness of possible effective screening offered by various metallic conductors in the vicinity.
In order to arrive at a more accurate value of MC, Low Frequency (LF) Induction test may be conducted by injecting 50 Hz low value earth currents in the power line and measuring the corresponding values of induced voltages on the paralleling telecom line. However, owing to the huge costs involved in making arrangements and conducting such test, it may be considered when there is any ambiguity in either the soil resistivity values or effective screening factors considered in theoretical calculations or variance in actual positions of telecom or power lines from the marking on the maps and the induced voltages calculated on paralleling telecom lines happen to be very high necessitating reengineering of either power or telecom lines.

Procedure
The Low frequency Induction test procedure may be divided into three parts as under.

Preparation for the test.

Conduction of the test.

Interpretation of test results

Preparation for the Test

General
DET (PTCC) of the region and the SDE (PTCC) of T&D Circle at the Telecom Circle Headquarters may do overall coordination with different agencies involved such as Power, Telecom, Railways, Defense etc in preparation for the test. Either a meeting among the representatives of the different agencies should be separately convened or in any State Level PTCC meeting the tentative date/period for the LF Induction test should be decided and intimated to all concerned.

Telecom Lines
The telecom lines involved in parallelism with the power lines on which testing is proposed should be got tested thoroughly and if there is any high resistance or low insulation fault, the same should be got removed by the Telecom authority incharge of these lines. A certificate to the effect that the involved telecom lines are in good condition and the line resistances are within prescribed limits should be given by the Telecom Authority accordingly to the concerned SDE (PTCC)/ DET (PTCC) before the due date of testing.

Power Lines
The Power authority incharge of the involved power line should also get the power line tested so as to ensure that it is free from low insulation and other faults and a certificate to that effect accordingly should be given to SDE (PTCC)/ DET (PTCC) before the due date of testing.

Soil Resistivity
SDE (PTCC)/ DET (PTCC) should arrange for a joint measurement of soil resistivity along the proposed power line at three or four test location so as to verify the data considered in theoretical calculations. In case of large variations in practically measured values from theoretically considered values, the soil resistivity should be got remeasured for every 2 to 3 kms to arrive at the actual value of soil resistivity. Procedure for measurement of soil resistivity is detailed in Appendix II to Chapter VII.
The soil resistivity data, thus obtained would be useful in analyzing the large variations, if any, in the induced voltage values arrived at by LF induction test from those calculated theoretically.

Instruments
Following is the list of important instruments to be checked and kept ready for testing by the concerned Power and Telecom authorities.
(i) Suitable transformers for injecting current in power line. Autotransformer is preferred.

Ammeters along with current transformer, if required, to cover the range for the experiment.

Voltmeters preferably digital type along with suitable rotary switch and connecting wires for connecting to telecom lines and earth.

Suitable connecting wires for connecting power lines to the transformer and through rotary switch and ammeter to earth.

Other Arrangements
Effective earthing for the power and telecom lines should be got prepared and kept ready before the testing. The testing locations should be identified and arrangements for intercommunication facility between testing parties and transportation of men & material should be arranged by the concerned Power & Telecom authorities.
3.0 Conduction of the L.F Test
3.1 Theory
For conduction of LF test the test setup as shown in Figure 1 may be used. The arrangement enables injection of currents up to 25 Amps, 400 V on the power line, the far end of which is earthed after bunching all the three conductors together. The isolating transformer is to segregate the earthing current on the line under test from affecting the power supply.
In case, with the above test setup it is not possible to drive even 8 Amps of current over and above the minimum current required to overcome the stray voltage, the test setup as shown in Figure 2 may be used. Here a special transformer, with 415 V winding on primary side and the secondary side having various tapings from 50 to 800 V in steps of 50V with a current driving capacity of 40 Amps, is used. The secondary voltage is impressed on the particular power line under test after bunching all the three phases. At the other end the three phases are bunched and earthed.
3.1.1 For earthing either the grid mat of the substation is used or a separate earth is prepared by driving a set of 3 numbers of 2.6M standard earth electrodes forming an equilateral triangle of 8 ft into the earth. Tower earth should not be used for this purpose. A similar earthing arrangement is made for the communication line under test. Measurements either limb to earth, on paralleling telecom line, whose remote end is also earthed, are made by a voltmeter as shown in Figure 3.
3.1.2 The mutual coupling can be derived from the relation V= M x I where M is constant for a given telecom circuit, the relationship between V and I has to be a straight line. The slope of the straight line, i.e. V/ I gives the value of M.

Detailed Procedure
3.2.1 One end of the power line with one or more phases bunched is to be connected to the earth and the other end may be connected in series with an ammeter to a suitable transformer for injecting the current.
3.2.2 Each wire of the telecom line should be isolated and one end may be connected to the earth. The other end is the testing end, where each limb may be connected through a suitable rotary switch to the voltmeter and earthed.
3.2.3 To overcome the effect of stray voltages due to other power lines in the vicinity, Central PTCC has recommended successive in feed from 3 phase line in its meeting held on 24.2.1982. It will enable more number of readings to be taken. Current may, therefore, be injected in the power line successively from three different phases RY, YB and BR and the corresponding induced voltages in each limb of the telecom lines may be measured by means of rotary switch and voltmeter. It is preferable to have 10 to 12 reading in between the minimum and maximum current which it may be possible to inject. No test will be valid if the number of readings taken is less than 8 (including ‘Zero’ current reading). The amount of current injected in the power line should be sufficient to overcome the static voltages already existing on the telecom line due to the presence of any other power lines in the vicinity. Feeding current should be 0 to 30A but in no case it is less than 8A over and above the minimum current required to overcome the stray voltages. If necessary, the length of power line under test may be suitably restricted to the extent of length of parallelism for this purpose. If the static voltages on paralleling telecom line continue to be high and there happens to be limitation as regards the current being fed on power line, shutting down of the other effecting power lines in the vicinity may be considered.

Induced voltage should generally increase with the increase in injecting current. In case where the induced voltage decreases with the increase in the injecting current, which may be due to the effect of other sources in the vicinity, it is suggested that the observation in such cases should be repeated.

Interpretation of Test Results
4.1 The respective current and corresponding induced voltage readings on each limb of telecom line for each phase are noted and tabulated. Average of induced voltage readings of both limbs is calculated for each telecom line. Actual Induced Voltage is arrived at as follows:
Actual Induced Voltage
Where V_{0} is induced voltage measured without injecting any current in power line.
V_{1},V_{2} & V_{3} are induced voltages measured with current injected successively in power line from RY,YB and BR phases.
Alternatively the direction of current flow may be reversed (Reversal of Current
Flow method) to overcome the stray voltages in which case the Actual Induced Voltage is given by:
Actual Induced Voltage
Where V_{0 }is induced voltage measured without injecting any current in power line
V_{1 } is the induced voltage measured with current injected successively
in one direction and
V_{2 } the induced voltage measured with injected current reversed by
180 degree.
4.2 Theoretically, the relationship between the inducing current & the induced voltage should be a straight line as explained in 3.1.2 above. However, due to possible experimental errors such as inaccuracies in the meters, imperfect earthing etc, such an ideal curve is hardly achieved in practice. Recourse is, therefore, taken to the best curve fitting method, whereby the nearest ideal straight line of the form y = mx + c is evolved. In this equation ‘m’ would give the slope of the straight line, which in this case would be the mutual coupling value.
The term ‘best fit’ is interpreted in accordance with Legender’s Principle of Least Square which consists in minimizing the sum of the Squares of the deviations of the actual values given by the line of best fit.
Considering the fitting of a straight line
y = mx + c …………………. (1)
to a set of n points (xi, yi), where i = 1,2……..n, the equation (1) represents a family of straight lines for different values of the arbitrary constants m and c. The value of m and c should be determined so that the equation (1) can be the line of best fit.
Let Pi (Xi, Yi) be any general point in the scatter diagram shown in Figure 4. P_{iA} is drawn perpendicular to Xaxis meeting the line (1) in H_{i}. Abscessa of H_{i} is x_{i} and since H_{I }lies on (1) its ordinate is (mx_{i}+c). Hence the coordinates of Hi are (x_{i}, mx_{i}+c).
P_{i}H_{i} = P_{iA} – H_{iA}
= y_{i} – (mx_{i}+c)
is called the error of estimate or the residual for yi. According to the principle of least square the values of m and c are to be determined so that
is minimum
From the principle of maxima and minima the partial derivation of E with reference to m and c should vanish separately, i.e.,
and
or …………………………... ( 2 )
or …………………………. ( 3 )
From equation (2)
…………………..……. ( 4 )
Substituting value of c in equation (3):
Thus,
………………….……… ( 5 )
From n number of observations for current injected in the power line xi Amps and corresponding induced voltages observed on the telecom line yi Volts all the quantities Σx, Σx², Σy and Σxy can be obtained and hence the value m can be calculated.
4.3 Due to the random nature of the actual curve obtained in practice, it has also been agreed in the PTCC that a statistical analysis of the results should be undertaken to establish the variation of the actual value of the result assuming student ‘t’ distribution and 90% confidence level. The later means that the result then obtained would be the correct value 90% of the time. Statistical table showing student ‘t’ distribution is given at Appendix I to Chapter VII. For a number of readings, degree of freedom to be considered is n2 and the value of ‘t’ for 90% confidence is given in Table at Appendix I to Chapter VII. The mutual coupling arrived at should be corrected for 90% confidence as shown below:
From Equation 5:
Mutual Coupling (MC)
(for 90% confidence )
5.0 Example
Name of power line: 132 KV D/C Jassore to Dehra
Name of telecom line: Narpur – Rehan
LF induction test was conducted and the readings as shown in Table 1 have been recorded.
From Table 1 various quantities are calculated as given in Table 2.
Average of induced voltage readings with ‘0’ Amps current
Table 1

Phase

Sl. No.

Feeding
Current
In Amps.

Induced Voltage
in Volts

Average Induced Voltage
in Volts

Limb1

Limb2

YB
RY
BR

1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
9
1

0
4
8
9.6
11.2
12.8
14.4
15.2
16.0
0
4
8
9.6
11.2
12.8
14.4
15.2
16.0
0
4
8
9.6
11.2
12.8
14.4
15.2
16.0
0

2.5
2.6
3.6
4.5
5.4
6.2
6.7
6.8
7.0
1.2
3.6
3.9
5.0
5.9
6.6
7.2
7.6
7.6
1.1
4.1
5.1
5.9
6.7
7.6
8.1
8.2
8.5
1.1

2.7
2.7
3.7
4.6
5.4
6.2
6.7
6.8
7.0
1.1
3.6
3.9
5.0
5.9
6.6
7.2
7.6
7.6
1.1
4.1
5.1
5.9
6.7
7.6
8.2
8.3
8.5
1.1

2.6
2.65
3.65
4.55
5.4
6.2
6.7
6.8
7.0
1.15
3.6
3.9
5.0
5.9
6.6
7.2
7.6
7.6
1.1
4.1
5.1
5.9
6.7
7.6
8.15
8.25
8.5
1.1

Table 2

Injecting
Current in
Amps.
(x)

Induced Voltage
In Volts


xy

x²

y²

YB
(V1)

RY
(V2)

BR
(V3)

0
4
8
9.6
11.2
12.8
14.4
15.2
16.0


2.65
3.65
4.55
5.40
6.20
6.70
6.80
7.00


3.6
3.9
5.0
5.9
6.6
7.2
7.6
7.6


4.10
5.10
5.90
6.70
7.60
8.15
8.25
8.50

1.48
3.17
3.99
4.96
5.84
6.66
7.22
7.43
7.58

0.00
12.68
31.92
47.62
65.41
85.25
104.0
112.9
121.3

0.00
16.0
64.0
92.2
125.4
163.8
207.4
231.0
256.0

2.19
10.05
15.92
24.60
34.11
44.35
52.13
55.21
57.46

From Table 2 Column 1 ∑x = 91.2
From Table 2 Column 5 ∑ y = 48.33
From Table 2 Column 6 ∑ xy = 581.07
From Table 2 Column 7 ∑ x² = 1155.84
From Table 2 Column 8 Σ y² = 296.03
From Para 4.2, equation 5:
= 0.394
From Para 4.2, equation 4:
= 1.378
Applying correction as per Para 4.3 above
Number of readings n = 9
Degree of freedom = 92=7
Value of ‘t’ for 90% confidence from Table at Appendix 1 to Chapter VII, i.e. t_{n2 }=1.89
Thus,
Mutual Coupling MC)
= 0.394 ± 1.895 × 0.0174
= 0.394 ± 0.033
= 0.427 or 0.361
In case the theoretically computed value of MC is greater than 0.427 then higher of the values, i.e., 0.427 should be taken as final value for mutual coupling. If the theoretically computed value is between 0.427 and 0.361 then that may be taken as final value for mutual coupling.
